Abundance-occupancy relationships of larval black flies (Diptera: Simuliidae) in temperate Nearctic streams

1. This study examines abundance-occupancy relationships of larval black flies in temperate streams of the south-eastern United States. It is the first study of abundance-occupancy relationships in lotic macro-invertebrate assemblages to focus on a single family. We address the following questions: (i) Do larval black flies show a strong inter-specific abundance-occupancy relationship? (ii) Do widespread and abundant species demonstrate greater niche breadth (NB) or niche position (NP) than do species with limited distributions and low local abundances? (iii) Does the abundance-occupancy relationship change between habitat generalists and specialists? 2. Species abundance was measured as the total mean relative abundance over all sites. Three measures of occupancy were used: occurrence (number of sites occupied), latitudinal extent of occupancy, and longitudinal extent of occupancy. Niche-based indices (breadth and position) were expressed as multivariate indices of habitat variables, reflecting variation in the niche of a species, as opposed to a simple range. 3. Non-ecological factors (sampling, analytical artefacts, phylogenetic non-independence) accounted for our results and, accordingly, we focus on ecological explanations of inter-specific abundance-occupancy relationships, grounded in niche and meta-population theory. When occupancy was measured as the number of sites at which a species occurred, a strong inter-specific abundance-occupancy relationship was found. Niche-based indices did not have a significant influence on shaping this relationship. When occupancy was measured as geographic extent, the abundance-occupancy relationship was weak or non-significant, although niche-based indices had a significant effect on occupancy. No evidence supported the notion that meta-population dynamics influence abundance-occupancy relationships. Abundance-occupancy relationship is a summation of individual species traits, a point that often receives little attention.