Does climate determine broad-scale patterns of species richness? A test of the causal link by natural experiment


David J. Currie, Ottawa-Carleton Institute of Biology, University of Ottawa, Ottawa, Ontario K1N 6 N5 Canada. E-mail:


Aim  Broad-scale spatial patterns of species richness are very strongly correlated with climatic variables. If there is a causal link, i.e. if climate directly or indirectly determines patterns of richness, then when the climatic variables change, richness should change in the manner that spatial correlations between richness and climate would predict. The present study tests this prediction using seasonal changes in climatic variables and bird richness.

Location  We used a grid of equal area quadrats (37 000 km2) covering North and Central America as far south as Nicaragua.

Methods  Summer and winter bird distribution data were drawn from monographs and field guides. Climatic data came from published sources. We also used remotely sensed NDVI (normalized difference vegetation index — a measure of greenness).

Results  Bird species richness changes temporally (between summer and winter) in a manner that is close to, but statistically distinguishable from, the change one would predict from models relating the spatial variation in richness at a single time to climatic variables. If one further takes into account the seasonal changes in NDVI and within-season variability of temperature and precipitation, then winter and summer richness follow congruent, statistically indistinguishable patterns.

Main conclusions  Our results are consistent with the hypothesis that climatic variables (temperature and precipitation) and vegetation cover directly or indirectly influence patterns of bird species richness.