Maintenance of an Exceptional Intertidal Grazer Biomass in South Africa: Subsidy by Subtidal Kelps

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Abstract
Dense populations of patellid limpets characterize the mid— to low—intertidal communities of the southwestern rocky shores of South Africa. Patella argenvillei and P. granatina reach average densities up to 200/m2, representing in some cases °10—13 kg/m2 of whole wet biomass, and attain size up to 100—mm shell length. Traditionally these limpets have been regarded as generalized grazers, but in facts they have highly specialized feeding mechanisms. P. granatina is found primarily on the mid— to low shore of sheltered boulder—bays, while P. argenvillei forms a conspicuous monospecific belt low on semiexposed shores in association with adjacent kelp beds. Both species are the dominant occupiers of space in their respective habitats. We have investigated the main pathways and supply of food necessary to maintain limpet biomasses as high as this. Epilithic micro— and macroalgae and the kelps Ecklonia maxima and Laminaria pallida are common sources of food for these two limpet species. Gut contents show that kelp presents >50% of the total diet of both species. An abundant source of food is provided to P. granatina by drifting kelp and seaweed debris, and to P. argenvillei by nearly attached kelp plants, which they actively prune. Differences in the timing and method of feeding exist. P. granatina captures drifting kelp and seaweeds while underwater during incoming and outgoing tides, and P, argenvillei prunes the fronds of nearby attached kelp plants during the rising tide. P. argenvillei feed collectively, several individuals sharing a piece of kelp trapped by a single limpet. This overcomes the intraspecific competition normally experienced in high density limpet populations, and the maximum size of P. argenvillei rises with density. P. granatina does not feed collectively and its maximum size declines with density. An experimental exclusion of kelp from the limpets' diets resulted in significantly increased mortality and a reduction in body mass. Subtidal production of kelp fronds subsidizes populations of both limpet species and is vital to the maintenance of their remarkably high biomasses. Supplied with a suberabundance of food, both species achieve such high "packing" that primary space becomes their limiting factor. The trophic subsidy provided by kelps has important secondary consequences since the dense limpet populations powerfully influence intertidal community structure and function.

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