Density-distribution relationships in British butterflies. I. The effect of mobility and spatial scale

1. Positive relationships between the density and distribution of species i n taxonomic assemblages are well documented, but the underlying mechanisms remain poorly understood. Two factors that are expected to be important in explaining variation in these relationships are the spatial scale of analys is and the relative mobility of the study species. 2. We examined density-distribution relationships in British butterflies at a variety of spatial scales. Distributions were proportions of grid square s occupied: 50 m grid within 0.25 km(2) areas (local), 500 m grid in 35 km( 2) (regional), 10 km grid across England, Wales and Scotland (national), 15 3 000 km(2) grid squares across Europe (European), and also seven categorie s of international distribution (Global; 1 = European endemic to 7 = in 5 continents). Densities were measured using transect counts at local, regio nal and national scales. 3. Different relationships between density and distribution occurred at dif ferent scales of analysis. When we controlled for the effects of mobility a nd /or phylogenetic association, a positive relationship between density an d distribution was apparent at local, regional and national scales. Species ' national densities in Britain were positively correlated with their Europ ean distribution sizes, but significantly negatively correlated with their global range sizes. 4. Butterfly mobility had a positive effect on distribution and a negative effect on density at all spatial scales. For a given total abundance, more mobile species had lower densities but wider distributions, i.e. they were less aggregated than more sedentary species. 5. The decreasing strength of the density-distribution correlation, and the eventual reversal of the pattern, with the increasing magnitude of differe nce between the scale at which density was measured relative to distributio n, suggests that some element of niche may be important in determining dens ities and distributions. However, the measure of niche breadth analysed did not explain significant variation in density distribution, or in the densi ty-distribution relationship.