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.