Discussions of the degree and causes of stability are frequently hampered by vagueness and inconsistency about what is meant by stability (Orians 1975). Stability may simply mean constancy, that is a low level of variation in some measurable property of the system. Stability may also refer to the resistance of the system to alteration by external perturbations (inertia), its speed of return to initial conditions following a perturbation (elasticity or resilience), the domain over which it returns to its initial state (global stability), and the tendency of the system to cycle in a predictable manner (cyclic stability). These varied properties, all of which are important components of overall stability, are often affected by different external factors, and these components of stability frequently respond differently to the same factors. In our analysis of the functioning of tropical forests, we attempt to address how al! of these components of stability may be influenced by losses of biological diversity.
An important result of the past three decades of theoretical and empirical research on the causes of patterns in species richness is the demonstration that these patterns are the products of complex interacting forces that vary in relative importance in both time and space (Solbrig 1991). The consequences of biological diversity for system-level processes are also the products of many factors operating at variable spatial and temporal scales, but these relationships have received much less attention than the causes of biological richness (Schulze and Mooney 1993).
In this discussion, we follow Lawton and Brown (1993) in treating "ecosystem processes", "behavior of ecological systems" and "ecosystem functioning" as equivalent terms. We do not use the term "ecosystem function" because we do not believe that ecosystems have goals or objectives. As a result of the activities of organisms living in them, ecosystems process materials and energy, and the efficiency and stability with which they do so is likely to be influenced by biodiversity. By "biodiversity" we mean not only the number of species (species richness), but also genetic variants within a species, evolutionary lineages, functional groups of organisms, and ecological communities. However, we concentrate here on species richness, in part because little is known about the genetic structure of populations of tropical species (but see Hamrick and Loveless 1989; Loveless and Hamrick 198?; Equiarte et al. 1993), but also because the task of erecting a classification of types of tropical forests and analyzing how biotic interactions differ among them remains to be accomplished.
In this chapter, we concentrate on relatively undisturbed lowland tropical moist forests, thai is forests that receive more than 2000 mm year-1' rainfall, but we briefly discuss the significance of moisture and elevational gradients for interactions between biodiversity and forest functioning. We also direct most of our attention to biotic interactions at local scales, realizing that landscape-level patchiness might modify the conclusions reached from a regional- versus local-scale analysis.
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