Ecology of fouling assemblages associated with mangrove's roots: An artificial substrate for manipulative experiments

Abstract Several of the most influential studies in marine ecology are based on results obtained from experiments using units of artificial habitats. However, there are few studies that have measured the effectiveness of those experimental substrates as surrogates of natural ones, in terms of reflecting natural patterns of colonization and development of the natural assemblages. Such considerations are necessary to extrapolate the described patterns and processes in artificial habitats to natural communities. This situation is particularly noticeable in ecological studies dealing with the identification of process that sustain the high levels of species diversity reported for epibiont communities that grow on the roots of Rizophora mangle in the Caribbean Sea. That is, no study has demonstrated the validity of artificial substrate as surrogate of mangrove roots. Consequently, in this study we tested whether wooden sticks can be used as surrogates for natural prop roots. Furthermore, and in order to propose possible mechanisms explaining the observed patterns, we also tested whether differences in epibiotic assemblages on natural mangrove prop roots were due primarily to intrinsic (biological) differences among roots. The experiment was conducted in four randomly chosen localities of La Restinga National Park, Venezuela, to test for the generality of the observed outcomes. We used two types of substrates: broomsticks as artificial mangrove roots (AMR) and natural mangrove roots (NMR) denuded from epibionts. We recorded the presence and estimated the abundance of sessile species growing on the experimental treatments after 30, 130 and 200 days. No significant differences in species richness between the two substrates at any locality and at any time were detected. However, the effect of the substrate on the composition and abundance of species varied among localities and increased over time. Multivariate dispersion was not different between substrates, indicating that intrinsic biological characteristics of prop roots are not responsible for the observed differences among neighboring roots. Despite spatial differences at the scale of localities, most of the species recorded in this study colonized both types of substrates, although some species were more abundant over AMR (e.g. cirripeds, bryozoans), while others were more abundant on NMR (e.g. oyster, algae). Typical mangrove sponges and tunicates colonized similarly both types of substrates. Results obtained in this work suggest that broomsticks are appropriate surrogates of mangrove roots to experimentally evaluate processes associated with species diversity, but should not be used to evaluate processes that affect the abundance and structure of epibiont assemblages.