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.