Like most ecologists whose gestation occurred during the 1970s, I have been intrigued with the role of competition in structuring natural communities, as well as with the subsequent controversies surrounding stochastic and deterministic mechanisms responsible for patterns. Much of my early work on bat communities in the semiarid tropics (e.g., Willig, 1986; Willig and Moulton, 1989; Willig et al., 1993) questioned extant paradigms, and suggested that stochastic events may intervene to preclude communities from exhibiting patterns consistent with competition theory. This interpretation of results was consistent with the overall assessment of the semi-arid Caatinga being an area characteristic for its environmental variability and stochasticity (Mares et al., 1985).
More recently, my MS student and I (Stevens and Willig, 1999a) examined the role of competitive exclusion in structuring bats communities based on a broad geographic selection of communities from throughout the New World. We show that considerable heterogeneity characterizes the degree to which communities exhibit ecomorphological patterns consistent with competitive exclusion. We subsequently develop a simulation model that explores the way in which density compensation rather than size assortment via competitive exclusion might structure communities, and document that the model can detect patterns in desert rodent communities, well-known for being influenced by competitive mechanisms (Stevens and Willig, 1999b). Finally, we show that heterogeneity characterizes the degree to which bat communities are a product of either size assortment or density compensation (Stevens and Willig, 2000a, b). Nonetheless, temporal variation in temperature, precipitation, and productivity influences the degree to which deterministic patterns in density or morphology occur in nature (Stevens and Willig, Unpubl. ms.). Hence, we are beginning to understand the environmental contexts within which we expect competition to play a dominant role in molding the structure of communities, and have a more sophisticated appreciation for ways in which competitive effects can manifest at the level of ecological guilds and communities.
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