Separating the elements of habitat structure: independent effects of habitat complexity and structural components on rocky intertidal gastropods

It has been difficult to understand the effects of habitat structure on ass emblages because the different elements of habitat structure are often conf ounded. For example, few studies consider that the effects of structural co mponents of a habitat (rocks, trees, pits, pneumatophores) may be separate from the complexity (e.g. surface area (SA)) they create. From prior observ ations and experiments, I developed three hypotheses about the effects of h abitat structure on gastropods on rocky intertidal shores in Botany Bay, Au stralia. (1) The complexity of habitats positively affects the density and richness of gastropods. (2) The fractal dimension (D) represents elements o f complexity that affect the density and richness of gastropods better than other indices of complexity. (3) The effects of specific structural compon ents on the density and richness of gastropods are independent of their com plexity. To test these hypotheses, treatments composed of pits and pneumato phores were used to independently manipulate complexity and structural comp onents in experiments repeated at five different times on two shores. There was support for hypotheses (1) and (3) at most times and places but not fo r hypothesis (2). Richness, total density, and the densities of two of the three most common gastropods were greater in treatments with greater comple xity. D was not definitively better than other indices of complexity, but D and SA were recommended for further consideration. When complexity was hel d constant, species richness and the density of most gastropods, except Aus trocochlea porcata, was greater in treatments with pits than with pneumatop hores. A common mechanistic explanation for the effects of habitat complexi ty on rocky intertidal gastropods relies on a specific characteristic of pi ts; they pool water and reduce desiccation stress. This assumption may be a ppropriate for many gastropods, but it was inappropriate for A. porcata. Ha bitat complexity affected its density, but this was not because of a charac teristic specific to pits. The complexity and structural components of habi tats have separate effects on assemblages, and it confuses the study of hab itat structure to combine them. (C) 2000 Elsevier Science B.V. All rights r eserved.