Local species richness reflects both regional (e.g., speciation, extinction, and dispersal) and local (e.g., interspecific interactions) processes. However, their relative influence on local species richness remains poorly understood. A common approach to discerning regional effects on local diversity has been to plot local richness versus the richness of the regional species pool. However, difficulties in distinguishing regional and local effects are partly due to variation in how studies define species pools. Here, we assess how the definition of the regional species pool influences the perceived interplay between regional and local characteristics in shaping variation in community diversity.
In the present study, we used data compiled by Leslie Holdridge for a network of forest plots distributed across Costa Rica, combined with herbariaum specimen records from throughout Central America. We defined “local” richness as the number of species in a particular forest plot, for which climate and edaphic variables were also available. We defined three types of regional species pool, comprising all species recorded within: 1) a circular area having a diameter of100-km (7854 km2) centered on each focal plot; 2) a countrywide 300-m elevational band centered on each focal plot; and 3) the Holdridge forest type to which each focal plot was assigned. Finally, linear models were used to assess the effects of the species pool (regional) and environmental conditions (local) on local species richness. Different models used different definitions of the regional pool, and the performance of models was compared using AIC values.
Results/Conclusions
In all models, local species richness was significantly related to local environmental variables. However, the definition of the regional species pool affected the degree to which variation in local species richness was influenced by variation in regional richness and local environmental variables. The two best models included the regional species pool defined as the species within 50 km of each plot. Overall, our results show how the definition of regional species pools can influence the local-regional relationship, and suggest an important regional effect on local species assemblages of woody plants across Costa Rica.