Stability Over Neontological Timescales

We are currently in a period of global change in which extinctions and invasions of new habitats are proceeding quickly. Thus, it is clearly important to understand if and how ecosystems react to physical and ecological perturbations. There are documented cases in which the introduction of exotic species into new ecosystems had little effect on the native fauna (e.g., Simberloff 1981), which suggests that stability is quite predominant. In other examples, the introductions have caused local extinctions and upheaval in communities (e.g., Vitousek 1986; Atkinson 1989). With such a diversity of responses to just one type of ecological perturbation (although the deletion or addition of species is considered to be a large "global" disturbance), it seems apparent that stability in modern communities is dependent on the many environmental, biological, and ecological factors in each ecosystem.

From the 1950s to 1970s, conventional wisdom among neoecologists professed that increased ecosystem complexity, which is related to high species diversity, large number of interactions between species, and other factors, was correlated with greater levels of stability. This would appear to be intuitively correct because one would assume that the removal or addition of organisms in a highly complex food web would not greatly disrupt the entire food web, whereas a food web with few pathways would appear to be easily perturbed (MacArthur 1955). Quantitative observations were forwarded to support and explain this idea of greater stability in diverse, complex ecosystems; for example, islands with few species appear to be more vulnerable to invaders than are species-rich continental ecosystems, and crop monocultures are more vulnerable to disease and invasion than are natural mixed associations (Elton 1958).

With the advent of advanced numerical modeling in the 1970s, however, it appeared that not only was the conventional wisdom that complexity begets stability incorrect, but initial results also suggested that the complete opposite was true: increased complexity was related to greater instability in model ecosystems (May 1972). Through the succeeding years, a variety of studies have shown that community stability can be affected by a number of factors, including number of trophic levels (e.g., Pimm and Lawton 1980), energy flux (e.g., O'Neill 1976), food web dynamics (e.g., DeAngelis 1975), and life histories (e.g., Pimm and Rice 1987). In addition, follow-up models have shown mixed results when considering larger perturbations (global rather than local stability) (e.g., Pimm 1979). The bottom line appears to be that numerical modeling has shown that no definitive relationship between complexity and stability can be clearly demonstrated.

Unfortunately, field-based studies also have not been able to provide definitive clarification on the relationship between stability, complexity, and other ecological factors. Tilman and Downing (1994) documented that increased species richness enhanced community stability (both resistance and resilience) in response to drought conditions in a terrestrial plant community, thus providing support for the original conventional wisdom (complexity begets stability). Similarly, Death (1996) found that more complex stream invertebrate communities were more resilient than less complex communities, and many recent studies have challenged the idea that complexity begets instability based on analysis of connectance (Fonseca and John 1996), food chain models (e.g., Sterner, Bajpai, and Adams 1997), and density-dependent populations (Dodds and Henebry 1996). In another example, however, high-diversity terrestrial plant communities exhibited lower resilience in response to changes in nutrient flux and grazing than did low-diversity associations (McNaughton 1977).

Therefore, results from both field studies and theoretical modeling are equivocal and seem to indicate that, as one ecology textbook states, "[N]o single relationship will be appropriate in all communities.. .[t]he relationship between the complexity of a community and its inherent stability is not clear-cut. It appears to vary with the precise nature of the community, with the way in which the community is perturbed and with the way in which stability is assessed" (Begon, Harper, and Townsend 1996). Despite problems with differences among field studies in the level of analysis, scope of study, and choice of analyzed variables (see Connell and Sousa 1983), as well as problems with taxonomic resolution (Hall and Raffaelli 1993), it generally appears that modern communities exhibit a range of stability.

0 0

Post a comment