Steven E. McMurray, Joseph R. Pawlik, and Christopher M. Finelli

It is often assumed that the functional roles of species within a community are fixed; however, these roles are often dynamic and influenced by the structure and dynamics of populations, including the abundance and size of organisms. Given recent and predicted changes in the populations of some species that play important roles in ecosystems, it is increasingly important to understand how these changes in demography may affect ecosystem function.

Much of the organic carbon available as food in marine ecosystems is in the form of microscopic particles suspended in the open ocean (i.e. pelagic) and suspension feeders that filter these particles from seawater are important in the transfer (i.e. flux) of pelagic carbon to communities of organisms that live on the seafloor. On coral reefs off Key Largo, Florida, the demographics of a particularly large and abundant suspension feeder, the giant barrel sponge Xestospongia muta, have significantly changed, with the density of sponges more than doubling since 2000 and the population becoming increasingly dominated by smaller individuals. To investigate how these demographic changes may affect ecosystem function, we parameterized a model of carbon flux by X. muta using measurements of sponge suspension feeding rates and data on the abundance and sizes of sponges over 12 years.

Carbon flux by X. muta increased over time with increasing sponge density and volume, with the largest individuals making the greatest contribution to total flux. Changes in the magnitude of carbon flux mediated by the population were found to be influenced by the growth of sponges of all sizes, the survival of the largest sponges, and reproduction by the largest sponges. Projections indicated that the transfer of carbon from the water column to seafloor communities on coral reefs by X. muta will continue to increase. Our results indicate that the magnitude of the functional roles of species can vary temporally and among size classes of a population, highlighting the need to consider demography in studies of ecosystem function.

Photo caption: The Caribbean giant barrel sponge Xestospongia muta. Photo Credit: Steven E McMurray, UNCW

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