Invasive Species

Invasive species are the second most important threat to biodiversity conservation globally, threatening individual species and even entire ecosystems. Furthermore, as humans carry species from one part of the world to another, we potentially endanger the fundamental root of the world's biodiversity—the evolution of new species. Spatial barriers that isolate populations from one another create and maintain biodiversity, fostering the evolution of new species. By transferring species to and fro across these barriers, we break down the natural process of evolution. The frequency, geographic scope, and sheer number of species that humans have carried from one area to another have increased tremendously as transportation and commerce have evolved.

Multiple terms are used interchangeably to describe invasive species; some of these terms are synonymous, while others are distinct. One major distinction lies between exotic species and invasive species. An exotic species lives outside its native range (Hunter, 2001). Terms such as nonindigenous, nonnative, alien, adventive, neophyte (for plants), and introduced are synonyms for exotic. Invasive species, on the other hand, can be exotic or native species whose populations have expanded dramatically and out-compete, displace, or extirpate native species, potentially threatening the structure and function of intact ecosystems. Not all exotic species are invasive. Many populations of exotic species do not survive for long in their new environment. Others become established but do not substantially disrupt their new host environment. Similarly, not all invasive species are exotic. Scientists are increasingly documenting native species whose populations grow out of control or substantially increase their range because of human-induced change in their environment. These range shifts may also be the result of natural changes, since it is difficult to distinguish between human-driven and natural changes. These species often prey on or parasitize species at a higher rate than previously, or they hybridize with or outcompete close relatives. Some examples from North America include the coyote in the eastern United States and the brown-headed cowbird throughout the United States, subarctic Canada, and northern Mexico. Cowbirds are a native species that were once restricted in their range to the Great Plains, where they followed bison herds. Human alteration of landscape has allowed their range to expand and thus impact other native bird species. Cowbirds are brood parasites that don't build their own nest but lay their eggs in the nests of other bird species. These birds then raise the cowbird young, often to the detriment of their own offspring. So little is known about native invasives that the rest of this section will focus mainly on exotic invasives.

Research into why and how some species become invasive is still preliminary, in part because of the complex process of invasion. The most comprehensive works on exotic invasive species (for example, Elton, 1958; Cox, 1999; Mooney and Hobbs, 2000) are compendia of case studies—predictive models still in development that detail the species most likely to become invasive and the potential consequences their invasion (Pimm, 1989). There appear to be three major stages in the process of invasion by exotic species: dispersal, establishment, and integration (Cox, 1999).

Dispersal Stage

The first stage—the dispersal stage—comprises how species move from one area to another. Characteristics of the donor region, of the dispersing organism, of the natural and human agents that affect dispersal, and of the colonized region all influence the success of exotic species at this stage. The inexorable globalization of human societies has afforded exotic species myriad avenues for dispersal into new environments. Humans have unwittingly brought "stowaways" along in containers such as ships' hulls and ballast water, on muddy shoes, or in our digestive tracts that have profoundly changed the face of their new environment. Classic examples of these stowaways include rodents, such as the Norway and black rats, and the house mouse, which have contributed to the demise of innumerable native species around the world, most notably island species; diseases such as smallpox and measles that decimated indigenous human populations when carried to the New

World and Australia by European explorers and colonists; marine invaders such as European green crabs that may have arrived in the United States in bilge water and are changing the structure of intertidal communities along the West Coast; and plant invaders such as the southern Russian or Ukrainian leafy spurge, which arrived in the United States as a contaminant of grain and is now crowding out the remaining native plant species in prairies.

Humans also deliberately brought exotic species with them when they settled new lands. These species served for food and commerce, fiber and fuel, medicine, sport, scientific interest, wind breaks or erosion control, and simple enjoyment. Sometimes species have been introduced to help contain outbreaks of other invasive species—unfortu-nately often with unanticipated consequences. On occasion, single individuals have caused the extinction of a whole species. A particularly predatory cat belonging to the lighthouse keeper on Stephen's Island, between the North and South Islands of New Zealand, evidently hunted and killed every last individual of the Stephen's Island wren, ironically just as it was identified as a species new to science (Hunter, 2001). Domestic cats in the United States (more than 60 million of which are pets and perhaps 30 to 40 million more that are feral) are estimated to kill more than a billion small mammals and at least 200 million birds annually. They are partly responsible the endan-germent of at least six species of North American birds and small mammals, and for the extinction of more than twenty animal species in Australia.


