Large scale variability in the structure of sessile invertebrate assemblages in artificial habitats reveals the importance of local-scale processes

- Artificial habitats are composed of low diversity, primarily non-indigenous assemblages different to nearby natural habitats
- Assemblages on artificial structure are more controlled by local factors than regional processes such as temperature gradients
- Complex interaction of local factors influence the structure of sessile assemblages, with implications for the spread of non-indigenous species

Natural communities are structured by a complex suite of interacting physical and biological processes that operate across multiple spatial and temporal scales. Documenting spatiotemporal variability in ecological patterns can yield insights into the key processes influencing the distributions of species and structure of communities. Many previous studies conducted in natural habitats have recorded systematic shifts in assemblage structure along broad-scale latitudinal gradients, largely because of individual species' thermal affinities. However, it remains unclear as to whether similar patterns occur in artificial habitats, where patterns could be decoupled from natural processes. In this study, we examined patterns of spatial variability in the structure of sessile invertebrate assemblages in coastal infrastructure at multiple scales, including along a large-scale latitudinal gradient in Western Australia (WA). We deployed settlement panel arrays to sample invertebrate assemblages at 5 regions (in 2 seasons) along a latitudinal gradient spanning about 16° and > 2000 km along the coast of WA. As sea temperature co-varies predictably with latitude in this system, the study also encompassed a temperature gradient of about 10 °C. We examined spatiotemporal variability in several assemblage-level metrics, including total biomass, total cover, taxonomic richness and multivariate structure, as well as variability patterns for individual taxa. Unlike assemblages associated with natural habitats along the WA coastline, sessile invertebrate assemblages on coastal infrastructure did not vary systematically with latitude/temperature. Assemblage structure demonstrated little predictability at the regional scale, driven by processes including variability in temperature and adjacent species pools. Rather local-scale variability (and presumably processes and conditions acting at this scale) was far more important. This is an important consideration for coastal managers as local factors (e.g. the design of coastal infrastructure, human activities, hydrodynamic processes and propagule pressure) are likely to be important determinants of ecological pattern, with implications for the spread and establishment of non-indigenous species, biofouling and general ecological structure and functioning.