Plant Reintroduction

Animal diversity depends on the plants that form the foundation of each food chain and define each of the Earth’s biomes. Trees and plants act as producers in the energy-matter cycle, so they are vital to animal life.

Plant reintroduction helps reestablish ecosystems that have been damaged or reduced by pollution, invasion, construction, or natural events such as fire. Like animal reintroduction, plant reintroduction uses only species that are natural to an ecosystem. In many cases, however, the natural plant has become endangered or threatened. Therefore, the second role of plant reintroduction is to reestablish in nature plant species that have become scarce in the biomes where they belong.

Restoration biology comprises a field of study that focuses on fixing degraded ecosystems. Preparation steps for plants that will be introduced into the environment consist of the following steps:

1. seed production
2. seed dispersal in the habitat
3. monitoring seed germination
4. establishment of young plants
5. nurturing young plants to maturity

Tree or plant reintroduction requires monitoring to ensure each species establishes itself in its new habitat and can reproduce there. The establishment period lasts a few weeks in which a plant gains a foothold in the ecosystem. To do this the establishment phase consists of a period in
which soil root systems develop, a period of steady growth rate, and then a period of slower growth, called closure, when plants compete for sunlight and soil nutrients. Closure leads to the eventual death of the tree or plant, though this may take many years. During this sequence, meanwhile, the plant life provides nutrients, shade, shelter, and other factors necessary to
the ecosystem.

Tropical forests hold most of the world’s biodiversity, but deforestation of those biodiversity hotspots has been especially hazardous to ecosystems. Tropical forests have therefore become a priority for worldwide efforts to reintroduce plant life to damaged ecosystems. The following table gives examples of plant reintroduction programs now underway in Asia.

The endangered species list contains four different plant groups: conifers and cycads, ferns, lichens, and flowering plants. Reestablishing an endangered plant in its natural habitat employs one of two approaches: complete reintroduction or enhancement. Reintroduction involves recovering seeds from an endangered plant and establishing seedlings in fields or hothouses. Biologists then return the plant to its native habitat. Enhancement endeavors to help plants that are on the brink of disappearing by adding nutrients to the soil or by introducing the same plant from another location. This second choice gives the population greater genetic diversity and so, one hopes, reestablishes a healthy population.

Today plant reintroduction takes place in every biome and in riparian areas, wetlands, and pollution-damaged sites. Plant reintroduction differs from animal reintroduction because it uses genetic engineering and tissue culture to create stronger, long-lived offspring. Plant scientist Carl
Leopold of New York’s Cornell University helped Costa Rica reestablish forests that had been nearly eliminated 50 years ago. Leopold pointed out the additional benefits to the local towns because of the program: “By restoring forests we are not only improving the native forests, but we are helping control erosion and helping the quality of life of the local people.” For these reasons and for the restoration of ecosystems in addition to forests, plant restoration is a high priority in saving biodiversity.

Animal Reintroduction

Natural-resource management uses animal reintroduction for the following two purposes: to restore balance to a damaged ecosystem and to return an endangered species to the wild. In many cases a reintroduced species can turn the tide on invasion, especially when eradication gives only partial success. Rather than put effort into eradicating difficult invaders, biologists can reintroduce animals that can restore balance to the ecosystem.

Reintroduction is done by either relocating animals from another community where their population is dense or by releasing them from breeding-in-captivity programs. Each method requires careful management of the animals during their capture, transport, and release. Reintroduction programs are more than trucking a group of animals to the wild
and opening their cages. Many animal advocates have concerns about reintroduction because they believe the process puts too much stress on both the released animals and the native species that have learned to live in their habitat without the added competition. Other critics fear for the safety of people, pets, and domesticated animals. The following case study on the return of gray wolves to Yellowstone National Park sheds light on this controversy.

Wildlife biologists develop a thorough understanding of the habitat before reintroducing any animal. Captivity breeding programs such as that run by many zoos have programs that prepare animals for release and ensure the best results for all members of the natural community. The Association of Zoos and Aquariums provides guidelines on the best
ways for carrying out a reintroduction that is successful both for the reintroduced animal and for the habitat’s existing species. According to the guidelines, a reintroduction should satisfy the following conditions:

1. aid a native population that can be self-sustaining once it is in the environment
2. preserve the existing ecosystem
3. take place in the animal’s normal habitat
4. take place in an area of the animal’s normal distribution
5. occur only in habitats not threatened by destruction
6. protect the reintroduced animal from aggression from animals in the habitat
7. not be a solution for disposing of surplus captive animals and orphans
8. not threaten the survival of species already living in the habitat

Mentor animals that live in captivity also help by teaching other members of their species about finding food and avoiding predators. Handlers teach young birds how to avoid power lines, and all activities at the training center try to minimize any connections a wild animal might develop
with humans.

Ideal reintroduction habitat should be free of overhunting, land development, fragmentation, pollution, exotic predators, or diseases that could infect the reintroduced species. Large animals such as wolves usually enter in numbers of 50 to 150 to help them build a social structure. Biologists furthermore try to release animals that are similar, genetically and physically, to the natural populations that once lived in the area. Veterinarians meanwhile make sure the animals are healthy and have no injuries; they are vaccinated before their release. Animals with infirmities or behavioral problems do not qualify for release in order to prevent them from being harmed in the wild.

Transport should involve as little noise and other stress as possible. Biologists prepare mammals for release by holding the animals in pens for a period of time at the release site. This acclimation period gives the animals time to overcome any stress caused by the trip, adjust to climate, recognize landmarks, and develop relationships in their group. Biologists provide food for certain released species, such as condors, to help their transition into new surroundings, while other species, such as wolves, simply leave their enclosure to begin their new life in the wild. Many reintroduced animals have transmitters attached to them so scientists can monitor them by radiotelemetry and learn about their behavior.

Even after all these precautions, animal reintroductions can be a tense time for biologists because no one can be certain of the success or failure of animals they have trained and come to know. Geneticist Hans Peter Koelewijn of the Netherlands summed it up this way at a conference on
wildlife reintroduction in 2008: “You can have a technical strategy, a scientific strategy, and a socioeconomic strategy, but the animals also have their own strategy.” Biology contains very few absolutes.

Zebra Mussels

Zebra mussels (Dreissena polymorpha) are small (fewer than 2 in. [5.1 cm] long), striped mussels native to the Caspian Sea. In the late 1980s a transatlantic cargo ship entered the Great Lakes and emptied its mussel-infected ballast water into Lake St. Clair. The mussels stayed there until 1992 but have now moved into all five Great Lakes plus the Huron, Mississippi, Tennessee, Ohio, and Hudson Rivers.

Like other invasive species, zebra mussels are prolific breeders: A single adult female produces between 30,000 and 1,000,000 eggs per year. The tiny larvae live as planktonic creatures, meaning they drift suspended in waters rather than settle to the bottom. This characteristic helps them travel in currents, and once they reach a new body of water, the young or the adults are not choosy about the aquatic ecosystem they invade, usually with human help. Invasive species expert Jim Carlton told National Geographic in 2005, “Before humans started moving around, the rate of species movement was a geologic rate. Now we’re moving species faster and farther than they ever would or could have moved in nature.” Adult mussels attach to surfaces and then secrete a strong, plasticlike fiber that sticks to living and nonliving things. The mussels then get transported from one body of water to another on boats or anything else that picks them up in the water.

Zebra mussel invasion creates five main ecosystem problems. First, mussels efficiently filter water for food and remove almost all the water’s algae and phytoplankton. In other words, they act as exploitative competitors because their actions remove an entire component from a food chain. Second, with phytoplankton gone, crustaceans, small fish, larger fish, and higher predators in the food chain each lose their main food source. Third, clear algae-free water allows more aquatic plants to grow, which benefits some species such as dabbling ducks but harms other aquatic species by crowding them out and blocking sunlight. Fourth, the mussels concentrate toxic pollutants in their bodies then excrete the pollutants in their feces, endangering native plants and animals near the mussels. Invaders that produce toxins harmful to ecosystems are called interference competitors. Fifth, zebra mussels have decimated the Great Lakes population of American unionid clams by attaching to their shells and restricting their movement, breeding,
and feeding. The native clams are now near extinction in the Great Lakes, and it is likely that zebra mussels will cause similar harm to other native species.

Zebra mussels have also made a surprising contribution to the ecology of the upper Midwest. Some migratory duck populations have rebounded because they have adapted to eating zebra mussels to fortify themselves during
migration stopovers. Fish such as catfish, sunfish, sturgeon, salmon, and yellow perch have also adapted to eating the mussels. Finally, smallmouth bass numbers have risen because the young have abundant aquatic plants in
which to hide and grow.



