Nested patterns of beta-diversity in forest Diptera

Are the assemblages of flies (Diptera) in local habitat patches predictable, or are they random sets of species? And if there is an obvious pattern, at what scale does it become apparent? The purpose of this study was to address these questions by describing the nested patterns of alpha-, beta-, and gamma-diversity in temperate forest Diptera. The diversity patterns were studied by determining the alpha richness and the regional pool; the nested patterns of beta-diversity; and the scale contributing the most to species richness. Fieldwork was carried out in June-July 2008 in sugar maple stands in three southwestern Quebec forest fragments, using three spatial scales. Each site had four randomly selected stands, with six trees per stand, and two traps per tree. Dipteran species diversity (239 species) and composition were non-random at all scales selected, and varied across scales. These scales were not equally important across different taxonomic and ecological groups, as shown by the different diversity patterns. Smaller scales seem to structure Diptera species composition (β1: between trees), as well as two taxonomic subgroups: Calyptratae and Acalyptratae. Common species were also more important at finer scales (α1: within trees), while rare species varied more at broader scales (β3: between sites). The scale contributing the most to γ-diversity varied across the different groups, but overall β1 was most significant; this was statistically supported by additive partitioning, NMDS, MRPP and similarity indices. Species composition was weakly supported by environmental variables.