Predicting the relationship between local and regional species richness from a patch occupancy dynamics model

1. A linear relationship between the number of species in ecological commun ities (local richness) and the species pools from which the communities are drawn (regional richness) suggests that species interactions are not suffi cient to limit local richness and that communities are not saturated with s pecies. Instead, this relationship implies that communities are open to reg ional influences and are interlinked by dispersal. 2. Here we show how the linear relationship between local and regional rich ness in real, noninteractive, assemblages of cynipid gall wasps on Californ ia oaks, can be predicted from a simple patch-occupancy model. 3. One cynipid assemblage has been surveyed for 3 years, allowing for crude estimates of colonization and extinction rates per patch. Using the mainla nd/island model of patch occupancy dynamics, these rates are combined with the observed number of cynipid species associated with each oak species (re gional richness) to predict the expected local species richness in each pat ch. Assuming that species are independently distributed among localities, t he expected variance in species richness among localities is also computed. 4. The model is then tested on an independent data set. When differences in sampling effort (number of surveyed trees per locality) were accounted for , the regression equation relating observed (n = 41) to predicted local spe cies richness does not differ statistically from the line of perfect agreem ent. The residuals are also distributed according to the predicted variance . 5. Although not statistically significant, the variance in local richness a ppears to be slightly underestimated by the model. One explanation may be t hat cynipid species display some positive covariance in their distribution among localities, that is, groups of species occur together in given locali ties more frequently than would be expected by chance. Variance ratio tests identified statistically positive covariance within cynipid assemblages fo r three oaks species. 6. The close fit of the model to the data supports the theoretical scenario for noninteractive communities, that the slope of the local-regional richn ess relationship and patch-occupancy processes are different expressions of the same phenomenon.