Scratching the niche: A continental-scale evaluation of the productivity hypothesis in explaining geographic variation of bird diversity across 25 years

Primary productivity and animal species richness are associated across spatial scales and extents. The productivity hypothesis had been credited for explaining these correlations by asserting that places with more energy fixed in the system by plants should support more individuals, whereby the community accumulates more species. Many have interpreted the spatial association between richness and productivity as support for the hypothesis. If true, bird richness should track productivity temporally, and there should be spatial and temporal relationships between productivity and both bird abundance and richness. We tested these predictions in the breeding season and winter across space and time. Using a remotely-sensed primary productivity proxy (NDVI) and avian data from two large North American citizen science surveys, the Breeding Bird Survey (BBS) and Christmas Bird Count (CBC), we evaluated the response of avian richness and abundance to interannual changes in plant productivity across 25 years. In the breeding season we found positive spatial relationships between richness and NDVI each year, but when evaluated temporally no support for the hypothesis was found. Richness and NDVI were positively associated at only half (49.3%) of BBS sites (mean r2 = 0.09). Further, total abundance and productivity were unrelated. Despite summer findings, it is possible that richness depends more on productivity in winter when birds are stressed by harsh conditions. Using CBC data we found that across several thousand communities the number of individuals was not spatially associated with productivity, providing no support for the productivity hypothesis. In these and earlier tests of the hypothesis, trait-neutral partitioning of resources across individuals is assumed, though it may be more realistic that partitioning is in unequal shares. To test for energetic trait differences, we regressed total avian biomass at sites against NDVI and found that avian biomass is independent of productivity. We conclude that primary productivity is unlikely the driver of bird diversity. Spatial relationships between productivity and bird richness may be spuriously arising via covariance between productivity and vegetation structural complexity, and the latter may be driving bird communities, consistent with the MacArthurs’ classic hypothesis that the vertical profile of foliage drives bird species diversity