Zebra mussels are marine and freshwater invasive species that provide an example of how globalization contributes to the movement of species around the world. These animals were brought to North America from Eurasia in cargo
ship ballast. Scientists have since learned that ship ballast is a major means of transport of invasive species. (ECHO, Lake Aquarium and Science Center, Leahy Center for Lake Champlain)

The zebra mussel invasion has put an economic burden on local businesses because of the cost of removing mussels from boats, ships, buoys, docks, and piers. The mussels have clogged the intake pipes of factories, power plants, water utilities, and other businesses that withdraw water from lakes and rivers. These places must now mussel-proof their pipes by installing strainers that keep out the intruders.

Jim Carlton summed up the issue of invasive species like the zebra mussel, which have migrated halfway around the world in ballast water: “Five thousand or more species could easily be in motion on any given day.” Though invasive species move around the planet today with ease, the problem is not new. Carlton explained in a 2005 National Geographic article, “Fouling is responsible for quite a few [in San Francisco Bay], especially from the gold rush, when so many old, heavily fouled wooden ships sailed in and were abandoned here. The oyster trade. Fish bait. Ballast water brought the rest.” New Zealand, Norway, and parts of the United States have now put legal restrictions on how and where ships may empty their ballast tanks, but that will not affect the zebra mussel invasion that has already spread.

Eradication

Eradicating invasive species from a habitat requires hard work with sprays, poisons, and backbreaking labor plus another obstacle: people’s opposition to eradication programs. Few people mourn the loss of a million or so zebra mussels from a lake, but they feel differently about hunts to reduce invading deer, pigs, golden eagles, hawks, rabbits, or foxes.

Eradication programs use chemical, physical, or biological methods. Chemicals have been tried on the Great Lakes zebra mussels, but it is difficult to attain a high enough concentration of any chemical that will not also injure native species. An eradication program in northern California’s Lake Davis has been used to remove northern pike released by fishermen more than a decade ago. The pike devour other fish in the Davis watershed along with frogs, crayfish, and ducklings. Biologists have tried the chemical rotenone in a long-term program to rid the lake of the pike, but the
poison also kills all the native fish. After eliminating the pike, biologists then must restock the lake with native fish. Residents near Lake Davis have been leery of this type of chemical eradication, especially since the pike have evaded more than one attempt to wipe them out. In 2007 USA Today quoted Bill Powers, mayor of a nearby community, as cautioning, “I agree with the scientists who say there’s no safe levels of carcinogens in drinking water,” when he discussed possible contamination of the aquifers near the lake. Store owner Tammy Milby gave her opinion on the economic harm caused by the pike’s invasion of one of California’s favorite fishing lakes: “Hopefully it’ll work this time. I don’t know if the community, the businesses, can handle another failure.” Powers and Milby both express the
concerns attached to eradication programs.

Physical eradication consists of hunting and trapping. Trapping programs capture invasive animals so biologists can humanely release them into a less sensitive habitat. Sometimes, however, exotic animals require speedy capture because they have entered an environment that does not suit them, and leaving them in the habitat would be inhumane. People release reptiles, amphibians, tropical fish, and tropical birds intentionally or accidentally into places that endanger the animal. Owners of exotic animals do not always understand the special needs of these pets, and the animals die soon after being released because they are not adapted to the climate, food, or other conditions in an unfamiliar environment. Though some exotic animals can go extinct for these reasons, others cause an opposite problem: They aggressively invade the new habitat. Swamps, woodlands, and grasslands have all been victimized this way by exotic species. Like other invasive species, exotic species prey upon native animals, threaten keystone and flagship species, and consume plants or prey needed by others in the natural ecosystem.

Hunting programs consist of tracking and killing invasive animals. Hunters licensed by the Fish and Wildlife Service (FWS) or private contractors hired by government agencies perform the eradication. Many people oppose hunting on ethical grounds, especially if leg-hold traps or dogs are used to catch the animals. Eradication is also criticized when
people have become accustomed to an invasive species and now consider them regular inhabitants. In 2005 a heated debate ensued over the hunting of feral pigs on the Channel Islands off California’s coast, brought there by farmers in the 1850s. Animal-protection advocate Scarlet Newton told a local university newspaper, the Daily Nexus, “The island pig has no malice. It is just gently looking for food and is not causing a fraction of the damage humans have caused. The best solution is to leave the ecosystem alone. It is as stable as it can be and still be vital.” Eradication by hunting will likely remain a very controversial way to stabilize animal populations.

Wildlife experts have also begun to use biological eradication in which they put a species into an imbalanced ecosystem with the intent to combat an invader. Vineyards, for example, contain only one type of vegetation (call a monoculture) that upsets the natural ecosystems of the original woods, meadows, or hillsides. Vineyards often draw gophers, ground squirrels, feral pigs, and birds that feed on vines, uproot plants, or build dens. Many vintners encourage natural predators to enter the area as an alternative to chemical poisoning, hunting, or trapping. Sustainable vineyards have relied on foxes, coyotes, and raptors living in the area to deter invaders.

The Consequences of Invasion

Invasive species interfere with food webs and disrupt ecosystems. The many interdependencies among species in an ecosystem can be radically upset by an invasion. If an invader eliminates one, two, or more of the members of a web, the entire web can potentially disappear.

Biodiversity provides a hedge for Earth’s biota from a variety of assaults. Ecosystems have always been buffeted by natural occurrences such as extreme heat and cold, storms, natural disasters, and epidemics, but they withstand such onslaughts because of the diversity contained within them. Ecosystems survive disruptions according to what was
described in 1955 by Princeton University biologist Robert MacArthur as the Diversity-Stability Hypothesis. According to the hypothesis, less complex food webs with fewer interactions between species are more vulnerable than more complex food webs with more interactions between species. This is because in simple food webs each predator’s livelihood depends on a small variety of prey. If the predator’s habitat becomes threatened, it has few alternatives for survival. Complex food webs, by comparison, contain greater diversity of species and more alternatives for food chains to function. In other words, complicated ecosystems withstand threats better than simple ecosystems, which can also be thought of as sensitive ecosystems. Complex ecosystems possess what is called a buffering effect against potentially harmful environmental changes. The sidebar “Zebra Mussels” examines a situation in which native species lack a buffering effect and cannot adapt to an invasion.



The great horned owl is such a skillful predator it might also be considered an invasive species. It dominates almost all of North and Central America and is moving south through South America. Many less adaptive owls disappear from territory invaded by the great horned owl. In addition to other bird species, the great horned owl eats reptiles, amphibians, fish, insects, invertebrates, and mammals such as rodents, rabbits, skunks, and cats. (Gary M. Stoltz, U.S. Fish and Wildlife Service)

Characteristics of Adaptive Species

Adaptation occurs when an animal develops one or more new traits through natural selection. These changes in the animal’s total genetic makeup, called its genome, come about by mutations in deoxyribonucleic acid (DNA). Evolution occurs on Earth today as it did for millions of years because of mutations, but some animal populations respond to environmental changes in an accelerated manner called microevolution. Microevolution refers to any set of small genetic alterations in a community’s population that allow the population to adjust to changes in the environment. Macroevolution, by contrast, is long-term development of a new species, like the evolution responsible for populating Earth.

Favorable adaptations pass from parents to offspring until the new traits become part of the genetic makeup of all successive generations. In a threatened habitat, the individuals most likely to survive are those possessing the advantageous traits. Animals and plants develop the following adaptations to gain a measure of protection against predators: camouflage, poisons, warning colors, smells, stingers, thorns, quills, bad taste, and mimicry. Microbes also adapt protective mechanisms, but they do this much quicker than multicellular organisms because microbes need only a few hours to produce a new generation; thousands of generations from a single microbe can grow literally overnight. The current problem of bacterial resistance to antibiotics illustrates the adaptive nature of bacteria.
Doctors began using antibiotics to treat infections in the 1940s, and within 20 years numerous pathogens had developed resistance to these antibiotics. The majority of today’s common pathogens are resistant to almost all antibiotics.



