Exotics As A Form Of Biodiversity

Exotic species affect biodiversity in two opposite ways. On one hand, through their invasion of a community they can reduce biodiversity by reducing populations of native species. On the other hand, through their invasion of a community they increase biodiversity by their own addition to the system. The former process (of exotics' reducing native biodiversity) is often seen as the central problem of the invasions. Reduction in biodiversity is sometimes difficult to attribute solely to exotics because other factors such as pollution, disturbance by humans, and habitat loss also may be involved. However, exotics certainly contribute to declines in native diversity to a greater or lesser extent through competition or predation when they invade natural systems.

The process of exotics' adding biodiversity to communities is much less studied and discussed than their role in causing biodiversity declines. Of course, exotics are biological species as are natives, and they are as intrinsically interesting and valuable as any species taken within an appropriate context. When an exotic invades a community, its addition represents an increase in the community's biodiversity. At least in some cases this process can greatly increase diversity. This phenomenon is especially characteristic of islands which naturally have few species due to dispersal limitations (see the discussion of the theory of island biogeography in Chapters 4 and 5). Fosberg (1987) cites a dramatic example of this situation for an isolated island (Johnson Island) in the central Pacific Ocean. When first visited by a botanist there were only three species of vascular plants on the island. The island became occupied by humans as a military base during World War II, and by 1973 the number of vascular plants had increased to 127. Fosberg (1987) termed this "artificial diversity" because it was attributable to species brought in by humans. He goes on to describe a "pantropical flora" of plants that "... are either commensals with man, cultivated useful or ornamental plants, or what have been called camp-followers, door-yard or garden weeds, or else aggressive pioneer-type plants that produce many long-lived seeds and thrive on disturbed ground, or even in bare mineral soil." This is not a particularly attractive description of biodiversity, but the new communities on Johnson Island and in other locations have higher diversity that deserves to be studied. A continental example for Arizona fishes was described by Cole (1983):

Thus by constructing artificial waters, we have increased diversity on one hand even as we have decreased it. The overall picture, however, is probably a lessening of diversity. Although the number of fish species in Arizona was originally about 25, exotic introductions have increased the state's fish fauna to more than 100 species (Minckley, 1973). Some of the original native species have disappeared or are endangered because of competition from the new arrivals and alteration of their fragile aquatic habitats.

This quote is instructive because it shows how exotics have increased biodiversity, but the author is quick to qualify the phenomenon by noting possible negative impacts. Ecologists generally have avoided the paradox (though, see Angermeier, 1994), but there is a need to take on the problem of understanding the new systems of exotics and native survivors, which may have more biodiversity than the old systems without exotics. Lugo (1988, 1990, 1994) seems to be the only ecologist who has discussed the problem in any depth. He has tried to take a balanced approach as reflected in the following quote (Lugo, 1988):

Although conservationists and biologists have an aversion to exotic species such as predatory mammals and pests (with good reason!), this may not be totally justified if the full inventory of exotic fauna and flora and certain ecological arguments are taken into consideration. For example, the growth of exotic plant species is usually an indication of disturbed environments, and under these conditions, exotic species compete successfully (Vermeij, 1986). They accumulate and process carbon and nutrients more efficiently than do the native organisms they replace. In so doing, many exotic species improve soil and site quality and either pave the way for the succession of native species or form stable communities themselves. There is no biological criterion on which to judge a priori the smaller or greater value of one species against that of another, and if exotic species are occupying environments that are unavailable to native species, it would probably be too costly or impossible to pursue their local extinction.

The paradox of exotic species invasion of islands with high levels of endemism is discussed by Vitousek (1988) in Chapter 20. He correctly points out that if the invasion of exotic species is at the expense of the extinction of local endemics, the total species richness of the biosphere decreases and the Earth's biota is homogenized since most of the invading exotics are cosmopolitan.

