Community assembly in marine macrofauna communities
A key question in community ecology is whether communities are a natural level of biological organisation, with characteristic emergent properties influenced by evolution and comprising species that have mutually influenced each others evolution, or whether they are simply haphazard collections of species adapted to similar physical conditions. Are they a real biological phenomenon or merely artificial constructs to help biologists order their thinking? If communities are real entities determined and constrained by evolution and with distinct emergent properties, then one place to look for evidence of this is during the assembly process. A community that displays a broadly consistent structure regardless of the order of arrival and abundance of taxa and thus whose structure reflects predictable post-settlement processes and not merely the accumulation of settlers, is likely to be a real community rather than a haphazard assemblage.
Community assembly in macrofauna communities developed in artificial kelp holdfasts was monitored at 1-month intervals over a 13 month period using a sampling design that used systematic patterns of temporal overlap and changes in start and collection dates. The aim of this experiment was to study the links between recruitment and community dynamics through the assembly process by comparing community trajectories for substrata deployed on different dates, and thus subject to different potential recruitment from the larval pool. The rate of settlement is thought to be an important determinant of marine community dynamics. The design is unique in its intensity of sampling and level of replication, which could only be achieved through the use of artificial habitats.
The hierarchical nature of the experimental design allowed several different approaches to analysis; by date of deployment and by date of collection of the artificial habitats, which enabled comparison of community assembly with and without the seasonal effects of the date of collection, and by community age to test whether there were alternative end-states to assembly depending on season or recruitment history.
While the process of assembly varied across the different deployment and collection dates, community structures defining the completion of assembly, were broadly similar. This was because the interactions between individuals (interspecific and/or intraspecific) were an important structuring force. This was particularly the case in older communities, and thus, the rate of supply of recruits (relative to the frequency and intensity of disturbance) can be a key determinant of community dynamics in macrofauna communities. Richness was lower than expected from accumulation of monthly recruits, suggesting the importance of pre- and/or post-settlement competitive and/or predatory interactions. However, results were taxon specific, with evidence of settlementdriven population dynamics (e.g. hiatellid bivalves), facilitation (e.g. serpulid polychaetes) and competition/predation (e.g. phyllodocid polychaetes), among the 100 families examined. The behaviours of a series of null models, which displayed increasing levels of ecological realism and which were based on observed data, showed that while the interactions among individuals already established in the community influenced community dispersion, the interactions occurring between settling individuals and the established community had the greatest influence on community structure. Results on a subset of taxa of sufficient abundance for further analysis suggested that the direction of these v interactions was consistent across communities of different age. These results have implications for the way we approach the study of macrofauna communities, because recruitment to bare space is likely to be a poor indicator of the actual recruitment of organisms to an established community.
The overall results suggest that the collection of organisms that establish within an artificial kelp holdfast are more than just assemblages of settling organisms, rather they should be considered real communities. This was evident in an analysis designed to identify and assess the performance of surrogates employed to monitor community structure through changes in season and community age. While surrogate performance varied depending on the surrogate examined, with careful selection, effective surrogates of community structure could be identified. Notably, good surrogates could be identified from as few as 10% of the total number of families, but surrogates based on familial diversity within abundant higher taxa did not perform any better than random selections of the same number of families.
Department/SchoolSchool of Natural Sciences
PublisherUniversity of Tasmania, School of Zoology