Spatial and temporal scale dependence in species coexistence: A quantitative demonstration of the change in spatial storage effect across spatial scales and between years
Revealing mechanisms that maintain competitive coexistence explains crucial drivers of biodiversity. However, there have been few rigorous tests of general coexistence mechanisms applicable to a broad spectrum of ecological systems. Moreover, multi spatial and temporal scale studies are much needed to reconcile the debate over whether small scale (e.g. local) or large scale (e.g. regional) coexistence drives diversity patterns.
We empirically test the hypothesis that population fluctuations driven by environmental variation at smaller scales give rise to a general coexistence mechanism, the spatial storage effect at larger scales promoting species coexistence. This mechanism links smaller and larger scale processes and include functional ingredients that express the degree of intensification of intra relative to interspecific competition. Direct measurements on those ingredients introduce unprecedented rigor and confidence to testing coexistence.
We focus on a community of desert winter annual plants co-occurring across a hierarchical range of spatial scales (neighborhood → subhabitat → habitat → landscape).We quantify the strength of the spatial storage effect for two years through multi-scale measurements of demographic variables of a few focal species under density and competition manipulation.
Our first year results show surprisingly a negative spatial storage effect that undermines species coexistence on the subhabitat scale. This destabilizing force then disappears on larger scales, potentially compensated by stabilizing force operating on those scales. Moreover, this negative storage effect no long exists in the second year on the same scale. The results suggest that spatial environmental variation can play both positive and negative roles in maintaining coexistence and its influence is strongly dependent on spatial scales. A quantitative approach is necessary to examine the actual consequence of spatial environmental variation on coexistence across multiple spatial scales. The annual change in spatial storage effect suggests more theoretical and empirical investigation is needed to understand the interaction between spatial and temporal environmental variation in promoting species coexistence.