Ecological succession in Maltese freshwater rockpools

Abstract

Processes of Gleasonian succession were investigated in five seasonally-flooded autumnal rainwater rockpools in the Maltese Islands over two partial wet seasons and one complete wet season. These pools support an r-selected resident biota that is depauperate relative to the pool of potential colonisers. This may be a consequence of the insular nature of the Maltese superhabitat and of the small size of the study pools, which thus present a restricted target for propagules. Dynamics of the pool communities were mainly regulated by climatic events. Broadly-predictable cycles were responsible for the seasonal flooding and desiccation of the pools, while unpredictable intraseasonal events, particularly droughts and freak storms, exerted significant intermittent effects on community structure and complexity. Early succession (up to one day after flooding) is dependent on the reservoir of cryptobiotic resting stages in the pool sediment, while later appearance of species is a function of the hatching response of resistant thick-walled eggs in the sedimentbank. In general, assembly of the resident community was largely completed within twenty days from filling of the habitat. Species entering the community after twenty days were mainly transient and semi-transient immigrants. In four of the study pools, a definite shift in community composition was detected, where the late-season community differed from the initial community. The temporal evolution of community structure was a consequence of the arrival into, and departure from, the community of a small number of non-resident species. Patterns of these species were superimposed on the generally constant background presence of an assemblage of taxa dominated by microcrustaceans (branchiopods, ostracods and copepods ). Autogenic processes such as interspecific competition and competitive exclusion were not significant for directing shifts in community composition. Competition between the most conspicuous organisms was muted by partitioning of the food resource and by insufficient duration of the habitat thereby precluding competitive exclusion of species from the community. Maximum species richness attained by the pool communities was found to be significantly correlated with hydroperiod duration and with presence of aquatic macrophytic vegetation.