Small-scale heterogeneity in temperate forest canopy arthropods: stratification of spider and beetle assemblages

Vertical gradients in biotic and abiotic factors may create small-scale spatial variation in arthropod communities, a phenomenon that continues to be understudied. We investigated heterogeneity in the vertical distribution of spider and beetle assemblages in the canopy of sugar maples (Acer saccharum Marshall) (Aceraceae) in a deciduous forest in eastern Canada. Comparisons across four strata (understorey [UN] through upper canopy [UC] crown) documented variation in density, diversity, and species composition. Density of all common families decreased significantly with height and overall species richness of both spiders and beetles was highest in the UN and lowest in the UC crown. We observed greater spatial variation in spider assemblages compared with beetle assemblages and documented differences in spider guild structure: web-spinning spiders were most common in the UN and jumping spiders dominated the canopy. Our results suggest that arthropod assemblages are not homogeneous with respect to vertical space and that heterogeneity exists even at the scale of several metres.