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 understanding of the effects of h a b ~t a t structure on species, because structural complexity 1s measured differently in most studies and habitats. There were 3 main objectives of this study: (1) to measure and compare structural complexity between rocky intertidal and mangrove habitats, (2) to examine whether structural complexity affected the density, richness and size of gastropods in these habitats, and (3) to determine whether one lndex of structural complexity [e.g. fractal dimension (D) and chain-and-tape] best represented features of the habltat that affected gastropods I used photogrammetric techn~ques to measure and to compare the effects of structural complexity in quadrats (1 m apart) nested withln sites (10 m apart), shores ( > l km apart) and habitats (rocky intertidal a n d mangrove) in Botany Bay. Australia. All indices showed that complexity was different between quadrats just meters apart in both habitats and was greater in mangrove than in rocky intertidal hab~tats. Two lines of evidence indicated that variation in complexity affected the density of gastropods In rocky intertidal but not in mangrove habitats. First, the density of gastropods varied 4-fold between quadrats withln habitats and, after gastropods were expenmentally removed, a similar density and size distribut~on of gastropods recolon~zed quadrats in the rocky intertidal. Second, this density was correlated with structural complexity. D was most often correlated with density and thus best represented features of the habitat that affected gastropods. The measurement and effects of structural complexity can b e compared between habitats, a n d these compa~isons help eluc~date the cond~tions in which habitat structure may exert strong effects on species.