Difficulties with estimating and interpreting species pools and the implications for understanding patterns of diversity

Evidence has been accumulating that species pools play a major role in regu lating variations in small-scale diversity. However, our ability to unambig uously estimate and interpret species pools remains a major impediment to u nderstanding the processes that control patterns of diversity. Two main app roaches have been employed to evaluate the relationships between species po ols and species diversity. The direct approach has been to estimate the act ual sizes of species pools by sampling discrete areas at larger spatial sca les and then relating these estimates to samples taken at smaller scales. T he indirect approach has been to search for correlations between abiotic en vironmental factors and patterns of diversity that are indicative of gradie nts in species pools. Both of these approaches have substantial predictive capability but also have limitations that impair our ability to draw unambi guous interpretations about causal factors. A primary difficulty for the di rect approach is in deciding which species in the larger pool of potential species are actually capable of living in a sample. In this regard, the ind irect approach requires fewer assumptions and has the ability to detect pre viously unsuspected gradients in species pools. As with the direct approach , assessing the causes for observed gradients in species pools remains a li mitation for the indirect approach, Consideration of experimental studies o f potential niches suggests that it may be valuable to distinguish between potential and observed species pools if the role of competitive exclusion i s to be fully assessed. This paper concludes by arguing for (1) an increase d use of multivariate studies that examine the effects of species pools ind irectly and (2) further experimental studies designed to determine potentia l species pools.