Ecological correlates of island incidence and geographical range among British butterflies

Abstract

The incidence of butterflies on British islands and their geographical (latitudinal) ranges are regressed on ecological and life history variables. The objective has been to investigate the contribution of individual variables and to incorporate information on phylogenetic links. The findings confirm the close relationship of species' incidence on islands with their geographical ranges on mainland Britain and that of species' geographical ranges with ecological variables, particularly migration capacity, hostplant type (variety) and breeding-habitat range. The results for island incidence considering phylogenetic links are virtually identical to those disregarding them. For geographical range, the results are similar. The key variable in each case is dispersal, scored in either one of two different ways. However, hostplant type takes precedence over breeding-habitat range when phylogenetic links are considered. Species categorized for upper and lower quartiles for geographical range form isolated clusters in the first two axes of a principal components analysis on a set of seven ecological variables. This result suggests the combined influence of a number of ecological variables on range size. Species with wide geographical ranges tend to have high migration indices, a wide variety of hostplants and ubiquitous hostplants, long flight periods and are often multi-brooded; those with narrow geographical ranges tend to have low migration indices, sparse and limited hostplant resources, short flight periods and are often univoltine. A number of life history variables are found to correlate significantly with geographical range, but account for only small amounts of variation. The lack of any association between range size and population abundance may well reflect the difficulty of obtaining adequate measures for abundance. However, we caution against expecting a strong correlation between range size and abundance.