The strength of cross-taxon congruence in species composition varies with the size of regional species pools and the intensity of human disturbance

AimOur aims were to determine whether cross-taxon congruence of species composition patterns varies across regions and human disturbance levels and to infer whether these patterns relate to the size of the regional species pool and the sorting of species along a gradient of human disturbance. LocationAlberta's Boreal and Grassland Natural Regions, Canada. MethodWe compiled presence-absence data of four biological groups (birds, vascular plants, bryophytes and mites) from low and high disturbance sites in upland habitat. The cross-taxon congruence across ecoregion and disturbance levels was analysed using Mantel and Procrustes tests. We applied resampling without replacement to generate confidence intervals to test for significant differences in strength of congruency between disturbance levels and ecoregions. We performed indicator species analysis to highlight how the species-level response to high and low disturbance influences the pattern in community-level cross-taxon congruence. ResultsCross-taxon congruence was stronger when all sites were considered than when high and low disturbance sites were considered separately. Congruency was relatively stronger in high than low disturbance sites in the Boreal ecoregion, but the pattern was reversed in the Grassland ecoregion. More species were indicators of undisturbed habitat than of highly disturbed habitats for all biological groups except for birds. Overall, biological groups that were poorly represented in a region and/or with few characteristic indicator species showed weak congruence in those sites. Main conclusionsWe conclude that a longer disturbance gradient can promote cross-taxon congruence by increasing the species pool characteristic of low or high disturbance levels. Moreover, regional context can influence (or even reverse) the relative strength of cross-taxon congruence in high and low disturbance sites, which may explain the inconsistent strength of cross-taxon congruence along the disturbance gradient. To use biodiversity surrogates across biogeographical regions, it is therefore important to account for the regional and disturbance-level dependence of cross-taxon congruence.