Density-distribution relationships in British butterflies.: II.: An assessment of mechanisms

1. The interspecific density-distribution relationship is a general and robust pattern that has been described as a rule in community ecology. Many theoretically plausible causes of the relationship have been described, but it is still disputed which factor(s) are most important. 2. Using data on the densities and distributions of butterflies and their host plants collected in a 35-km(2) area of north Wales, and data on butterfly mobility, niche breadth, habitat breadth and distance from range margins, we examined five of the principal explanatory mechanisms. 3. We found that several variables were significantly correlated with density or distribution. Habitat breadth, mobility and distance from range margin had significant positive effects on butterfly distribution. Host-plant density was significantly positively related to butterfly density; mobility was significantly negatively related to density. 4. Despite these results, we could not unambiguously demonstrate that one hypothesis (or several interacting hypotheses) generated density-distribution correlations. The most conclusive evidence was that statistical patterns of distribution (aggregation models) underpinned the positive density-distribution relationship seen amongst the more mobile butterflies. The results provided evidence against the metapopulation dynamic explanation, and were equivocal with respect to the contributions of range position, niche breadth and resource availability. 5. An alternative approach was to explore deviations from the underlying relationship between density and distribution, rather than concentrating on the correlation itself. This approach was much more successful: we demonstrated that species that occurred at high densities relative to their distributions used aggregated resources and were relatively sedentary; whereas those that occurred at low densities relative to their distributions used less aggregated resources, and were more mobile. Mobile species had less aggregated distributions than did relatively sedentary species. 6. Given that the interspecific density-distribution pattern appears to be almost ubiquitous and that the proposed explanations are not mutually exclusive, faster progress may be made by examining deviations from the pattern than from further analysis of the pattern itself.