More individuals or specialized niches? Distinguishing support for hypotheses explaining positive species–energy relationships.

Aim: Positive productivity–richness relationships have been observed across taxa and ecosystems. We assessed support for two hypotheses explaining these relationships, the More Individuals Hypothesis (MIH) and the Niche Specialization Hypothesis (NSH) in two complementary datasets of avian communities in North America, the Breeding Bird Survey (BBS), which has been widely used to study productivity–richness relationships, and the Breeding Bird Census (BBC), which has been used rarely yet offers accurate population density estimates and more homogeneous landscapes, eliminating confounding variables that may make it more difficult to test predictions of the NSH and MIH in the BBS alone. Location: Coterminous United States, 1988–2009. Taxon: Birds. Methods: We first evaluated the strength of productivity–richness relationships in the BBS and BBC, and tested predictions from the MIH and NSH in the two datasets by examining patterns in community abundance, productivity niche breadth and number of foraging guilds along a gradient of normalized difference vegetation index (a measure of vegetation productivity). We also implemented a null model to examine the contribution of sampling effects due to increasing species richness in explaining patterns in number of foraging guilds in both datasets. Results: We found that the BBS had a much stronger productivity–richness relationship than the BBC, potentially as a result of increased landscape diversity with increasing productivity at BBS survey sites. Although the MIH and NSH may not be mutually exclusive, we found weaker support for the MIH in the BBC, and stronger support for the NSH in the BBC and BBS. Main Conclusions: Researchers should consider the role of confounding variables such as landscape diversity and focus on developing direct measurements of food resources at macroecological scales to determine the relative importance of mechanisms driving productivity–richness relationships. [ABSTRACT FROM AUTHOR]