• Additive partitioning;
  • community ecology;
  • community structure;
  • scale;
  • species composition

Abstract.  1. We examined whether local assemblages of temperate forest Diptera are structured or simply random sets of species, and at what scale any patterns become apparent. Nested patterns of α-, β-, and γ-diversity in higher Diptera (Schizophora) were described using additive partitioning, to determine the spatial scale contributing the most to species richness. Patterns were examined for Schizophora, two subordinate taxonomic groups (Calyptratae, Acalyptratae) and common versus rare species.

2. A hierarchically nested design was used with three spatial scales: three sites, four stands per site, six trees per stand. Flies were sampled using trunk traps and flight intercept traps in June-July 2008 in sugar maple stands in southwestern Quebec forest fragments.

3. Species diversity and composition were non-random at all scales, and varied across scales and among subgroups. Smaller scales (β1: between trees) seem to structure species composition of Schizophora, Calyptratae and Acalyptratae. Common species varied more at finer scales (α1: within trees); rare species varied more at large scales (β3: between sites). The scale contributing the most to γ-diversity varied across the groups, but β1 was the overall trend.

4. Diversity patterns differed from those in other forest arthropod taxa, in which larger scales drive overall patterns. This may be explained by the high ecological diversity in Diptera, in which species occurrence is often dictated by the presence of ephemeral, patchy resources within larger sites. The overall similarity from site to site is difficult to explain without genetic evidence as to the extent of dispersal of Diptera between sites.