Natural densities of mesograzers fail to limit growth of macroalgae

Natural densities of mesograzers fail to limit growth of macroalgae

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Journal of Ecology 2009, 97 , 164–175 doi: 10.1111/j.1365-2745.2008.01457.x © 2008 The Authors. Journal compilation © 2008 British Ecological Society Blackwel Publishing Ltd Natural densities of mesograzers fail to limit growth of macroalgae or their epiphytes in a temperate algal bed Alistair G. B. Poore 1,2 *, Alexandra H. Campbell 1,2,3 and Peter D. Steinberg 2,3 1 Evolution and Ecology Research Centre; 2 School of Biological, Earth and Environmental Sciences; and 3 Centre for Marine BioInnovation, University of New South Wales, Sydney, NSW 2052, Australia Summary 1. Herbivory is particularly intense in marine environments, with a higher proportion of primary productivity removed than in terrestrial habitats. Experimental manipulation of large herbivores (Fsh, urchins) has clearly documented their grazing impacts on algal and seagrass beds. Grazing impacts of mesograzers (small invertebrates such as amphipods and isopods) are, however, less understood due to the practical difFculties in manipulating their abundance in Feld conditions. 2. We developed a novel technique that successfully manipulated the abundance of herbivorous amphipods on macroalgae without the potential artefacts associated with exclusion cages or mes- ocosms. We then used this technique to test the effects of reduced amphipod grazing over extended periods on the structure of a temperate algal assemblage. We tested grazer effects on growth rates and epiphyte cover of the brown alga Sargassum linearifolium , and on developing assemblages on bare substrates. 3. Large reductions in the abundance of herbivorous amphipods affected neither the growth rates of S. linearifolium , the cover of its epiphytes, nor the structure of algal assemblages. This result contrasts strongly to previous studies in mesocosms documenting strong impacts of mesograzers on community structure, and we discuss differences in the experimental approaches and biology of the systems that could give rise to the observed differences in grazer impacts. 4. Synthesis . Marine macroalgae and seagrasses support very high densities of small herbivores whose ecological role in these habitats is poorly understood. We have provided the Frst, replicated experiment that directly manipulates their density in situ to quantify grazer impacts without caging artefacts. Our results indicate that strong impacts are not likely with the naturally occurring amphipod densities in the temperate algal bed studied. ±urther such experimental tests in Feld conditions are required to understand the properties of grazer and plant communities that can predict grazer impacts. Key-words: algae, amphipods, community structure, epiphytes, grazing, herbivory, marine, mesograzers, plant–herbivore interactions, seaweeds Introduction Herbivory is a fundamental ecological process that regulates the biomass and composition of primary producers, and thus the structure and functioning of natural systems. The intensity of herbivory, and its importance relative to other processes, varies widely among habitats, with herbivores removing
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Natural densities of mesograzers fail to limit growth of macroalgae

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