The American crow is one of the most adaptable generalists in the animal kingdom. Crows are scavenger omnivores, meaning they eat plants and animals and also feed on carrion. Some inventive crows have been known to stand on a sidewalk, waiting for traffic to roll over acorns in the road. The crow then saunters out to pick up an easy meal. (Jason Finley)

Adaptive species in the animal kingdom possess characteristics enabling them to remain healthy and reproduce in spite of damaged habitats, noise, traffic, pollution, or other disturbances. In many ways, the characteristics of adaptive species resemble those of invasive species: Adaptive species breed prolifically, produce many offspring, grow fast,
possess genetic variability, and are generalists. Generalist adaptive species live in many different habitats, tolerate wide ranges in climate, eat a variety of foods, and resist disease and parasites. Familiar examples of adaptive species found living in proximity to people are cockroaches, flies, mice, rats, raccoons, coyotes, and crows.



The Galápagos Islands were one of many places that Charles Darwin visited to observe the world’s wildlife. Several subpopulations of finches have been associated with Darwin’s studies because they demonstrate the principles of evolution and adaptation. In general, the size and shape of related finches differ depending on the islands on which they live and the type of food they eat on each of those islands. Of about 13 different types of Galápagos finches, sharp-billed varieties probe for insects in tree bark, medium-billed birds catch insects on leaves, and stout-billed finches crack open cactus seeds as their main diet.

Humans are generalists, adaptive, and also invasive. People live in virtually every place on Earth, exist on a variety of diets and in a wide range of living conditions, and can adjust their behavior to avoid predation, pollution, and disease. To withstand changes in the environment, humans simply invent a new technology that ensures their survival. Humans
are predators in a number of food chains, and they invade habitats because they proliferate to large numbers rapidly and outcompete native species. Finally, humans play the role of pioneer species when urban areas encroach into pristine wilderness. Mountain lions, grizzly bears, wolves, lions, and sharks are all predators that naturally steer clear of humans. Rare attacks from these animals on humans usually involve mistakes or because the animal feels that a person threatens it or its young. The familiar phrase “He is more afraid of you than you are of him” describes the reaction of most natural predators when confronted with the novel sight of a human.

Adaptations having the most effect on biodiversity are changes that lead to adaptive radiation. Charles Darwin noted in the Galápagos Islands in 1835 that characteristics of one bird species was common to all the islands. The birds—ornithologists later identified them as finches—seemed to be of the same species, yet each had slightly differentshaped
beaks, depending on the island on which they lived. Darwin had surmised that an ancestor of the finches had once lived on one of the islands, but occasionally visited the other islands. Darwin wrote in his 1839 memoir The Voyage of the Beagle, “One might really fancy that, from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.” Darwin had drawn his conclusion from his observation that all the islands’ finches possessed slight variations in beak size and shape to master the food in their local habitats. “Darwin’s finches,” as they became known, adapted to their local habitat, yet they shared the entire archipelago without interfering with each other’s well-being.

How Resource Partitioning Preserves Species

Nature uses resource partitioning as a clever way to allow species seeking the same resources to thrive side by side in the same habitat. Rather than trying to build a relationship in which they split the habitat’s resources in equal halves—humans have a difficult time doing this also—each species evolves traits for using the resources in a way that will not threaten its competitor. For example, hawks and owls both live as predators at the top of their food chain and hunt rodents (rats, mice, voles, squirrels, etc.), small mammals (skunks, rabbits, gophers, etc.), small birds, snakes, and amphibians. To partition these food resources, hawks hunt during the day and owls hunt at night.

Resource partitioning comes about not by choice but through a genetic change in the species so that it develops a trait favorable in its new niche. This occurs over a time period long enough to allow natural selection for favorable traits, a process called adaptation. Adaptations may be physical features, physiological processes, or behavioral characteristics. In 1859 British naturalist Charles Darwin described the process by which species either survive through natural selection or disappear because they cannot adapt to changes in their environment. In his book On the Origin of Species, Darwin wrote, “Though Nature grants long periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will be exterminated.” Competition initiates natural selection, which gives rise to adaptations that in turn enable resource partitioning.

Individuals that develop traits allowing them to partition resources occupy a new niche for their species, and the next generations may do the same. Eventually an entirely new species, or subspecies, fills the new niche. This process of filling new niches over generations is called adaptive radiation, and it is the main reason for biodiversity on Earth today.

Migrations

Animals migrate to escape floods and fires, human activities, competition, or invasion from other animals, or when habitat reaches its carrying capacity. Migration is thus a natural way for animals to avoid starvation and find land that provides resources to sustain their population. Competitive exclusion causes some migrations when one species, often a generalist, outcompetes another species, often a specialist. If one species excludes the other from the habitat forever, this is called one-way migration. In the animal world, one-way migration is sometimes the only way to survive when an aggressive nonnative species invades a habitat.

Migrations aid the environment in four ways. First, grazing herbivores contribute to the health of grasses by grazing and by fertilizing the soil during migration. Second, when migrating birds or fish leave an area, insects, flowering plants, plankton, small fish, and invertebrates receive time to reestablish their numbers. Third, migrating animals often restore balance to ecosystems they enter by controlling prey numbers or providing food to predators. Fourth, migration contributes to genetic diversity within a species by allowing members of groups to mingle, split off from the herd, or join the herd.

Migration patterns consist of the times, distances, and routes taken, called migration corridors. Migration corridors comprise strips of land, air, or water where a migrating group travels unimpeded, and they are influenced by three things: resources for food and shelter, habitat destruction along the route, and population density—both animal and human. Migrating animals need places along their routes to rest and eat before continuing their trek. (Some species migrate nonstop, however, over remarkably long distances.) Migration corridors provide space for the migration’s numbers, safety from predators, and food.

Many migration corridors have been destroyed or blocked, and biologists have noted a decline in migratory species in the ocean, in air, and on land. Pipelines, roads, housing developments, and walls and fences all act as barriers to migrations. (Roads are a critically hazardous obstacle to California’s endangered tiger salamander.) When the United States–Mexico border wall was proposed, Christine Haas of the Arizona Audubon Society said, “If the wall goes up, it will be a complete and absolute barrier for terrestrial wildlife.” Haas’s prediction has played out at the border fencing in the Sonoran Desert that now blocks migrations of jaguar, pronghorn, and owls. Biologist Emil McCain told Audubon in 2007, “It’s [the border fence] probably the finest example of habitat fragmentation you can think of.” Other human-caused actions have exerted subtle effects on migrations, such as nighttime light emitted by cities that upsets bird migrations, which take place at night. This disturbance is called light pollution or photopollution.



Caribou migrations are among the largest migrations in the Western Hemisphere. These caribou in Alaska’s Togiak National Wildlife Refuge migrate in herds, behavior characteristic of animals that must eat while they watch for predators during their migration. Habitat fragmentation and blocked migration corridors present very dangerous threats to migrating species. (Aaron Collins, U.S. Fish and Wildlife Service)

The instincts that guide animals along their migrations have evolved over the millennia, yet Princeton University ecology professor David Wilcox stresses that migration in these times represents an extraordinary “act of faith.” Migrating animals do not know what might have become of their wintering grounds, spring nesting sites, or stopovers that have provided food and water for centuries. If migration corridors continue to erode, animals will have one less option for survival.

Species Competition

Species competing for living space, water, and food is called interspecific competition. When resources are abundant in a habitat, competition is minor or does not take place at all, but when resources become scarce, interspecific competition becomes a critical factor in determining whether an endangered animal lives or dies. Invasion may cause resources to become so scarce, in fact, that members of the same species begin to compete to stay alive, an event known as intraspecific competition. In addition to food, water, and space, plant or animals also compete over the following things in both healthy habitats and invaded habitats: sunlight, grazing land, soil nutrients, migration corridors, nesting sites, den sites, shelter, and hiding places.



Two or more species that live in the same habitat and use the same resources can coexist without competing. Specialists do not have much flexibility in resource use, but generalists can adjust their uses to avoid competition. Niche overlap occurs when two species use the same resource in identical ways. No overlap results in resource partitioning, which is advantageous for the strict Specialist 2 in this diagram.

Severe direct competition between species causes extinction of one of them in a situation known as competitive exclusion. In competitive exclusion, one species’ population outcompetes another species’ population vying for the same space and food, leading to a local extinction of the loser. Perhaps the two species simply cannot devise a way (by adapting) in which both can thrive in the same ecosystem. Usually the more adaptive species wins, and the loser is then considered the less fit organism for the ecosystem. The defeated species must then migrate to a new habitat or
become extinct.

Migration may not favor some animals, however, because it leaves them vulnerable to new environmental stresses, diseases, and predators. For that reason some animals have devised ways to avoid competition altogether. This avoidance serves two purposes: They save energy by not competing, energy that can be used for other daily activities, and they avoid mass migrations. Two means by which animals avoid competition are by sharing resources or by finding a unique niche.

