Peak beta diversity occurs at regional spatial grains

Abstract

Beta diversity (β) quantifies the dissimilarity between communities and is thus fundamental to biogeography and spatial ecology. However, multiple and conflicting hypotheses exist to explain how β varies with spatial scale (here grain within a constant extent). Some resolution on spatial scaling of β may help clarify long-standing debate about: (a) β itself and (b) the ecological community concept, because the clearest spatial size of an ecological community corresponds to the scale with maximal β. Here we test five alternative hypotheses for β spatial scaling by calculating β for four very different taxa (ants, birds, diatoms, and trees) at spatial grains ranging from one to one million km2 in a constant spatial extent (contiguous USA). We also test repeatability for: (a) summer and winter bird β in six consecutive years; (b) trees through time (4 years, spaced 5 years apart). Specifically, we organize point data into hexagonal polygons, and then calculate mean pairwise β between each hexagon and its neighbors. We also calculate the deviation of β from two null models (assuming either spatial heterogeneity or homogeneity). For multiple reasons, β deviation based on spatial heterogeneity was preferable to that based on spatial homogeneity. For all taxa, β peaks at regional scales: trees ~300 km2; winter birds ~500 km2; summer birds ~2000 km2; ants ~2000 km2; diatoms ~11,000 km2. Also, spatial scaling is consistent inter-annually (birds, trees) and sensitive to seasonality (birds). Results broadly support the regional community concept and, based on the variety of taxa evaluated here, similar patterns are likely across the tree of life. Local ecological and evolutionary forces scale up to form regional community patterns and results here support efforts to coordinate local conservation efforts in regions (e.g., corridors and networks) to best conserve biodiversity. Beta diversity peaked at substantial spatial scales for all taxa evaluated here (ants, birds, diatoms, trees), suggesting a general pattern. The regional community concept advanced by Robert Ricklefs is supported empirically, in preference to four other hypotheses also tested. Beta diversity calculations used a relatively new deviation from a spatially heterogeneous null, which modestly adjusted observed beta diversity. Results support use of this newer beta deviation rather than the older beta deviation based on spatial homogeneity. Given that ecological communities appear regional in scale, local biodiversity conservation work can best ensure success if it is regionally coordinated.