Distinguishing direct from indirect effects of grazers in intertidal estuarine assemblages

Previous work demonstrated that gastropod grazers (Bembicium auratum and Austrocochlea porcata) are important in structuring ecological succession in an intertidal estuarine assemblage in New South Wales, Australia. This paper describes work designed to test the hypothesis that the effects of grazers an the intertidal assemblages are primarily indirect, due to their removal of algae. The experiment done to distinguish direct from indirect effects included independent and replicated removals of grazers or algae id a crossed design, along with controls for potential artifacts associated with removals. In addition to specific tests on individual species, a new non-parametric multivariate randomization test (distance-based redundancy analysis, db-RDA) was used to test the hypothesis that simultaneous effects of grazers on the whole assemblage of species were indirect. Results showed that the grazers had indirect positive effects on oysters (Saccostrea commercialis) and indirect negative effects on some species of barnacles (Balanus variegatus and B. amphitrite). Other species of barnacles (Hexaminius sp. and Elminius covertus) were not affected, either by algae or grazers. Multivariate analyses showed that the effects of grazers on the entire assemblage of species were indeed indirect, through their removal of algae. This study highlights the importance of considering the implications of indirect effects of herbivores on the ecology of whole communities, in addition to their impacts on single-species populations. (C) 1999 Elsevier Science BN. Ail rights reserved.