Biotic filtering and mass effects in small shrub patches: is arthropod community structure predictable based on the quality of the vegetation?

1. Community assembly is affected by four processes: dispersal, filtering effects (selection), ecological drift and evolution. The role of filtering relative to dispersal and drift should decline with patch size, hampering possibilities to predict which organisms will be observed within small‐sized patches. However, vegetation structure is known to have a marked impact on species assemblages, and plant quality may act as a biotic filter. This challenges the assumption of unpredictable species assemblages in small‐sized vegetation patches. 2. Using 32 stands of five shrub species in south‐west Finland, this study investigated whether biotic filtering effects caused by patch‐forming plants are strong enough to overcome the mixing of mobile arthropod assemblages across small patches. 3. Stochastic variation did not hide the signals of biotic filtering and dispersal in the small shrub patches. Habitat richness around the patches explained a three times larger share of variation in the species composition than did the identity of the patch‐forming plant, but it had less effect on the abundance of arthropods. A radius of 50–100 m around a patch explained the species composition best. 4. Abundance patterns varied between the feeding guilds; the patch‐forming shrub influenced the abundances of detritivores and leaf‐feeding herbivores, whereas the abundances of flower‐visiting herbivores appeared to track the flowering phenology of the plants. Shrub identity had little effect on omnivores or predators. Predator abundances were correlated with the abundance of potential prey. 5. The results of this study suggest that community composition within a vegetation patch may be predictable even if dispersal overrides local filtering effects, as suggested by the mass‐effects paradigm.