The roles of extinction and colonization in generating species-energy relationships

1. Positive correlations between energy and species richness are frequently observed, but the causal mechanisms of such species-energy relationships have rarely been identified conclusively. 2. The more individuals hypothesis (MIH) describes one possible cause of positive species-energy relationships. It suggests that greater resource availability in high-energy areas increases population sizes, reducing local extinction rates and promoting species richness. It predicts that extinction rates will be lower in high-energy areas and that, because a given change in population size has a greater influence on extinction risk when initial populations are small, such relationships will be more pronounced in numerically rare species than more abundant ones. 3. Colonization rates may also influence local species richness, and they may respond positively to the greater resource abundance in high-energy areas. 4. We provide the first empirical test of relationships between extinction/colonization rates and energy availability and of the influence of population size on these relationships. We use data on the changing distributions of British birds, in which positive species-energy relationships have previously been documented. 5. We find that extinction rates are lower in high-energy areas, but that such patterns are stronger in more abundant species. Spatial variation in colonization rates is influenced less markedly by energy availability, but such patterns are also more marked in numerically abundant species. While these results provide little overall support for the MIH, nor some of its alternatives, they add to increasing evidence that common species drive much of the spatial variation in species richness.