Biodiversity exists at several scales (Whittaker, 1977), and exotics can increase alpha or local (within habitat) diversity. Thus, during the invasion process, a community adds one or more exotics. Biodiversity goes up if there are fewer local extinctions of native species than there are additions of exotics. Beta (between habitats) and gamma (regional) diversity can go down, even while alpha diversity goes up, if local endemic species are driven to extinction. The reductions in beta and gamma diversities with concurrent increase in alpha diversity characterize the homogenization phenomenon mentioned earlier. Although there have been few studies of this phenomenon with sufficient depth to document simultaneous change in diversity at different spatial scales, these kinds of biogeographical surveys are needed. Is homogenization actually happening? How many species have been added through introductions and how many species have gone extinct because of these introductions? If invasions of exotics are proceeding in all geographical directions, perhaps the actual net losses in species diversity are small. For every Asian species that invades North America, is there a North American species that invades Asia? In reality, there seem to be few studies spanning the geographic dimensions of biodiversity (alpha, beta, and gamma) that document changes solely attributable to invasions of exotics. Known losses in biodiversity are perhaps best thought as resulting from cumulative impacts of a number of factors which include exotic invasion, pollution, habitat loss, and others. In this context, it would be interesting to know the contribution of the different factors, especially for decision makers who must allocate scarce resources to mitigate separate impacts, such as invasions of exotic species.

Grazing Intensity

FIGURE 7.2 Classification of rangeland plant species based on adaptation to grazing intensity. Exotic species are like increasers or invaders. (Adapted from Strassmann, B. I., 1986. Energy and Resource Quality: The Ecology of the Economic Process. C. A. S. Hall, C. J. Cleveland, and R. Kaufman (eds.). John Wiley & Sons, New York.)

Grazing Intensity

FIGURE 7.2 Classification of rangeland plant species based on adaptation to grazing intensity. Exotic species are like increasers or invaders. (Adapted from Strassmann, B. I., 1986. Energy and Resource Quality: The Ecology of the Economic Process. C. A. S. Hall, C. J. Cleveland, and R. Kaufman (eds.). John Wiley & Sons, New York.)

As a form of biodiversity, exotics seem to generally share certain traits, but they are also a diverse group. It is sometimes even difficult to state definitely whether a species is even an exotic (Peek et al., 1987). The problem with defining these kinds of species mirrors the related challenge of defining a "weed." Herbert G. Baker (1965) defined a weed as a plant which grows "entirely or predominantly in situations markedly disturbed by man (without, of course, being deliberately cultivated plants)." The relation between exotics and human disturbance is a key in this definition and it will be explored in more depth in a later section of this chapter. Terminological challenges to defining weeds can be seen in the long lists of alternative definitions given by Harlan (1975) and Randall (1997).

The old range plant terminology (Ellison, 1960) also is instructive for defining exotic biodiversity. Rangeland plants were classified as increasers, decreasers, or invaders depending on their response to grazing. Thus, with increasing grazing intensity, increasers increase in density, decreasers decrease in density, and invaders invade from outside the community (Figure 7.2). This is a common-sense kind of classification that is value-free and that relies on a species response to perturbation.

Exotic species range in size from microbial diseases to wide-ranging wildlife and canopy-level trees. Most are fast growing with wide dispersal capabilities ("r-selected," see Chapter 5) but they have other qualities that allow them to be invasive. Some authors have tried to characterize "ideal" invaders (Baker, 1965, 1974, 1986; Ehrlich, 1986, 1989; Mack, 1992; Noble, 1989; Sakai et al., 2001), but many kinds of organisms can take on this role.

One fairly general feature of successful exotic invaders is preadaptation for the conditions of their new community (Allee et al., 1949; Bazzaz, 1986; Weir, 1977).

Preadaptation is a chance feature for unintentional introductions but a conscious choice for those species intentionally introduced by humans. In many cases invasive exotic species are preadapted to the disturbances caused by humans.

A final note on exotics as a form of biodiversity deals with the context of human value judgment. There is an underlying subjective feeling that natural ecosystems should have only native species. In this context, exotic species represent biodiversity in the wrong place. There are anachronistic exceptions such as the feral horses on several U.S. east coast barrier islands (Keiper, 1985), but exotics generally have a negative connotation. In the U.S. this is appropriate for national parks (Houston, 1971; Westman, 1990) where the objective is to preserve natural conditions despite changes in the surrounding landscape. However, in other situations exotics could be viewed with less negative bias. For example, Rooth and Windham (2000) document the positive values of the common reed (Phragmites australis) along the eastern U.S. coast, where it is regarded as one of the worst exotic plant species by many workers. These values include marsh animal habitat, water quality improvement, and sediment accumulation, the last of which is especially significant in terms of the impacts caused by the global rising of the sea level. The case for introducing an exotic oyster into Chesapeake Bay for reef restoration provides another case study (Gottlieb and Schweighofer, 1996). Brown (1989) summarizes ideas on value judgments about exotics with the following statement:

Unless one is a fisherman, hunter, or member of an acclimatization society, there is a tendency to view all exotic vertebrates as "bad" and all native species as "good." For example, most birdwatchers, conservationists, and biologists in North America view house sparrows and starlings with disfavor, if not with outright loathing; they would like to see these alien birds eliminated from the continent if only this were practical. There is a kind of irrational xenophobia about invading animals and plants that resembles the inherent fear and intolerance of foreign races, cultures, and religions. I detect some of this attitude at this conference. Perhaps it is understandable, given the damage caused by some alien species and the often frustrating efforts to eliminate or control them.

This xenophobia needs to be replaced by a rational, scientifically justifiable view of the ecological role of exotic species. In a world increasingly beset with destruction of its natural habitats and extinction of its native species, there is a place for the exotic. Two points are particularly relevant. First, increasing homogenization of the earth's biota is inevitable, given current trends in the human population and land use. ...

The second point is that exotic species will sometimes be among the few organisms capable of inhabiting the drastically disturbed landscapes that are increasingly covering the earth's surface. ...

It has become imperative that ecologists, evolutionary biologists, and biogeographers recognize the inevitable consequences of human population growth and its environmental impact, and that we use our expertise as scientists not for a futile effort to hold back the clock and preserve some romantic idealized version of a pristine natural world, but for a rational attempt to understand the disturbed ecosystems that we have created and to manage them to support both humans and wildlife. .

The current sentiment among most ecologists and environmentalists is that invasive exotics are "bad" species. However, it must be remembered that this is a subjective assessment. Perspective on the degree of this subjectivity comes from a consideration of a historical case. From the early 1900s until the 1950s, the U.S. government conducted a predator control program on public lands including national parks. Professional hunters and even park rangers were specifically employed in this program to kill wolves, coyotes, and many other mammalian predator species because they were judged to be "bad" species. This situation is described, with an emphasis on national parks, by McIntyre (1996):

Our country invented the concept of national parks, an idea that represented a new attitude toward nature. In the midst of settling the West, of civilizing the continent, some far-sighted citizens argued for setting aside and preserving the best examples of wild America. Public opinion supported the proposal, and Congress established a system of national parks, including such crown jewels as Yellowstone, Yosemite, Sequoia, Rocky Mountain, Grand Canyon, Glacier, and McKinley. The natural features and wildlife found within these parks would be protected as a trusted legacy, passed on from one generation to another.

But the early managers of these national parks defined preservation and protection in ways that seem incredible today. The contemporary attitude classified wildlife species as either "good" or "bad" animals. Big game species such as elk, deer, moose, bison, and big-horn sheep fell into the favored category. Park administrators felt that national parks existed to preserve and protect those animals. Anything that threatened them, whether poachers, forest fires, or predators, had to be controlled. Based on that premise, predators, especially wolves, became bad animals, and any action that killed them off could be justified.

Besides wolves, many other animals were also blacklisted and shot, trapped, or poisoned during the early decades of the national park system: mountain lions, lynx, bobcats, red foxes, gray foxes, swift foxes, badgers, wolverines, mink, weasels, fishers, otters, martens, and coyotes. Amazingly, rangers even destroyed pelicans in Yellowstone on the premise of protecting trout.

The predator control program in the national parks was just an extension of a national policy to rid the country of undesirable species. ...

This control program stopped in the 1950s, and many are questioning its wisdom to the degree that wolves are now being reintroduced to the national parks. Thus, the judgment of these species as being "bad" and needing to be controlled has been reversed as attitudes have changed. Will a similar reversal in attitudes happen with invasive exotics some day? Chase (1986) in his critical review of management policies at Yellowstone National Park labeled the old predator control program as an example of "playing god" with the species. The comparison is striking with current exotic control programs.

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