Resource partitioning allows two different animal species to share the same resource in a slightly different manner. For example, wolves and coyotes compete, yet they find ways to share the habitat by focusing on different hunting techniques. Coyotes often hunt alone and for small mammals; wolves hunt in packs to prey on much larger animals such as elk. Though each species can subsist on the other’s main food source, they focus on the resource less likely to interest their competitor.

Other species avoid competition by seeking a unique niche in a process called niche specialization. In niche specialization, two species develop specialties that allow them to occupy separate niches in the same ecosystem.
As they evolve toward these distinct niches, they reach a point in which neither species need compete directly with the other. For example, plovers and sandpipers are birds that both live along beaches. Plovers have evolved large eyes and short, strong beaks for finding crustaceans on the sand’s surface. Meanwhile, sandpipers hunt the same sands as the plovers. Sandpipers have much smaller eyes than plovers so depend less on sight for hunting, but they do possess much longer beaks that probe deep into the sand for food the plovers cannot reach. The following table describes
the discrete adjustments that species make to share an ecosystem or a habitat.



The associations between different species represent symbiosis, which is a cooperative relationship between dissimilar species; it may be thought of as the opposite of competition. Symbiotic relationships may be between two animals or they may be plant-animal, plant-plant, microbe-animal, or microbe-plant. Two main types of symbiosis between species in ecosystems are mutualism and commensalism. In mutualism, two species interact in a way that benefits both. Bacteria living inside an animal’s intestines provide an example of a mutualistic relationship because the bacteria receive nourishment while helping the host digest food. In commensalism, one species benefits and the other is neither helped nor harmed. When barnacles attach to gray whales, they catch food as the whale swims, but the whale is unaffected. Parasitism occurs when one species benefits but the other is harmed, such as ticks living on a coyote.

Ecosystems Invaded

Nonnative species may be viruses, bacteria, protozoa, algae, fungi, multicellular plants, or multicellular animals. Many familiar plants and animals in North America began as nonnatives that humans introduced centuries ago. For instance, most of today’s cultivated crops and domesticated animals were brought onto the continent by early settlers as food and sources of nonfood products, such as wool and leather. Other nonnative plants served as ornamental additions to gardens, and nonnative animals came as companion animals. These imports arrived in North America from Europe, Asia, and South America centuries ago, but on occasion unwanted imports came, too.

For centuries the holds of ships have carried new animals to new lands: insects, rodents, fish, reptiles, amphibians, and aquatic crustaceans. These travelers disembarked at the same time that settlers and explorers set foot on new continents. Similar accidental releases of nonnative organisms take place today when oceangoing cargo ships transport nonnative organisms in their holds. Many of today’s most troublesome invaders have been introduced into aquatic ecosystems when ships empty their ballast tanks in each new port at the end of a transcontinental trip. Parasites and pathogens also get into new environments with infected crew members and tourists.

Invasive species threaten all animals in a habitat, but they are of special concern when they threaten endangered species. Usually the nature of the invader is more suited for success than the endangered species



Endangered hawksbill turtle populations are declining in the Atlantic and Pacific Oceans. Adults live in reef habitats where they feed on sponges, clearing space for other reef species to settle. Hatchlings must complete a harrowing migration from their nests on the beach to the water. Gulls and invasive raccoons, rats, and even dogs feed on thousands of hatchlings every year during this time. (Ron Mesessa)

because, unlike many endangered species, invasive species adapt readily to new conditions. In other words, they are generalists, while today’s endangered species tend to be specialists. Invasive species hold an advantage over endangered species because they often thrive in places affected by a high degree of human activity, and at the same time they outcompete native species for habitat. The major characteristics of invasive species that lead to their success are the following:

• fast growth due to high reproductive rate and short generation time
• long-lived
• easy means of transport or natural dispersal
• no natural predators (predator animals, pathogens, or parasites) in the invaded habitat
• high genetic variability
• ability to adapt

Adapting to new environmental conditions becomes a vital survival tool for any species, not just invasive species. Native species that have a generalist lifestyle withstand invasion better than native specialists. That is, adaptability and the ability to live as a generalist give an animal its best chances for survival. This becomes especially important when the presence of an intruder changes conditions within an ecosystem. When this occurs, native generalists may be able to adapt to new food sources, new breeding grounds, or new shelters for their newborn.

Even generalists sometimes cannot overcome the most aggressive invasive species. Some invasive species simply overwhelm habitats or ecosystems so quickly that native species have no time to adapt. For instance, fast-growing invasive plants can take over a lake so quickly that other plant and animal life simply cannot keep up with changes in nutrient supply, water temperature, light penetration, or any other factor introduced by the invader. As a consequence, the energy-mater cycle falters as normal producer plants disappear, then animals that feed on these plants suffer. Predators soon decline because they have less prey for maintaining their populations. An entire food chain becomes disrupted and in turn upsets the ecosystem’s natural food web.

Invasive animals differ from invasive plants in the way they upset food chains. Animals tend to affect the food chain at the point where they enter it. For example, the European wild boar came to Hawaii about 400 c.e. with Polynesian sailors, and explorers brought these hardy, prolific breeders to the North American continent in the 1500s. Now called feral pigs, these animals thrive in the southern United States from the Carolinas to the west coast. As feral pigs prey on native rodents, populations of large raptors and other mammals that eat rodents lose one source of nutrition. At the same time, the pigs devour natural plant life and uproot the earth in foraging, causing soil erosion. Invasive plants, by contrast, almost always enter food chains at the producer level and may or may not affect the organisms below them in the food chain.

In 2008 the Nature Conservancy scoured databases on ecosystems worldwide to determine the extent of invasive species. Their research revealed that every habitat in the world’s temperate climates had some degree of invasion, but the world’s most invaded habitat is California’s San Francisco Bay. Several ports line this aquatic ecosystem, and each year more than 3,000 ships plus millions of tourists arrive from all over the world. The invasive species probably enter the bay by a number of conveyances such as the following: on the hulls of oceangoing freighter ships, in ballast water (extra water carried in large ships to provide stability, then emptied when the ship enters a port), on the hulls of pleasure craft, attached to recreational equipment such as kayaks and wind-sailing boards, and even when people discard aquarium water into streams that lead to the bay. Local marine biologist Andy Cohen told KQED television in 2007, “Ballast water is currently the largest mechanism bringing organisms in. From our studies, somewhere between half and 90 percent of organisms now arriving in San Francisco Bay seem to be arriving in ballast water.” Ballast water probably accounts for most of the spread of aquatic plants and animal life to other parts of the world as well.

More than 250 species now live in the bay, and as many as 90 percent are thought to be invaders. Though not all of San Francisco Bay’s invasive species harm its ecosystems, many do, and the overall effect on biodiversity can be devastating. Cohen added, “Before these arrived, the flora and fauna of the bay were different from what we find in bays in other parts of the world. Increasingly now, as we go from one environment to the other, we see the same organisms in one place after another, and so what we have is this homogenization of the world’s flora and fauna.” The following table of San Francisco Bay’s major invasive species gives examples of how invasion harms this and other ecosystems in other bays around the world.

Sometimes environmental factors conspire to help invasive species build a stronghold in their new territory. First, invasive species very often play the role of pioneer species. Pioneer species are rugged first-colonizers of an environment, usually situated low on food chains. Bacteria, fungi, algae, mosses, and lichens provide examples of pioneer species. Higher organisms also act as pioneer species when they are the first of their kind to inhabit an ecosystem. By being the first of their kind, pioneers enjoy an advantage, as competitors, predators, or disease or parasite carriers. Some pioneer species bring additional characteristics that help them gain an advantage in a new habitat. First, even when a pioneer species could serve as food for native predator species, the native predator may simply not recognize the pioneer as a potential meal and leave the invader alone. Second, most invasive species succeed in climates similar to their native climates, and this helps reproduction and rearing offspring without a need

to acclimate. Third, some invasive plants ward off competition from native plants by depositing chemicals into the soil that inhibit the growth of the native plants.

Protecting Native from Invasive Species

Invasion of natural habitats by nonnative species contributes to biodiversity loss and extinctions. Native species consist of plants or animals that normally thrive in a particular community. Invasive species, by contrast, do not naturally belong in the environment they invade; they are also called nonnative, alien, or foreign species. Introduced species are those that have been deliberately or accidentally released in a new, nonnative environment. Exotic species also arrive deliberately or accidentally, but these species tend to be rare, unique, and highly valued. An orchid known to grow only in Brazil would be an exotic species in Connecticut for instance. By any name, these plants and animals oft en create biological havoc in the community they invade. Th is invasion of new habitats can occur by three diff erent means: migration, deliberate introduction, and accidental introduction.

