Evidence for ecosystem engineering in a lentic habitat by tadpoles of the western toad
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A number of aquatic organisms have demonstrated an ability to modulate sediment deposition in tropical and temperate stream systems, but little is known about the impact of similar benthic grazing processes in temperate pond systems. Using exclosure experiments in mesocosms, we used tadpoles of the western toad (Anaxyrus boreas) to investigate two questions: (1) can grazing activities of tadpoles of the western toad in ponds reduce sediment accrual on grazing surfaces (e.g. via bioturbation or ingestion); and (2) do these grazing activities contribute to enhanced periphyton biomass and chlorophyll a content of benthic matter. Mesocosms received pulses of fine sediment weekly over 8 weeks to simulate allochthonous surface inputs following rainstorms. Accumulated sediment and periphyton matter on exposed and grazing-restricted substrate surfaces were measured weekly. Tadpole grazing activity significantly reduced the standing stock of periphyton on open grazing surfaces by 80% in controls. Under sediment-additions, tadpoles effectively removed settled particulate matter from exposed grazing surfaces, but no corresponding increase in periphyton biomass was observed. These results suggest that tadpoles can impact the dynamics of sediment distribution in ponds as well as streams, providing support for a role as “ecosystem engineers” in these habitats.
KeywordsAnaxyrus boreas Ponds Benthic grazing Sediment Mesocosm
Thank you very much to Nancy Hofer, Aya Reiss, Isabelle Deguise, Mona Matson, Ashlee Alford and Dorota Klimek for their assistance in the field, and to Peter Arcese and Laurie Marczak for their valuable comments. We also thank British Columbia Provincial Parks, in particular Hugh Akroyd at Alice Lake Provincial Park, for permission and ready access to park facilities and to Mark Bomford for coordination at University of British Columbia South Campus. We would like to also thank the reviewers for their helpful comments. Animals involved in this study were handled in accordance with the Canadian Animal Care Committee, approved protocol A06-0080 from the University of British Columbia. We are grateful for funding supplied by the Forest Sciences Program (British Columbia) and a National Science and Engineering Research Council scholarship to S.Wood. All experiments carried out complied with the laws of Canada.
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