The second stage in the process of invasion— establishment—encompasses how biotic and abiotic factors in the colonized region affect the initial survival, reproduction, and expansion of invasive species in a new area. An overwhelming majority of exotic species are unsuccessful in establishing populations when introduced to mainland settings. Some scientists have argued that tropical oceanic islands are more susceptible to invasions, but the evidence for that seems equivocal. Many organisms arrive in a new region and are swiftly eliminated by any number of physical or biotic agents. Others survive but do not expand their populations or become harmful to the environment.


The final stage in the invasion process—integration—embraces how exotic species interact with the communities and ecosystems they invade, and the factors that affect their rates of expansion both in population size and over space. As noted above, some exotic species settle into their new environment and become naturalized. They do not depend upon reimmigration from their natural range to persist. A few of these species then become invaders. Scientists estimate that, of every 1,000 species that reach a new region, some 100 will settle temporarily, 10 will establish long-term populations, and 1 will become a problematic invasive species (Cox, 1999). The transition from naturalized immigrant to invader often encompasses a long delay, or lag phase, followed by a phase of exponential increase that diminishes only when a species reaches the boundaries of its new range. Many extinctions of immigrant populations take place during this time. It is often difficult to predict which species will remain as naturalized immigrants and which will become invasive.

What Makes an Invasive Species Successful? Unfortunately, it is currently next to impossible to predict the success of invasives, nor can we even come up with a comprehensive list of attributes of common invaders. Some evidence points to greater success for species with higher numbers of invaders, for those that are widespread in their native habitat, as well as for those that settle in already heavily disturbed areas. The European starling was released in New York City around 1890 by someone who wanted to bring all the birds mentioned in the writings of Shakespeare to North America. The initial release of starlings was unsuccessful, and several more individuals were brought over. From these relatively modest beginnings, the European starling has become arguably the worst invasive bird in the United States. Fifty years after its release, its population size was estimated at 120 million birds. Starlings are displacing native birds, particularly cavity nesters, throughout the eastern United States.

In terrestrial ecosystems, the most successful intruders seem to be those that are significantly different from native species. For example, Myrica faga, an exotic tree, has successfully settled in Hawaii and is affecting the entire ecosystem that it invaded. This tree is a member of the legume family and harbors symbiotic bacteria that convert atmospheric nitrogen to ammonia, a trait not present in native plants in the ecosystem. The introduced tree forms dense canopy stands beneath which other plants don't grow. Because of its nitrogen-fixing capabilities, it can colonize nutrient-poor volcanic sites faster than native plants and prevent native communities from establishing.

Ecological Consequences of Invasive Species The list of consequences of invasive species on their host environment is as lengthy as it is depressing. Invasive species can cause local or global extinction of species as well as complete disruption of an ecosystem's structure and function. As predators (including herbivores such as cows, goats, pigs, and rabbits), invasive species often benefit from encountering "naive" prey that have not yet developed appropriate defenses. A well-known example is the brown tree snake, which was introduced inadvertently on many Pacific islands and subsequently caused the extinction of a number of native birds, bats, and lizards. Similarly, the newness of exotic species to a region may also mean that they themselves escape predation, as potential predators do not yet look upon them as prey. Freed from their natural predators, competitors, and diseases, populations of exotic species flourish in their new environment.

Exotic invasive species often outcompete native species for food, water, shelter, nutrients, light, and space. The North American gray squirrel is outcompeting and replacing the native red squirrel in Britain and mainland Europe. Particularly successful invaders include zebra and quagga mussels, which were introduced into the Great Lakes region of North America sometime in the 1980s. These mussels can achieve densities of 524,000 per square meter or greater, blanketing whole lake bottoms and other surfaces. In Lake Erie, where zebra mussels extirpated a healthy population of native, freshwater bivalves (Unionoidea), some shells were covered by 15,000 zebra mussels—the equivalent of five times the weight of the living bivalve (ibid.). Zebra mussels are extremely efficient filter feeders, and as a result they substantially modify the aquatic systems they invade.