Invasive species threaten native species by crowding the native populations out of their space and using up their water and food sources. Th ey do this by outcompeting the native species in the natives’ own habitat, perhaps aided by any foreign parasites that the invader also brings into the community. In many cases, the native species have no defenses against an unfamiliar pathogen or parasite, and their numbers plunge due to disease.

Invasion’s main threat to biodiversity resides in ecosystem imbalance, in which normal food webs and other interrelationships cannot work because one or more of the ecosystem’s components have been eliminated. Native species in invaded habitats disappear for the following reasons: forced migrations to new areas, disease, starvation, and unnatural behaviors. Behavioral changes often include conflicts between native animals and the invader, or against other members of the same species. Invaders usually bring aggressive mannerisms with them into their new territory, and, as a consequence, native species can offer little resistance to the takeover.

Preservation Technology

Advanced technologies aid in today’s conservation programs like those at the San Diego Center for Conservation and Research for Endangered Species (CRES). As the largest zoo-based conservation center in the world, CRES develops technologies for helping endangered animals survive and reproduce in the wild or propagate in captivity.

These technologies make up five main disciplines: sustainable populations, bioresource banking, wildlife health, habitat conservation, and restoration biology.

Conservation biologists work for the day they can release wildlife into its natural habitat. After conservation technology, biologists turn their attention to monitoring the animals by manual tracking and counting of animal populations. Various techniques have been adapted to hard-tostudy habitats and animal lifestyles, and these methods range from basic
data collection on clipboards to instrument-aided monitoring. Radiotelemetry is one instrument method that has become widely accepted for monitoring rehabilitated or released animals.

Radiotelemetry played a role in the recovery of California condor populations from the brink of extinction. In 1987 biologists captured AC9, the last wild California condor, and brought it to a specialized breeding center







at the San Diego Zoo’s Wild Animal Park. At the time only 27 condors remained in North America and all lived in captivity. Fortunately, these birds bred well in captivity, and by the early 1990s adults had raised and nurtured several healthy chicks. Human handlers trained the youngsters to avoid power lines and taught the adults to avoid picking up trash items, such as bottlecaps, glass, and shell casings, for feeding to their nestlings. (Condors naturally pick up bits of earth to bring back to their young as a calcium source.) Finally, mentor birds taught the youngsters how to evade
danger and to interact with flocks in the wild.

Rescuers returned AC9 to its natural habitat on May 1, 2002. Biologists followed AC9 along with other new releases, and in 2004 a pair of released birds successfully fledged the first condor chick in the wild since the capture-release program began. Today’s population of California condors totals more than 300 birds, living along California’s coast range, the Grand Canyon in Arizona, and Baja California, Mexico. Many continue to send signals from their transmitters to relieved scientists miles away. At the close of 2007, nature writer John Moir wrote for Birder’s World magazine, “Not long ago, I saw AC9 myself, a magnificent bird soaring over the chaparral-covered mountains of his ancient home, his mighty wings a symbol of hope that the majestic species will survive.” The California condor program exemplified the worth of conservation biology for capture, protection, and safe release into the wild of species on the brink of extinction.

Watching a Species Disappear

About 22,000 polar bears live on and near the Arctic Sea ice in northern Canada, Greenland, Norway, and Russia. This flagship species reigns atop the frozen habitat’s food web, composed of seals, fish and sea birds, krill and small crustaceans, and finally zooplankton. Polar bears are the world’s largest terrestrial carnivore (775–1,400 lb. [350–635 kg]), but today their average weight is decreasing due to a lack of food and a melting habitat.

Climate change has raised temperatures in the Arctic about five times faster than warming in the rest of the world. As a consequence, the Arctic Sea ice that builds and thickens each winter has decreased year by year, leaving less platform on which polar bears hunt ringed, harp, and bearded seals. Less ice means longer swims across open water for the bears that are already receiving fewer calories. In 2005 the marine biologist Charles Monnett alerted the world to the problem at the top of the world. He told Britain’s Sunday Times, “We know short swims of up to fifteen miles are no problem, and we know that one or two may have swum up to 100 miles. But that is the extent of their ability, and if they are trying to make such a long swim and they encounter rough seas they could get into trouble.” Meanwhile, hungry bears on land have started to wander inland to find food; some have broken into houses in northern Canadian villages. In 2006 Interior Secretary Dirk Kempthorne proposed listing the polar bear as threatened on the endangered species list, reasoning, “We are concerned the polar bears’ habitat may literally be melting.” Computer-generated models used in polar research today predict the sea ice will have completely melted by 2080 due to global temperature increases.

Polar bears have long confronted other challenges from hunting to the accumulation of toxic chemicals in their bodies, yet 25,000 bears still roam the Arctic and may represent a healthy population. In May 2008 Kempthorne nevertheless announced that the polar bear would receive protections under the Endangered Species Act. This represents the first listing of an animal whose numbers have not yet begun to decrease and also the first listing related directly to global warming.

Opponents of the polar bear’s new protected status come mainly from oil interests; drilling companies eye the Arctic as a potential new oil source. M. Reed Hopper of the Pacific Legal Foundation voiced just such opposition of the listing: “Never before has a thriving species been listed nor should it be.” Kempthorne admitted to the difficulties of listing the bear as protected, and at the same time explained the polar bear’s uncertain future: “When the Endangered Species
Act was adopted in 1973, I don’t think terms like climate change were part of our vernacular.” No one knows at this time whether the polar bear’s habitat will soon include tankers, drilling rigs, and possible oil spills or remain an undisturbed spot at the top of the globe.

Extinction

Animals and plants go extinct for many different reasons, but human activities in the last two centuries have had the greatest impact. Since the industrial revolution, the human population has grown exponentially. As a consequence, habitats have been destroyed, fragmented, or polluted, while ecosystems have been injured by climate change, pollution, and invasive species. Poaching, illegal hunting, and trade in exotic wildlife have contributed to the downward spiral of vulnerable species.

To be sure, other factors contribute to extinction, and many of these are natural events that would happen even if humans did not populate the Earth. Fossil records show that some species followed a path of evolution to a point in which new life forms emerged and transformed habitats. The original species did not adapt to the new conditions and so vanished. Species may also go extinct due to natural disasters, natural climate change cycles, competition, and overspecialization. Any extinction caused by things other than a natural event is called a premature extinction, because it occurs at a faster rate than would naturally take place. Premature extinctions occur mainly because of the following five human-related factors:

• habitat loss, destruction, or fragmentation
• invasive species
• human population growth and expansion
• pollution
• overharvesting (overhunting, overfishing, poaching)

The above factors plus natural events cause three different types of extinction: local, ecological, and biological. A local extinction may occur when a species no longer inhabits a certain area, even though it can be found in other parts of the world. Ecological extinction means that so few members of a species remain, the species can no longer play its normal role in ecosystems. Finally, biological extinction occurs when a species no longer exists on Earth.

Animal extinctions that have taken place in human history have mirrored the migration of humans from Africa and Asia to Europe, and then to the Americas. Some of the species known to have disappeared during humans’ westward migration are the following: the dodo, great auk, passenger pigeon, dusky seaside sparrow, Carolina parakeet, Steller’s sea cow, and aepyornis (elephant bird).

Scientists estimate 99.9 percent of all species that have ever existed are now extinct. Some animals succumbed to mass extinctions, which occur periodically over a course of millions of years. Other extinctions take place more frequently, about every few to several centuries, and are called extinction spasms, such as the disappearance of dinosaurs. Most biodiversity experts agree on extinction rates of between 0.1 percent and 1 percent per year throughout the world. To put these numbers in perspective, at 0.1 percent, Earth loses 14,000 species per year if there are a total of 14 million species; at 1 percent, Earth loses 140,000 species of the 14 million. Furthermore, at a rate of 1 percent lost per year, at least onefifth of the world’s plants and animals could be gone by 2030 and half by 2100. Perhaps most troubling of all, an extinction rate of 0.1 to 1 percent may be a conservative estimate, which means the extinction rate could be significantly faster. Edward O. Wilson has pointed out perhaps the main obstacle to finding a true extinction rate: “The vast majority of species are not monitored at all.” Scientists clearly cannot study things that they do not know exist.

