Does Solar Energy Control Organic Diversity? Butterflies, Moths and the British Climate

In Britain, the diversity of butterflies, measured as the number of species present in an area of 900 km2, is highly correlated with sunshine and temperature during the summer months May to September. These two variables explain nearly eighty percent of the variance in diversity, and also explain it to some extent independently of latitude. Diversity shows a negative correlation with the average temperature for the three winter months December to February. Diversity (species number) therefore appears to be strongly influenced by the amount of energy available during the favourable season. The simplest explanation for this relationship is the extreme ectothermic behaviour of adult butterflies, which depend on both warm air and direct sunshine to maintain normal activity. Heavy bodied, endothermic, nocturnal moths however show a very similar correlation with the climatic variables, suggesting that larvae as well as adults benefit from sunshine and warmth. Butterfly and moth diversity is shown also to be influenced by habitat diversity. It is suggested that these findings support the species-energy hypothesis of D. H. Wright, which explains global latidudinal gradients in diversity by the gradient of direct and atmospherically transported solar energy. They appear also to explain some temporal fluctuations in butterfly diversity.