Invasive species may impact native species abundance through hybridization. Mallard ducks in North America have spread through introduction by sport hunters into new areas and by expansion of their range as natural areas are converted to agricultural lands. As mallards encounter closely related ducks in these new areas, they interbreed with them, often coming to dominate the gene pool of smaller populations, as is the case with the Mexican duck and the mottled duck. North American mallards are also hybridizing with the New Zealand gray duck and the Hawaiian duck. Similarly, "new" species generated through genetic engineering may pose a threat via hybridization after their release into the wild, as is suggested for escaped fish from aquaculture operations.

Although not often considered in discussions of invasives, diseases are a special class of invasive species that affect wildlife and human populations the world over. In the Hawaiian Islands, avian pox and malaria have led to an almost complete extermination of endemic birds in lowland forests. Similarly, emerging infectious diseases may play a role in the recently noted global amphibian decline.

Invasive species have transformed whole ecosystems, affecting fertility, productivity, and stability. For instance, plant invaders can alter ecological processes such as fire regimes, nutrient cycling, and hydrological cycles, or replace the dominant species in a community. In the fynbos ecosystem of South Africa's Cape Province, native plant species have evolved to withstand the difficult environment. These plants efficiently process water— particularly during dry summer months. They can live in nutrient-poor soils, and their roots bind the soil and minimize erosion. Much of the vegetation has fire-resistant leaves or bark, and their overall low biomass minimizes the impact of the fires that occasionally move through the area. Eucalyptus, pine, acacia, and other invasive species in the fynbos scrubland are heavy water users. They have threatened the extinction of many endemic plants, increased the overall biomass and water demands in the ecosystem, raised fire intensity, and reduced the amount of water for agricultural production and the limited water available to Cape Town and Port Elizabeth.

Invasive species seem to be particularly successful in establishing in and significantly changing the structure and function of freshwater lakes and stream ecosystems. Introduction of game or commercial fish in lakes and streams around the world has wreaked havoc on local fish species. The San Francisco Bay and Delta was rated the most invaded aquatic system in North America by the U.S. Fish and Wildlife Service. In many bay and coastal communities from Canada to Mexico, exotic species outnumber native species.

Biotic invasions affect economic systems in two major ways. First, they influence potential economic output—that is, causing loss in crop and livestock production and fisheries profits. Second, the cost of battling invasions (including invasives that are threats to human health), from quarantine to control to eradication efforts, is enormous. Accurate assessments of these costs are difficult to calculate, but estimates exceed $138 billion per year (Pimentel et al., 2000).

Controlling Invasive Species There are two major ways to limit the effects of invasive species. The first is to prevent new invasions, and the second is to minimize their impact once they have colonized a region. Initial invasions can be held in check by using quarantine techniques, but these techniques are hampered by our inability to predict which species might become invasive. In addition, countries such as the United States and Australia apply an "innocent until proven guilty" approach to incoming species, mostly to avoid limits on trade (Mack et al., 2000). In other words, all species are let in until we know they are harmful. A major problem with this approach is that once we know a species is harmful, it is often too late to control its spread.

Attempts to control invasive species have focused on chemical methods (for example, herbicides), mechanical methods (for example, hand removal of giant land snails), and biological methods (such as introduction of native parasites). Each of these has a suite of problems associated with it, ranging from cost (mechanical) to limiting the side effects of applications (chemical and biological). Control efforts have been most successful at the beginning of an invasion, when populations are smaller and localized. Once populations take hold and produce huge numbers of individuals, any attempt at control is usually unrealistic financially and logistically. Efforts to control invasive species are frequently stymied by public concern, either for the species being eradicated (as in the case of mute swans or feral cats) or the proposed method. Considerable effort should be directed at educating the public about invasive species and their effects on biodiversity.

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