Fossil records help fill in the gaps regarding species that have disappeared. Fossils consist of mineralized sections of things that once lived: bones, teeth, entire skeletons, shells, leaves, or seeds. Sometimes the biological matter has degraded and only an impression in sediment remains, but these impression fossils are valuable because they show what an animal or plant looked like millions of years ago. In addition fossil structures compared with skeletons of present-day animals or plants shed light on how a species may have evolved.

Fossil studies have drawbacks that make them useful only as complements to other technologies. First, some species may have left no fossil records, or, second, their fossils have not yet been discovered. Third, many fossils decompose to a condition that yields little information about the creature that had lived millions of years ago. Despite these disadvantages fossil records have been used to show the following animals became extinct in prehistoric eras: mastodon, mammoth, wooly rhinoceros, saber-tooth cat, and dire wolf.

Three additional technologies supplement the information from fossil examinations. The first technique is radioisotope dating. An isotope is a form of an atom that spontaneously emits an alpha or beta particle or gamma rays. American chemist Willard Libby of the University of Chicago won the 1960 Nobel Prize in chemistry for developing radioisotope
dating, which measures the isotopes emitted by fossils. Libby devised a method based on the radioisotope carbon-14, which the atmosphere makes continuously when neutrons from the Sun bombard nitrogen molecules. In one reaction, a Sun neutron displaces a neutron from nitrogen and emits carbon-14 and a proton (N is nitrogen, C is carbon, and H is
hydrogen):



A small portion of Earth’s carbon is carbon-14, which, when it decays, returns to 7N14 by emitting an electron. Carbon dating makes use of these reactions plus the two following pieces of information: carbon-14 decay has a half-life of 5,730 years, and living organisms contain a constant amount of carbon-14 (about 1 atom of carbon-14 per 1012 atoms of normal carbon-12) in their cells. When an organism dies, it no longer metabolizes carbon, and its carbon-14 content gradually decays. Carbon-14 degrades at a known rate of 693 disintegrations/hour/gram of total carbon, so an estimate of the age of an object becomes



where ln is a natural logarithm, Nf/No is the percent of carbon-14 in the sample compared with that in living cells, and t1/2 is the half-life of carbon- 14. When it is put all together in the following example, a fossil containing
5 percent of its total carbon as carbon-14 can be dated as follows:



A second method used in dating fossils involves deoxyribonucleic acid (DNA) analysis. Analysts compare the DNA recovered from a fossil with the DNA from a similar present-day organism, and they use the assumption that mutations occur in DNA over time at a constant rate. By this method, scientists have estimated the age of Siberian permafrost to be 50,000–100,000 years old.

The third method of dating ancient life is done by core sampling of glacial ice. This technique has helped scientists correlate the depth of the core sample to the number of years ago in which the ice formed. The U.S. Geological Survey’s (USGS) National Ice Core Laboratory has recovered cores from polar sites that date to at least 100,000 years.

Assessing Habitats

Preserving species in the wild requires preservation of the species’ habitat as well. Abundance or scarcity of habitat plus the species’ characteristics determines how species distribute across the Earth, and this distribution follows six different patterns. The first is continuous distribution, in which an animal exists almost all over the world. Crows, which are generalists, are an example of continuous distribution. Second, a disjunctive distribution takes place when a species must live in a highly fragmented habitat. The Indian tiger once lived in a continuous band extending from northern China, along the eastern portions of Asia, and over the Indian subcontinent. Today the tiger’s disjunctive habitat includes small, isolated pockets in Southeast Asia. The third type is restricted distribution, in which fragmentation has not occurred but the total area of the habitat is greatly reduced. The mountain gorilla has been restricted to two national parks in Africa, the last habitat available to this endangered species. Fourth, an evolutionary distribution describes animals that were once dominant in their habitat but have now lost most of their population. The scattered survivors of the American bison are an example of this type of distribution. Fifth, a climate-affected distribution is one in which species have been affected by climate change. In some areas of the world, their habitats have been severely damaged or completely destroyed, such as marine coral habitat. The “Watching a Species Disappear” sidebar on page 58 describes another effect of climate on a specialized habitat. Finally, the sixth pattern
is endemic distribution. Endemic distribution, called endemism, defines species found only in one place on Earth, but they may be abundant and not threatened in that place. Wallaby species found only in Australia provide an example of endemism.

Certain species, called flagship species, serve as a symbol of a habitat and are fairly easy to monitor. These species help biologists gauge habitat health and the potential risks to other species living in the habitat, and they act as public relations ambassadors. Conservationists rely on flagship species to symbolize the plight of all animals in the environment, because the public recognizes these animals and may feel more inclined to help them avoid extinction. The Mother Goose Syndrome describes this type of thinking in which people feel a closer relationship to animals they
perceive as friendly or cuddly (pandas, otters) and less connection with animals that are dangerous or frightening (snakes, vultures).

Flagship species are usually large animals that people identify easily and represent certain traits in people’s minds. Therefore lions are rulers, panthers are stealthy, pandas are cute, and so forth. Conservation biologists also refer to these species as charismatic species, perhaps best represented by polar bears. Global warming and rapidly melting polar ice have created a flagship story for the environment in the past decade. Polar bears need stable ice floes for hunting seals, and, therefore, for their survival. Polar bear images adorn greeting cards, stuffed toys, and many other items in Western society, so people worry more about these animals’ fate than they might worry about the other species dependent on the ice. The Canadian ecologist Ian Stirling described to the Associated Press in 2008 a study he
led on the effect of global warming on polar life other than bears: “What we wanted to do was look at the whole picture because there’s been a lot of attention on polar bears. We’re talking about a whole ecosystem. We’re talking about several different species that use ice extensively and are very vulnerable.” Indeed, an ecosystem contains many roles or niches. The following table summarizes the main types of species, in addition to flagship species, used in habitat assessment.

Habitat assessments often uncover one of the main threats to species today: habitat fragmentation. Fragmentation results from incursion of humans into animal habitats, causing the habitats to break into smaller,



isolated areas. Some of the major human activities that fragment habitats are roads, sound walls and border walls, fences, canals, pipelines, cultivated land, and suburban areas. Industries, too, such as strip mining and logging fragment habitats, and natural events such as floods, earthquakes, erosion, and lava floes can also cause fragmentation. Any large, continuous habitat may suffer from these occurrences, but the habitats now under the greatest threat from fragmentation are forests, riparian areas, ocean dunes, and open grasslands.

Fragmentation causes two threats to the species living in them. The first threat comes from a separation of a population into two (or more) smaller populations. If animals cannot reach one another, they cannot congregate and breed, so the species loses genetic diversity. Over time mutations and natural selection in two separated groups of the same species make them so dissimilar they can no longer successfully breed with each other. Two geographically separated species then arise from a common one, an event called speciation.



Deforestation by clear-cutting destroys large areas of forest habitat within days. In the Amazon forest shown here, people clear-cut forests to increase area needed for cultivation or grazing cattle or for obtaining wood for heating. Clear-cutting may also occur when corporations set up mining, logging, or large-scale agriculture. (Rhett A. Butler/Mongabay.com)

The second problem caused by fragmentation comes from the edge effect. Edges in the environment are places where two habitat types meet— a forest meeting an open meadow, for instance. Some species thrive on edges because the edge increases their chances of getting food. But edges may also harm species in the following ways: increased threats from predators that hunt along edges, vulnerability to parasites not found deeper within the habitat, threats from harsh weather conditions—wind and temperatures tend to be more extreme at edges—may threaten newborns
and decrease the density of insects for insect-eating species. Edges also open up opportunities for some species. Brown-headed cowbirds prefer living along edges where they can act as brood parasites, meaning they infiltrate other birds’ breeding areas. The female brown-headed cowbird invades a nest when a parent is away, lays her eggs next to the native eggs, and then flies off. Cowbird young have particularly large, gaping mouths, so native parents bringing food back to the nest tend to feed them more than their own offspring. In the worst circumstances the native chicks
starve to death and the cowbird chicks thrive. Fragmentation therefore causes a change in ecosystem behavior.

Case Study: The March of the Argentine Ants

Some species by their mere presence indicate that an ecosystem is under threat. The spread of Argentine ants from South America to North America illustrates how one species directly and indirectly can endanger other species by taking advantage of a quirk in genetic diversity.

