COMPARISON OF THE MEASUREMENT AND EFFECTS OF HABITAT STRUCTURE ON GASTROPODS IN ROCKY INTERTIDAL AND MANGROVE HABITATS

Ecologists have had little success in the development of a synthetic u nderstanding of the effects of habitat structure on species, because s tructural complexity is measured differently in most studies and habit ats. There were 3 main objectives of this study: (1) to measure and co mpare structural complexity between rocky intertidal and mangrove habi tats, (2) to examine whether structural complexity affected the densit y, richness and size of gastropods in these habitats, and (3) to deter mine whether one index of structural complexity [e.g. fractal dimensio n (D) and chain-and-tape] best represented features of the habitat tha t affected gastropods. I used photogrammetric techniques to measure an d to compare the effects of structural complexity in quadrats (1 m apa rt) nested within sites (10 m apart), shores (>1 km apart) and habitat s (rocky intertidal and mangrove) in Botany Bay, Australia. All indice s showed that complexity was different between quadrats just meters ap art in both habitats and was greater in mangrove than in rocky interti dal habitats. Two lines of evidence indicated that variation in comple xity affected the density of gastropods in rocky intertidal but not in mangrove habitats. First, the density of gastropods varied 4-fold bet ween quadrats within habitats and, after gastropods were experimentall y removed, a similar density and size distribution of gastropods recol onized quadrats in the rocky intertidal. Second, this density was corr elated with structural complexity. D was most often correlated with de nsity and thus best represented features of the habitat that affected gastropods. The measurement and effects of structural complexity can b e compared between habitats, and these comparisons help elucidate the conditions in which habitat structure may exert strong effects on spec ies.