Why empirical tests of theory mandate more attention to ecological context: Using local versus regional drivers of local species richness as a case-study.
Russell, Roly*,1, Wood, Spencer2, Fuhlendorf, Sam3, Muldhavin , Esteban4, Woiwod, Ian5, 1 Oregon State University, Corvallis, Oregon, U.S.A.2 University of British Columbia, Vancouver, British Columbia, Canada3 Oklahoma State University, Stillwater, Oklahoma, U.S.A.4 University of New Mexico, Albuquerque, New Mexico, U.S.A.5 Rothamsted Research, Harpenden, Hertfordshire, U.K.
ABSTRACT- It seems as though empirical data are often blithely and opportunistically used to test ecological theory without due regard for ecological context. Here we conceptually and then empirically address how a) spatial scale, b) species groupings, and c) habitat types alter predictions of theory relating local species richness to ecological interactions and regional species pools. These three contextual gradients are predicted and shown to have strong influence on the relative strength of effects of local ecological interactions versus regional species pools on local species richness. We use data on marine intertidal algae and invertebrates, dessert plants, and terrestrial insects to empirically assess these relationships and test our conceptual predictions. First, we conclude that spatial scale of a defined 'local' community determines whether we expect to see the signature of saturation (an upper ceiling to local species richness imposed by local ecological interactions) or not. Species richness of spatially extensive local communities is expected and demonstrated to be driven more by regional species pools than by local ecological interactions. Similarly, searching for saturation in weakly or non-interactive groups of species is incompatible with the proposed mechanism (local ecological interactions)--it is thus predicted, and shown, that saturation is more likely to be observed in more strongly interactive groups of species. Thirdly, we conclude that different habitat types have potential for dramatically different saturation dynamics. Finally, we discuss the implications of incorporating the roles of facilitation and stress into a predictive framework relating local species richness to local and regional influences. In sum, we argue that conclusions of some past studies are artifacts of unrecognized contextual influences, and that a reconciliation of past conclusions is possible through an explicit awareness and incorporation of ecological context of biological communities.
Key words: context dependence, regional species pools, local species richness, spatial scale
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