Argentine ants arrived in New Orleans in the late 1700s aboard coffee-carrying ships from South America. By 1820 the insects had spread throughout New Orleans, and within the next century the ants began an inexorable march northward beyond Louisiana. By 2000 they had spread into the following states: Louisiana, Mississippi, Alabama, Georgia, Florida, South Carolina, North Carolina, Tennessee, Arkansas, Texas, and Oklahoma. Argentine ants have now spread into the Pacific Islands, Europe, and southern Africa.

Like many successful invaders, Argentine ants display aggressive tendencies when they take over new territory. They fight and defeat native insects and soon overrun natural ant colonies, the hives of native stinging insects, and even birds’ nests. Genetic studies of these far-flung populations have turned up a surprising phenomenon that helps them as invaders: The ants have almost no genetic diversity. A single small colony, perhaps from a single ship in the port of New Orleans, may have provided the ancestors for all the generations that have now spread around the globe.

The Argentine ants passed through what is called a genetic bottleneck in which very little genetic diversity exists. Usually such bottlenecks confer a disadvantage on species, but the Argentine ants manage to use their relatedness as a distinct advantage. Normal ant colonies have organized social structures so that members work in a cooperative fashion, and when one colony invades the territory of another, the two families fight for the spoils. Argentine ants, however, act as one big family of close cousins. Because they all recognize each other as kin, they form huge cooperative colonies that overtake species normally able to repel attacks. When Argentine ants invade, they actually make biodiversity go backward because they eliminate all other various ant species and replace the territory with their one supercolony. The biologist Andrew Suarez noted in 2000 in the New York Times, “Some people say, ‘Big deal; you’re just replacing some ants with others.’ But the thing is, you’re displacing twenty species. And all the roles they play are wiped out: dispersing seeds, pollinating plants, providing food for other animals. All the functions are totally
lost when Argentine ants move in.” These ants wreak all this havoc by creating what is called a trophic cascade.

A trophic cascade occurs when a change in one species’ population leads to major disturbances of other unrelated species. California’s coast horned lizard and the similar Texas horned lizard both live a life evading their predators: birds, mammals, and snakes. As their defenses, the lizards rely on spiny armor and cryptic coloring to blend in with their desert habitat. Each lizard species also reproduces slowly, so they are not likely to develop adaptations fast enough to meet changes in environment. Enter the Argentine ants. Both lizards depend on native harvester ants
as their main food source, but when the Argentine variety decimates native ant colonies, the lizards cannot adapt to new food sources. When the lizards roam out of their natural habitat in search of harvesters, their camouflage does not match the surroundings and predators pounce. Meanwhile plants that depend on harvester ants to spread their seeds also decline. When the native vegetation declines, so too do insects that depend on the plant life. The birds that eat the insects disappear, and next the animals that prey on birds go hungry.

The biologist Ted J. Case of the University of California at San Diego has offered some words of comfort on the unusual ability of Argentine ants to form huge, homogeneous colonies. The ants’ relatedness, he explained to the New York Times, “may be useful in the short term, as it is for Argentine ants and some other invasive ants, but in the long run it appears to be an evolutionary dead end because social behavior could not evolve.” Meanwhile the Argentine ants demonstrate the intricate ways in which biodiversity can be threatened.

Measuring Species Loss

Animal species decline may be estimated using mathematical models, which are built on vast data collections on the number of species found in each habitat. Three complementary methods supply those data: field surveys, global mapping of plant diversity to assess animal habitats, and estimates of animal numbers based on the effect of humans on habitat loss, in a technique called the biodiversity intactness index (BII).

Field studies consist of manual counts of animals in the wild followed by the creation of species lists. Local wildlife officials in national parks and university research teams usually do the counting and they compile the data from the study areas to build a picture of species distribution in a habitat or across larger regions such as counties and states.

Species counts provide valuable information on areas containing a high degree of biodiversity, as well as areas with little biodiversity. Areas having the greatest biodiversity are called hotspots, and these places have drawn worldwide interest as the most critical for protection. Species counts also help biologists determine the effects of habitats damaged by reduction in size, fragmentation, pollution, and other forms of destruction.

Two methods provide biologists with useful information in addition to that received by manual counts. The first method, camera surveillance, enables scientists to observe animals that live secretive lifestyles or in remote places. The Wildlife Conservation Society currently has set up 15 cameras in northeastern Cambodia along trails used by rare Asian tigers. As an animal passes between paired cameras, it breaks a detection beam and triggers the cameras. (Two cameras are used for each tiger because tiger stripes are not symmetrical and the combined images from each
flank help researchers identify individual animals.) Despite this high-tech approach to species monitoring, cameras have caught only one tiger, but the project leader Ed Pollard dreams of the time when the tiger’s habitat is sufficiently preserved to sustain a population. “In twenty years,” he said in 2007 to the Associated Press, “people will come here and drive along the road and there’s a distinct chance there’ll be a tiger trotting along in front of you.” This of course represents the goal of every conservation project.

The second method, fossil records, provides fewer technological results than remote-controlled cameras but offers a historical view of species rise and decline. Fossils give evidence of ancient organisms as well as a physical description of these organisms. Fossil records do not, however, give scientists all the information they need today about biodiversity becaus today’s available fossil records may represent only about 1 percent of the species that have ever lived on Earth.

Global mapping techniques are used for building databases on plant diversity, which in turn provide clues to the likely animal diversity in specific habitats. Any confined study area may be mapped by combining field study data with high-resolution satellite images. On a larger scale, global plant diversity mapping depends on statistics to predict where biodiversity is rich and where it is scant.

Despite the varied manual and technological means of assessing biodiversity, putting a quantitative value on biodiversity remains a difficult task, and no single method does the entire job. For this reason the Convention on Biological Diversity introduced in 2005 a tool for describing biodiversity in a single, standard manner. This BII quantifies the abundance of a diverse population relative to a well-studied reference species. The BII requires only a few weeks to gather data, compared with previous databases on species that have taken decades to compile. Biologists then calculate an index value as a percentage and using three pieces of information: the current way an ecosystem is being used (wilderness, agriculture, ranching, etc.), the area of the land being measured, and the species richness in that area. Using the grasslands of Kansas as a hypothetical example, a BII of 80 percent means that, when averaged
against all plant and animal species in the region, Kansas’s grassland populations have declined to 80 percent of their preindustrial (about the year 1800) numbers.

The BII assesses general trends in biodiversity rather than exact values, so that biologists learn about functional groups like insect-eating birds, small burrowing mammals, large grazing mammals, or rain forest amphibians. The BII also takes into account the types of human activities on the land, which of course greatly impact the status of the species living in the region. In summary, the BII indicates where biodiversity is disappearing the fastest and where it might be increasing.

The BII may soon supplement the information already gained from field studies, fossil records, and mapping. To date, these methods have together shown the following trends in biodiversity:

• Populations have declined 84 percent since the preindustrial period.
• Mammals declined the most in this period, 71 percent.
• Species associated with a specific habitat declined the most, 26 percent, in grasslands.
• Ninety percent of habitat loss leads to the extinction of about 50 percent of the habitat’s species.
• Greatest biodiversity losses in Africa are in countries with the continent’s greatest population density: Lesotho and Swaziland.

Georgina Mace, director of science at the Institute of Zoology in the United Kingdom, explained to Nature magazine in 2005, “Biodiversity assessments need to move away from species lists and species extinction rates, because often the existence and proximity of local [human] populations matters more. [The] biodiversity intactness index makes a start in satisfying many requirements and provides a robust, sensitive and meaningful indicator.” For this reason comprehensive indicators such as the BII give the best picture of biodiversity losses and gains.

Environmental Indicators

Environmental scientists would learn very little about habitat and diversity by monitoring crows or other generalists. Specialists, however, often fluctuate in numbers in response to changes in the environment. For this reason many specialists are called indicator species because they react dramatically to changes in the environment and so serve as early warnings of environmental decay. The following table lists some indicator species and the information they provide in environmental science.

Like a canary carried into a mine to detect deadly gases, birds serve as harbingers of danger in the environment before humans sense it. Certain species require very specific habitats, so monitoring them is the best way to monitor that habitat, whether it is a wetland, beach, riparian area, woodland, or forest. Birds therefore serve as excellent biodiversity indicators for the following reasons:

• live in every climate and biome
  
• participate in almost every terrestrial and aquatic ecosystem on the earth
• migrate between climates and biomes
• respond quickly to changes in habitat
• easy to track and count
• give behavioral clues to threats
• most species play a central role in numerous food webs
• different species depend on certain terrestrial plants, aquatic grasses, trees, seeds, insects, rodents and other mammals, and marine species as food
• reproduction is sensitive to pollution

Environmental Ethics

Earliest human society consisted of hunter-gatherers in which members of a settlement ventured afield to collect plants and fruits, to fish, and to hunt meat-producing animals. In these societies, humans behaved as predators in a sustainable manner, meaning they hunted to sustain their village but they did not decimate wildlife populations.

