Fine-scale structure and cross-taxon congruence of bird and beetle assemblages in an old-growth boreal forest mosaic

Nestedness occurs when species present in depauperate sites are subsets of those found in species-rich sites. The degree of congruence of site nestedness among different assemblages can inform commonalities of mechanisms structuring the assemblages. Well-nested assemblages may still contain idiosyncratic species and sites that notably depart from the typical assemblage pattern. Idiosyncrasy can arise from multiple processes, including interspecific interactions and habitat preferences, which entail different consequences for species co-occurrences. We investigate the influence of fine-scale habitat variation on nestedness and idiosyncrasy patterns of beetle and bird assemblages. We examine community-level and pairwise species co-occurrence patterns, and highlight the potential influence of interspecific interactions for assemblage structure. Cote-Nord region of QuEbec, Canada. We sampled occurrences of ground-dwelling beetles, flying beetles and birds at sites within old-growth boreal forest. We examined the nestedness and idiosyncrasy of sites and sought relationships to habitat attributes. We analysed non-random species co-occurrence patterns at pairwise and community levels, using null model analysis and five 'association' indices. All three assemblages were significantly nested. There was limited congruence only between birds and flying beetles whose nestedness was related to canopy openness. For ground-dwelling beetles, nestedness was related to high stand heterogeneity and sapling density, whereas site idiosyncrasy was inversely related to structural heterogeneity. For birds, site idiosyncrasy increased with canopy cover, and most idiosyncratic species were closed-canopy specialists. In all assemblages, species idiosyncrasy was positively correlated with the frequency of negative pairwise associations. Species co-occurrence patterns were non-random, and for flying beetles and birds positive species pairwise associations dominated. Community-level co-occurrence summaries may not, however, always reflect these patterns. Nestedness patterns of different assemblages may not correlate, even when sampled at common locations, because of different responses to local habitat attributes. We found idiosyncrasy patterns indicating opposing habitat preferences, consistent with antagonistic interactions among species within assemblages. Analysis of such patterns can thus suggest the mechanisms generating assemblage structures, with implications for biodiversity conservation.