| Summary: | Microarthropod diversity patterns were investigated in southwestern British Columbia, Canada. We surveyed soil microarthropods associated with moss carpets on exposed rocky outcrops. Our survey identified 352 morphospecies in 32 sites spanning a 130 km 60 km area. Previous studies have interpreted strong correlations of species composition with environmental factors as evidence of niche limitation, and strong correlations with spatial factors as evidence of dispersal limitation. Here, we examine 18 ecological variables relevant to either spatial location or environmental aspects of ecological processes, and evaluate their influences on the microarthropod community. We tested whether the relative importance of spatial and environmental factors was concordant between various community attributes including composition, abundance and species richness, and between different taxonomic groups of microarthropods (Oribatida, Mesostigmata, Collembola). We used two different methods (distance-based Mantel and raw data-based ordination methods) to show that spatial variables could not explain composition or compositional turnover for most microarthropod groups, except Collembola. Dispersal limitation of Collembola is surprising given the high dispersal ability of this group. Although environmental factors explained a large amount of spatial variance in composition (raw data-based ordination method) for all microarthropod groups, environmental similarity (distance-based Mantel method) was a poor predictor of compositional similarity for Oribatida and Mesostigmata. Total abundance and species richness could also be explained by combinations of environmental factors, particularly those relating to tree cover and soil-relevant microhabitat variables (i.e, water content/mass, total soil mass and particle mass), but total abundance and richness were themselves only weakly correlated across space. The most important environmental influences on microarthropod communities were tree cover and water mass, followed by distance-to-sea. At the same time, there was a lot of unexplained variance in the composition of microarthropod communities (especially for species incidences) which could not be explained by the available ecological variables. As richness hotspots were dispersed across different habitats for different taxonomic groups, we suggested that species interactions might be equally important as environmental filtering and spatial autocorrelation in shaping microarthropod community structure, especially for patterns in species incidence.
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