Does plant productivity really influence the richness of bird assemblages? A re-appraisal of the energy-richness hypothesis for North America

Background/Question/Methods

Plant biomass/productivity and the species richness of bird communities are associated across a range of spatial scales.  Species-energy theory is generally assumed to explain these correlations.  But if true, bird richness should also track productivity temporally, and there should be spatial and temporal correlations between bird abundance and both productivity and bird richness.  Using summer NDVI from 1982 – 2006 and the North American Breeding Bird Survey, we evaluated the response of avian richness and abundance to temporal changes in plant biomass/production. 

Results/Conclusions

We found positive spatial relationships all 24 years.  However, we found that richness and vegetation were temporally positively associated at 1,579 survey sites and negatively associated at 1,627 sites (mean slope = -0.7 and mean r2 = 0.09).  Further, total abundance and NDVI are uncorrelated spatially (r2’s spanning 0.00 and 0.03) and temporally (mean r2 = 0.09).  We find no evidence that productivity drives bird richness beyond the spatial correlations, and neither prediction arising from species-energy theory is confirmed.  Spatial correlations between productivity and bird diversity may be spurious, arising via relationships between plant biomass/productivity and vegetation structure, and it is the latter that matters, consistent with Robert MacArthur’s classic explanation for the diversity for bird communities.