Effects of early recruits on temperate sessile marine community composition depend on other species recruiting at the time.

In many environments recruitment of dispersive propagules (e.g. seeds, spores and larvae) can vary from situations when particular taxa recruit in relative isolation to times when they recruit simultaneously with other, functionally quite different taxa. Differences in the identity and density of recruiting taxa can have important consequences on community structure, but it is still not clear how the effects of individual taxa on communities are modified when they recruit together with other species. Using an experimental approach we compared early development of a temperate marine sessile community after the recruitment of mixtures of botryllid ascidians and barnacles to that when barnacles or botryllid ascidians recruited alone. Communities exposed to recruitment of botryllid ascidians in isolation differed from those that received barnacles, a mixture of botryllids and barnacles or no recruitment in 2-week-old communities. These early differences were driven by higher abundances of the species that were present as initial recruits in experimental treatments. After 2 months communities also differed between barnacle and mixed recruitment treatments but not mixed and botryllid or botryllid and barnacle treatments. These differences were not directly due to differences in the abundances of our manipulated taxa but occurred because of two abundant arborescent bryozoans, Bugula dentata, which occupied more space in communities that initially received mixed recruitment than in those that received barnacle or no recruitment, and Zoobotryon verticillatum, which occupied more space in communities that initially received only barnacle recruitment than those that initially received botryllid or mixed recruitment. These effects did not persist, and communities did not differ after 6 months. These results suggest that, more generally, species may influence community dynamics differently when they recruit alongside other species than when they recruit in relative isolation.

Contrasting effects of variable species recruitment on marine sessile communities.

The species composition, density, and frequency of recruitment into any given habitat are highly variable in most biological systems that rely on dispersive propagules (larvae, seeds, spores, etc.). There are few direct experimental studies of how recruitment variation between single species influences the composition and assembly of whole communities in many of these systems. We manipulated recruitment of a variety of single taxa and followed their effects on the subsequent development of hard-substrate communities of sessile animals living in temperate marine waters. The effects of recruitment on communities were complex. Patterns of recruitment of individual species influenced community structure, but these effects varied greatly depending on the identity of species recruits, the time of community development, and location across three different sites. Variable recruitment of arborescent bryozoans and didemnid ascidians had little effect on community structure. At one site, recruitment of the colonial ascidian Botryllus schlosseri had short-lived effects on community structure, while barnacles had more persistent effects. At another site, recruitment of B. schlosseri and the bryozoan Watersipora subtorquata had strong persistent effects on community structure, dominating space where they recruited and influencing the abundances of a variety of different taxa. Differences in the effects of species recruitment on communities appear to be caused by differences between the ecology and life history of recruiting species as well as differences in background processes between sites. These results demonstrate that discrete recruitment events that vary between single species can be important drivers of community composition but are likely to be heavily influenced by the local environment, even within a single species.

Effects of pulse versus steady recruitment on sessile marine communities.

Variation in patterns of propagule establishment (recruitment) has important effects on population dynamics and the structure of some communities. Most experimental studies have varied recruitment by changing the nature of a single event early in community development, but recruitment can also vary from steady rates of arrival to highly episodic 'pulse' events, causing differences in the temporal spacing of individuals recruiting into patches. We examined whether two different temporal patterns of recruitment of sessile invertebrates affected temperate marine communities in southeastern Australia in two experiments that were run at different times at the same site and that manipulated several different species. Target species entered communities as either a single pulse of recruits within a 2-week period or steady input of the same total number of recruits over a longer time period (5-6 weeks). The pattern of recruitment had variable effects on community structure. The colonial ascidian Botryllus schlosseri did not have a strong influence on community structure whether it recruited in a single pulse or steadily. The cover of B. schlosseri was higher when recruitment occurred as a single pulse. In a second experiment, botryllid ascidians caused changes in the composition of communities when they recruited steadily compared to when they did not recruit or didemnids recruited, but caused no differences in communities when they recruited in a shorter pulse. In contrast, recruitment frequency of didemnid ascidians had little effect, though their presence/absence caused community differences. Though we found that different temporal recruitment patterns can alter community composition, the life history and ecology of particular taxa as well as differences in environmental background processes are likely to influence the strength of these effects.