The role of community structure for invasion dynamics in marine fouling communities in the Baltic Sea

Abstract

Biological invasions occur increasingly and worldwide due to anthropogenic interconnections between previously isolated biota. In marine environments, fouling organisms that attach on ship hulls are transported into new habitats and can have a severe impact on the local species pool and thus on ecosystem functioning. The success of non-native species depends on several factors. Within a fouling community, there can be positive interactions that increase the stability of fouling communities, thus enhancing the possibility of a species within the community to persist long enough to complete its life cycle. In consequence, the risk of dispersal and establishment in the new environment increases. To examine the persistence of fouling communities under changed environmental conditions, I mimicked the transport of marine fouling communities to a new environment. Artificial hard substrata were submerged in the Gulf of Finland at two different sites to allow local species to settle for 2 and 4 months. After that, half of the communities grown on the substrata from one site were transported and paired with communities of the same successional stage from the other site, and vice versa. Subsequently, the communities were sampled bi-weekly to measure the abundances of all macrofouler species and the amount of available settlement substratum. This method provided the raw data for the Bray-Curtis similarity index, which allowed quantifying the convergence of the transplanted towards the non-transplanted communities over time. I used the speed of convergence as a measure for the structural stability of communities in the face of environmental change. Despite a low convergence rate, a small species pool, and similar abiotic conditions, the age of the fouling communities and the site conditions significantly affected the convergence process in the Gulf of Finland. I examined the impact of different properties of community age on the convergence process and found no correlation with diversity, which is discussed to be one of the most important factors determining community stability. Instead, single species identity determined the convergence process, while the level of exposure to wave action apparently influenced the speed of convergence between transplanted and non-transplanted communities at the two sites in general. My results suggest that attention should be paid to species interactions within fouling communities to assess the possible invasion success of single fouling organisms that arrive as part of these assemblages