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.
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