Today Earth approaches 7 billion people, which is beyond its carrying capacity. Population densities in Africa, Asia, and South America have forced some people into a far more menacing predator role in which wildlife numbers and habitat disappear in the face of human activities. Environmental ethicists have confronted the underlying cause of this problem: poverty. The International Union for Conservation of Nature (IUCN) and the European Commission published this viewpoint in an undated briefing titled Biodiversity in Development: The Links between Biodiversity and Poverty: “Poor people themselves are often the cause of biodiversity degradation and loss, especially if lack of income alternatives drives them to over-exploit the resources.” This statement emphasizes the complex association between human poverty and biodiversity.

Environmental ethics involves the search for a solution for two opposing needs: human hunger and wildlife survival. Many African communities depend on their native wildlife for food (called bushmeat) and for income. On a small scale this practice was at one time sustainable, but an increased demand for food and space far exceeds the capacity of wildlife populations to keep up. Threats to the survival of African wildlife now include the following: people hunting for food, destruction of habitat for agriculture or urban development, disappearance of prey animals, retaliatory or preventive killings to protect villages from predator animals, and illegal wildlife smuggling and poaching for income. Some residents sell their native animals as sources for medical drugs, nonmedical cures and supplements, aphrodisiacs, religious and ornamental items, and exotic foods. Wildlife protections exist in many nations but, unfortunately, as governments strengthen the protections, black-market prices for animal products soar. As a result, the extinction of some animals hastens rather than slows.

These forms of legal and illegal hunting have been driven by hunger. Starvation is an immediate crisis for people in many regions of the world in addition to Africa, and things like bushmeat often provide a family with its only protein source. So a need to protect endangered wildlife faces another equally critical need: preventing human starvation.

Some corners of the world rich in biodiversity have revised their relationship with native endangered wildlife. For instance, hunters in Thailand and Costa Rica refrain from capturing rare birds, reptiles, amphibians, or fish because they can make more money keeping them alive (instrumental value) for ecotourism. These two countries now earn more from ecotourism than they would by destroying animal habitats.

Ethicists must also consider conflicts between biodiversity and cultural needs that may not have an obvious instrumental value. The black rhinoceros’s habitat in sub-Saharan Africa has stayed about the same size in the past 30 years, yet 90 percent of the animals have disappeared. Two related factors have contributed to this tragedy: a black market that deals in rhino horns and petroleum. In Yemen, young men earn daggers with elegantly carved handles made from rhino horn as a symbol of status and wealth. As oil-rich Yemen’s wealth has grown in the past few decades, the country has become the world’s largest importer of black rhino horn to make these ceremonial pieces. The Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES) has put a global ban on the import of black rhino horn, but as a result the black market for rhino horn has flourished. Armed guards now protect many rhinoceros herds against poaching, and conservationists have even resorted to tranquilizing the animals and cutting off their horns to dissuade poachers. The dehorned animals confront another problem: adults use their horns to establish herd hierarchies and to protect their young. As with most ethical dilemmas, more than one aspect exists to each problem.

In 2005 conservationist Adam Oswell said in a radio interview in Australia, “In countries where people don’t make a lot of money, they’re not concerned about killing animals; they just want to feed their family and make money.” The link between poverty and biodiversity cannot be explained any better.

Gene Pools and Niches

A gene pool is the collection of all the genes in a particular population of individuals. Genetic diversity enhances biodiversity overall by improving the traits carried in a species’ gene pool. Therefore today’s biodiversity programs rely on the knowledge gained from genetic studies.

Genes control the traits of every plant and animal, and each generation transfers its genes to the next through asexual or sexual reproduction. Sexual reproduction gives an advantage to organisms because it creates more diversity in their gene pool by combining the traits from two unrelated parents. A greater variety of potential parents and greater variety of pairings in a breeding season therefore increase genetic diversity in offspring. Over time an animal population acquires advantages for survival in two ways: variations in its gene pool and random gene mutations.

Even a minute change in an individual’s genes might give an animal a better chance of adapting to environmental change. Within a few generations, the advantageous gene has been passed on to many of the group’s offspring. For example, peppered moth populations living in London, England, changed their coloration during the 1950s from light gray to sooty black. The reason? Each generation of moths had gained genes for dark color so that the moths could blend into a landscape marred by pollution, smoke, and dark soot. This process increased the moths’ fitness, a species’ ability to sustain health and reproduce in its environment. Small genetic changes that enhance fitness in any specific population—London moths compared with the same species in Liverpool—over a few generations is called microevolution.

Animals evolve to meet changes in their environment by acquiring adaptations, and these adaptations may also determine a species’ role in an ecosystem. This role of a species within an ecosystem is called an ecological niche, or simply niche. Students often assume that to “occupy a niche” means an animal occupies a specific location. This is actually the definition for habitat; a niche is a species’ lifestyle or role in that habitat.

To occupy a niche, a species depends on certain foods, plants, and physical and chemical conditions within its ecosystem. Fundamental niche refers to the combination of potential physical, chemical, and biological factors that animal species use for survival. The concept of potential is important in this definition, especially when many niches overlap. Earth’s species have evolved to occupy fundamental niches that eliminate competitions, but when a species occupies a niche that overlaps with another species’ niche, it has two choices for survival: compete or adapt.

Competing directly with another species may increase the number of deaths in both species’ populations, so for the benefit of both, species often adapt to avoid competition. When different animal species adapt in this way, they are said to occupy a realized niche, a specialized portion of the fundamental niche. For example, elks in North America digest woody plants that grow on high mountain slopes, as well as plants that grow on flat lowlands. Elks are therefore capable of occupying a fundamental niche as a general grazer. Wolves have evolved to hunt elk on flat terrain to make the best use of the pack’s ability to chase a herd and separate out a single individual. Elk therefore increase their survival chances by spending as much time as they can (other than to find water) on mountain slopes, where wolves tend not to hunt. They therefore occupy a realized niche, that of high-slope grazer of woody plants. Wolves benefit too because they conserve the energy it would take to chase elk over mountainous terrain. The wolves can carry out more successful hunts by targeting elk herds that descend for water.

North American elk make a behavioral change to occupy a realized niche, but other animals undergo a physical adaptation to accomplish the same thing. For example, two lizard species may look and act similarly and both may prefer to feed on the same types of insects. In the same habitat these species would compete directly for the same food, but through microevolution one species becomes slightly larger than the other. The larger variety of lizard ingests larger insects and leaves the tiny meals for the smaller lizard. Rather than compete, each lizard conserves its energy by feeding differently instead of competing. This behavior is called niche differentiation, or niche splitting, and it occurs only between two similar species.

How do niches affect biodiversity? The more specialized the niche, the more vulnerable an animal is to change in its habitat. Conversely, the best survivors, called generalists, thrive in broad niches; that is, they survive on many different types of food and tolerate a wide range of environmental conditions: crows, coyotes, cockroaches, and humans live as generalists. The “Case Study: The March of the Argentine Ants” gives an example of what happens when a generalist invades a habitat. Specialists, by contrast, are not as versatile and occupy narrow niches. Specialists tend to live in only one type of habitat, on a single type of food, or in a narrow range of environmental conditions. The northern spotted owl, discussed earlier, occupies a narrow niche. Other examples of specialists are giant pandas, polar bears, tiger salamanders, and red-cockaded woodpeckers. The woodpecker illustrates the precarious lifestyle of some specialists. This bird nests by carving holes only in longleaf pines that are at least 70 years old. Old longleaf pines have become limited to the southeastern coastal plain of the United States from the Carolinas to Louisiana. If the remaining longleaf pine forests disappear, the woodpecker will disappear, too. Meanwhile birds of the family Corvidae, more commonly known as crows, are generalists with no such restrictions. Crows range to just about every landmass on Earth and eat—with only slight exaggeration—anything in sight!


Two distant relatives that may have very different futures. (a) A three-toed sloth hangs in the foliage in a Costa Rican rain forest. This species has a fairly restricted habitat in only certain tree species that live in northern and central South America. (Keith Sirois) (b) A raccoon is a generalist that adapts to varied environments throughout North and Central America, parts of Europe, and Japan. (David Menke, U.S. Fish and Wildlife Service)