A number of community variables can be examined from the standpoint of their variability with respect to diversity. Among these are species composition and food web structure. Simpler communities, in terms of species composition and food web structure, often appear to be more stable than complex communities (e.g., May 1973,1983). Boucot (1990) noted that simple marine communities in the fossil record continue across sedimentary discontinuities more often than do complex marine communities. Boucot (1990) also noted that particular taxo-nomic associations usually recur over larger areas for 106-107 years, indicating a high degree of stability within environmental constraints. The variety of succes-sional pathways leading to multiple endpoints (H. Horn 1981, Whittaker 1953) also has indicated that many communities do not necessarily recover their pre-disturbance composition or food web structure, although some mechanisms lead to positive feedback between disturbance and community organization (Schowalter 1985, Schowalter et al. 1981a, Shugart et al. 1981).
Modeling approaches have led to contrasting conclusions. May (1973,1983) and Yodzis (1980) reported that more complex communities were more vulnerable to disruption by perturbations in any particular species population because of their propagation through the network of interactions involving that species. However, de Ruiter et al. (1995) incorporated the patterning of interaction strengths in real communities and found that simultaneous occurrence of strong top-down regulation of lower trophic levels and strong bottom-up regulation of higher trophic levels imposed stabilizing patterns on interaction strengths. E. Evans (1988) found that grasshopper assemblages converged toward significantly greater similarity in structure following fire in a grassland ecosystem than predicted by a random model. Fukami et al. (2001) modeled compartmentalized communities and demonstrated that increasing biodiversity increased similarity in composition among local communities and that greater similarity improved reliability of community structure and function.
Diversity may dampen the spread of insects or pathogens that could threaten some species, hence disrupting community structure. For example, the diversity of pines and hardwoods in the southern United States reduces spread of southern pine beetle populations (Schowalter and Turchin 1993). Ostfeld and Keesing (2000) found that the number of human cases of Lyme disease, caused by the tick-vectored spirochete, Borrelia burgdorferi, declined with species richness of small mammals and lizards but increased with species richness of ground-dwelling birds (Fig. 10.10). These data indicated that disease epidemiology may depend on the diversity of reservoir hosts, but disease incidence generally should decline with increasing dilution of reservoir hosts by nonhosts.
To some extent, lack of a clear correlation between diversity and stability of community variables may be an artifact of the duration of succession or the number of intermediate stages that can generate alternative pathways. More frequently disturbed communities may appear to be more stable than infrequently disturbed communities because the ecological attributes of ruderal species favor rapid recovery, whereas longer time periods and more intervening factors affect recovery of tree species composition. Furthermore, if maximum species diversity occurs at intermediate levels of disturbance (the intermediate disturbance hypothesis), then the lower species diversity of earlier and later successional communities is associated with both high and low stability in terms of frequency and amplitude of departure from particular community structure.
A major source of diversity is the variety of community types and the regional species pool maintained in a shifting landscape mosaic of patch types. Although the community of any particular site may appear unstable because of multiple factors interacting to affect its response to perturbation, the landscape pattern of local communities minimizes the distance between population sources and sinks and ensures proximity of colonists for species packing and assortment during site recovery. Even if the community does not recover to the same endpoint, that predisturbance endpoint likely appears in other patches.
Furthermore, diverse, or complex, communities can be compartmentalized spatially as a result of heterogeneity of resources and population growth patterns (see Chapters 7 and 9).As Boucot (1990) noted, fossil communities rarely persist long at a site but recur over larger areas for long time periods, indicating a high degree of stability at a landscape scale. Hence, species composition and community structure may be conditionally stable at the landscape scale but not at the site scale, with the landscape-scale community represented as a shifting mosaic of local component communities.
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