Spatial effects on predicted patterns of abundance and diversity.
Gardner, Robert*,1, 1 Appalachian Laboratory, Frostburg, Maryland, USA
ABSTRACT- The unified neutral theory of biodiversity and biogeography (UNT) describes patterns of diversity within ecological communities "structured entirely by ecological drift, random migration, and random speciation" (Hubbell 2001 Princeton University Press). This provocative theory assumes that individuals of all species are ecologically equivalent and has been explored using non-spatial mathematical analyses. However, the formation of isolated (island) communities involves a number of spatial processes including island size and distance, species dispersal and establishment, and disturbance and mortality events. Even if all individuals are ecologically equivalent, as assumed by the UNT, community formation may be dominated by these spatial effects. A spatially explicit model of plant dispersal and establishment was used to examine this possibility. Monte Carlo experiments (N = 20, t =10,000 y) showed that: (1) Species richness increased linearly with island size (square-root of area) until the potential for immigration from the mainland metacommunity was exhausted. This two-phase process is not described by a power-function relationship. (2) Log-linear declines in species richness occur with increasing distance from the mainland community. (3) Small disturbances increased richness by preventing dominance by a single species. Richness continued to increase with disturbance extent, failing to reveal an optimal (i.e., intermediate) level of disturbance. (4) Local dispersal enhanced the probability of persistence and increased species richness while long-distance dispersal resulted in an increased frequency of establishment but at reduced densities that were not competitively viable. These results indicate that species richness on islands is a long-term persistence rather than a simple migration issue. The spatial effects on community formation were consistent with the assumptions of the UNT, offering an opportunity to define neutral models of spatial effects on species diversity and abundance.
Key words: biodiversity, neutral model, spatial effects
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