The influence of quantitative and qualitative aspects of habitat complexity in tropical sea-grass meadows

Abstract

In sea-grass meadows of Islas Los Roques, Venezuela, densities of turtle grass ( Thalassia testudinum (König)) and the understory of calcareous green algae ( Halimeda opuntia (Linnaeus)) were manipulated in nine experimental treatments and controls to examine the influence of both quantitative (biomass and total surface area) and qualitative (architectural) aspects of habitat complexity on crustacean assemblages. Small, motile epifaunal crustaceans quickly colonized manipulated habitats, with numbers of individuals and species becoming asymptotic within 5 days. Greatest abundance of crustaceans occurred in plots with highest macrophyte biomass. Number of individuals per unit plant surface area, however, was uniform over all vegetated treatments, suggesting that surface area of the habitat controlled total abundance of epifaunal Crustacea, regardless of architectural type available. Rarefaction indicated that highest species richness was found in treatments containing high standing crops of Thalassia and Halimeda, suggesting that quantitative aspects of habitat complexity have an important influence on species richness. Responses of individual species to architectural type were variable; of the 15 most abundant taxa, eight species were significantly associated with Halimeda, while only two were associated with Thalassia. Because of the high number of animal species associated with Halimeda, addition of this architectural type to Thalassia beds had a pronounced influence on faunal species richness in the grass bed. It is now clear that both quantitative and qualitative aspects of habitat complexity affect abundance, species richness, and composition of crustaceans in tropical sea-grass meadows. Increased habitat complexity associated with the occurrence of Halimeda and other “supplemental substrata” in tropical sea-grass meadows may be responsible for large-scale biogeographic patterns such as gradients in species diversity.