Effects of variable recruitment on the structure and development of marine sessile communities

The effects of recruitment on communities were complex. Different 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 across three different sites. Variable recruitment of arborescent bryozoans and didemnid ascidians had little effect on community structure. Recruitment of the colonial ascidian Botryllus schlosseri had short lived effects on community structure, while barnacles had more persistent effects at one site. At another site, recruitment of B. schlosseri and the bryozoans Watersipora subtorquata had strong persistent effects on community structure, dominating space where they recruited and influencing the abundances of a variety of different taxa.

The results of this thesis demonstrate that species composition and frequency of recruitment can have an important influence on the formation and structure of marine sessile communities but that these effects can vary greatly between species, sites and times. These findings are likely to have implications for our understanding of a range of communities that rely on recruitment of dispersive propagules and may respond to varying recruitment patterns in ways similar to those found in this thesis.

When two different groups of taxa, a bryozoan (Conopeum seurati) and barnacles recruited in the presence of an established dominant solitary ascidian, Pyura stolonifera, they caused differences in community structure. Established P. stolonifera did not alter the effects of recruitment of these taxa at the community level, but did alter the effects of recruitment on some individual taxa that subsequently arrived in communities. The combined effects of bryozoan and barnacle recruitment and established P. stolonifera on subsequently arriving taxa were complex and non-uniform.

When species recruited together at the same time rather than as single species they also had effects on community dynamics. These effects were not simply an additive of the effects of single species, suggesting that mixtures of recruits can have synergistic effects on communities. However, different recruitment effects did not persist into later community stages.

The species composition, density and frequency of dispersive propagules (e.g. seeds, spores and larvae) that recruit into any given habitat are highly variable in most biological systems. Such variation in recruitment has been shown to have important consequences on populations, but little is known about recruitment effects on communities of organisms occupying the same habitat. In particular, few direct experimental studies exist that examine how recruitment influences the composition and assembly of whole communities. For this thesis, I used a range of experiments where I manipulated the species composition, density and frequency of recruitment for a variety of taxa and followed the subsequent development of hard-substrate communities of sessile animals living in temperate marine waters over 6 months periods occurring in 2009, 2010 and 2011.

The frequency at which recruits entered communities also had variable effects on community structure. When Botryllus schlosseri and didemnid ascidians entered communities in a single large event or steady recruitment of the same number of individuals it did not have a strong influence on community structure. In a separate experiment botryllid ascidians caused changes in the structure of communities when they recruited steadily compared to when they recruited in a single recruitment pulse.