In significantly nested assemblages, species found in species-poor sites are proper subsets of species-rich ones. If, within a given system, different assemblages exhibit similar pattern of nestedness, it may indicate that they respond to a common underlying causal factor or to different factors that covary over space. On the other hand, within assemblages, certain species may markedly depart from assemblage-level nestedness, such that they may occur most frequently in species-poor sites (outliers) but rarely as part of species-rich communities (holes) or they may have a segregated pairwise species association. Such species are termed as “idiosyncratic species”, and sites with many idiosyncratic species are “idiosyncratic sites”.  The species-specific and site-specific idiosyncrasy score by nestedness analysis using the temperature metric (NTC: Atmar & Paterson 1993, BINMATNEST: Rodríguez-Gironés & Santamaría 2006) reflects varying degree of species’ and sites’ departure from the overall nestedness pattern. We examined fine-scale patterns of nestedness and idiosyncrasy of bird and two beetle assemblages (ground-dwelling and flying) in ‘irregular’ boreal forest mosaic, in Côte-Nord, Québec, Canada. We determined if fine-scale between stand variation in forest composition and structure can explain these patterns. We applied null model analysis of species pairwise co-occurrences to examine whether species-specific idiosyncrasy score is related to species’ frequency to have negative associations.

Results/Conclusions All three assemblages were significantly nested, but we found limited (between birds and flying-beetles) or no congruence in their nestedness patterns or the associated fine-scale forest attributes. Specifically, stands with a high nestedness rank for ground-dwelling beetles had higher heterogeneity of forest structure and composition (PC1) and sapling density (PC3), while for flying beetles and birds had low canopy cover and low density of live trees (PC2). For bird and ground-dwelling beetle assemblages, the relationship of stand-specific idiosyncrasy scores to habitat attributes contrasted that of nestedness rank. Species that departed markedly from assemblage-level nestedness had more negative species pairwise association than nested ones. Our study suggests that nestedness of different taxa may show, albeit limited, congruence if they share similarity in life history, e.g., vagility. More importantly, however, nestedness pattern of different assemblages, even belonging to the same taxa, may not correlate because they differ in their response to underlying habitat attributes. This difference could be prevalent also within assemblages between species that conform to (nested) and depart (idiosyncratic) from assemblage level nestedness. Also, negative interspecific interactions may partly account for idiosyncratic species distribution.