Habitat Heterogeneity and Invertebrate Species Richness and Abundance in Tropical Seagrass Meadows

Wetstone, Gregory; Heck, Kenneth

Two measures of habitat heterogeneity, plant species number and plant biomass, are considered in relation to the species richness and abundance of invertebrates inhabiting tropical sea-grass (Thalassia testudinum) meadows. Plant biomass is significantly correlated with both invertebrate species number and abundance, while the number of plant species is not significantly related to either of these factors. The significant correlation between plant biomass and invertebrate species number is probably a result of the additional sheltered habitats that are formed among the foliose branches of certain plant species when they are very dense. The significant plant biomass invertebrate abundance correlation is most likely due to the protection from predators that thick vegetation provides, as well as the greater plant surface area that is available for habitation in heavily vegetated areas. The lack of correlation between plant species number and either invertebrate species number or abundance supports the thesis that these tropical marine species are not specialized feeders on macroplants. This stands in sharp contrast to the highly specialized feeding habits ascribed to many terrestrial herbivores in the tropics. A primary concern of ecology is to determine why different areas support different numbers of species. One of the earliest explanations involves differing structural characteristics of the environment; intuition dictates that a more complex environment will support more species. In recent years this idea has been termed the theory of spatial heterogeneity. Mechanistically, complex environments could support a diverse array of species because they would contain many resources capable of sustaining large numbers of species. The correlation between habitat complexity and species number has been documented for a wide variety of terrestrial organisms including birds, mammals, lizards, insects, and plants. In the marine environment Abele demonstrated that over a wide geographical range the number of shallow water decapod species is a function of substrate complexity. Kohn found a similar relationship for gastropods of the genus Conus. In general, however, marine habitats have been poorly investigated with regard to this phenomenon, especially when the habitat complexity is comprised of living organisms. The seagrass habitat, which is widespread throughout tropical and temperate latitudes and exhibits varying degrees of complexity in relatively small areas, is a convenient system for investigating habitat heterogeneity in marine environments. In this study we discuss the relationship between habitat complexity and both the number of species and the number of individuals of epibenthic invertebrates living in subtidal seagrass meadows.