Relative influence of resident species and environmental variation on community assembly

Abstract

Prior residence by a species can affect subsequent community assembly. However, previous studies insulated their focal communities from additional sources of variation, and the role of resident species in the context of environmental heterogeneity is rarely considered. If environmental and resident species effects act independently, then each should be broadly predictable, and their contribution to community assembly should be quantifiable in relation to each other. Alternatively, if effects interact, their combination may explain more of the differences in com munities than the additive influence of each alone. We estimated the effects of a common, early-colonising resident (the encrusting bryozoan Hippopodina iririkiensis) on community assembly relative to substrate orientation. Some species showed complex responses in association with orientation, with positive responses in one orientation, negative in the other. Variation in orientation explained the majority of variation in overall community assembly. Variation among the panels holding replicates of our resident species, a blocking factor in the analysis, permitted us to consider small-scale spatial variation. Abundances responded to resident species effects but interacted with spatial variation: the impact of the resident species on community assembly varied with orientation and space. Functional groups showed similarly idiosyncratic responses to the prior resident. Overall, we found that resident species effects were weak relative to the effects of environmental variation on community assembly. Furthermore, those resident species effects that we did detect were inconsistent across environments, suggesting that this species has little predictable influence on community assembly. Environmental variation may be an important contributor and requires more widespread consideration to better understand how resident species effects act in nature.