Furthermore, Vitousek (1988) suggested that ecological theory can benefit from studies of exotic invasions:
Better understanding of biological invasions and their consequences for biological diversity on islands will contribute to the development and testing of basic ecological theory on all levels of biological organization. ... An understanding of the effects of invasions on biological diversity in rapidly responding island ecosystems may give us the time and the tools needed to deal with similar problems on continents; it may even contribute to the prediction and evaluation of the effects of environmental releases of genetically altered organisms.
Ecologists are just beginning to explore the use of exotic invasions as unplanned and uncontrolled experiments. Simberloff (1981) used historical records on introductions to examine two relevant ecological theories (equilibrium island biogeog-raphy and limiting similarity of competing species). He found little support for the theories in his analysis, and generated discussion about how to use historical data sets on introductions (Herbold and Moyle, 1986; Pimm, 1989). While a few other attempts at using exotics to examine ecological theories have been made (MacDonald and Thom, 2001; Mack, 1985; Ross, 1991), many relevant topics, such as assembly theory, keystone species, and the role of indirect effects, could be examined. Here, two theories are discussed as examples.
Catastrophe theory is a branch of mathematical topology which describes dynamic systems that can exist in alternative stable states and that can dramatically change between states over short periods of time in a discontinuous fashion (Thom, 1975). Although the mathematical basis of the theory was criticized soon after it came out (Kolata, 1977), catastrophe theory has been profitably applied to several kinds of outbreak-type systems including forest insects (Casti, 1982; Jones, 1975; Ludwig et al., 1978), Dutch elm disease (Jeffers, 1978), algal blooms (Beltrami, 1989, 1990), and others (Loehle, 1989; Saunders, 1983). The theory is receiving renewed attention for understanding alternative stable states in ecosystems (Allen, 1998; Scheffer and Jeppeson, 1998) and it may offer a language for understanding invasion and dominance of natural communities by exotic species. For catastrophe theory to apply to exotic takeover, the system must have a certain structure of control variables that results in an equilibrium surface or a map that tracks a periodic outbreak-type of dynamic behavior. Several kinds of maps are described by the theory; most common are the fold and cusp catastrophes, which depend on one and two control variables, respectively. Thus, for catastrophe theory to be useful for understanding exotic invasions, the structure of control variables must be understood. Phelps (1994) suggested that a cusp catastrophe might help explain the invasion of the Potomac River near Washington, DC, by Asiatic clams (Corbicula fluminea), and perhaps other exotic invasions can be understood with this approach.
The maximum power principle may also be useful for understanding exotic invasions. This is a systems-level theory that states that systems develop designs that generate the maximum useful power through self-organization (Hall, 1995b; H. T. Odum, 1971, 1983). The concept is based on the premise that "systems that gain more power have more energy to maintain themselves and ... to overcome any other shortages or stresses and are able to predominate over competing units" (H. T. Odum, 1983). The general systems design that tends to maximize power is one that develops feedbacks which increase energy inflow during early successional stages or which increase energy efficiency during later successional stages. Feedbacks are performed by species within ecosystems, so the maximum power principle also is a theory about how species composition develops. The theory suggests that those species that are successful and dominate a system must contribute to the system's ability to maximize power. Exotic species that invade a system then should lead to an increase in power flow, if the maximum power principle holds. Thus, exotic invasions may allow a test of the theory by examining power flow or metabolism of systems before invasion and after invasion. For example, the theory predicts that a natural Chesapeake Bay marsh dominated by Spartina or Scripus would have lower energy flow than the same marsh after invasion by Phragmites. This test has not been formally made yet but the work discussed by Vitousek seems to be consistent with the maximum power principle (Vitousek, 1986, 1990; Vitousek et al., 1987) as does the analysis of exotic Spartina marshes in New Zealand (Campbell et al., 1991; H. T. Odum et al., 1983).
Existing ecological theory may not be completely adequate to understand exotic invasions (Abrams, 1996), and entirely new ideas may be needed for their description and explanation. The prospects are good for new theory to be developed from the study of exotics. Much new quantitative modelling has focused on how exotics spread across landscapes (Higgins and Richardson, 1996; Shigesada and Kawasaki, 1997), but the best prospects for new theory may be with invasibility of communities. This subject was first treated by MacArthur and Wilson (1967) in the context of islands using equilibrium approaches to theory. Invasion is the process of species entering an established community. It differs from colonization, which is the process of species entering a community while it is being established. Ewel (1987) noted the importance of this topic when he suggested that invasibility is one of the five most important criteria for assessing newly restored ecosystems. The concept of invasion is receiving increasing attention with empirical studies (Burke and Grime, 1996; Planty-Tabacchi et al., 1996; Robinson and Dickerson, 1984), review articles (Crawley, 1984; Fox and Fox, 1986) and application of existing theory (Hastings, 1986). Elton's (1958) old concept of resistance to invasion is more or less the inverse of invasibility (Orians et al., 1996; Pimm, 1989; Rejmanek, 1989). Resistance of a community to invasion is sometimes found to be proportional to its diversity (Kennedy et al., 2002), but in other cases "invasional meltdowns" can occur where the invasion rate accelerates as more species are added (Ricciardi and MacIsaac, 2000; Simberloff and von Holle, 1999). The invasional meltdown concept has only recently been introduced and may be explained by facilitation interactions between exotic invaders. This is an example of new ecological theory that is being developed to understand exotic invasions.
A final value of exotic invasions as a stimulus to learning would be if knowledge generated from their study can help deal with new problems facing society. The connection between invasions of exotic species and releases of genetically engineered or modified organisms (GMOs) has been made (National Research Council [NRC], 1989b) and similar theories may apply to both problems (Kareiva et al., 1996;
Purrington and Bergelson, 1995). There are many risks associated with the release of GMOs. For example, adding genes for disease resistance to crops is risky because they may pass these genes on to weeds, creating superweeds with enhanced growth potential (Kaiser, 2001b; Snow and Palma, 1997). Moreover, the disease-resistant crops may themselves become weeds (Rissler and Mellon, 1996)! Understanding degrees of weediness in exotic species may help assess the risks associated with GMOs. Products derived from genetically altered food crops have been called "fran-kenfoods," referring to Mary Shelley's story of Frankenstein. This reference is evocative because in the story the man-made monster escapes and kills his creator. Another issue deals with possible biological cross-contamination caused by extraterrestrial space travel. The concerns are that missions to other planets may infect them with organisms from the Earth and that missions that return from other planets may infect the Earth with alien organisms. Assessment of this risk began with lunar missions in the 1960s and protocols for planetary quarantines were established by NASA (Lorsch et al., 1968). Interest became more intense with planned Mars missions because life on Mars was then thought to be a definite possibility (Pittendrigh et al., 1966). An interesting controversy about the need for quarantines and space craft sterilization developed between some engineers who thought the probabilities of cross-contamination were too remote for concern, and some biologists who understood the ability of living organisms to grow and spread even under harsh environmental conditions. Carl Sagan was a vocal supporter of the need for precautions, and the controversy between engineers and biologists is discussed in depth in one of his biographies (Poundstone, 1999). There is now renewed interest about the issue of cross-contamination because of the chance of false-positive results in planned extraterrestrial life detection experiments caused by Earth organisms (Clarke, 2001) and because of the chance of alien invasion from samples of rocks and soils that are planned to be returned from space (Space Studies Board, 1997, 1998). Perhaps NASA would be well advised to include ecologists specializing in exotic species invasions on committees and advisory boards dealing with planetary cross-contamination.
Was this article helpful?