Demography alters carbon flux for a dominant benthic suspension feeder, the giant barrel sponge, on Conch Reef, Florida Keys

Abstract

Abstract A challenge to understanding ecosystem processes is that the functional roles of species are linked with the demography of populations. In many systems, and particularly on coral reefs, the dynamics and structure of populations of many functionally important species has significantly changed over recent decades due to a variety of disturbances, and the changes in the ecological processes mediated by them are poorly understood. Benthic suspension feeders are allogenic ecosystem engineers that contribute to the flow of materials and energy in aquatic systems. There is increasing evidence that sponges dominate this functional group on Caribbean reefs. Using demographic data from 2000 to 2012 and measurements of filtration rates of particulate and dissolved organic carbon, we parameterized a stage‐based matrix model of population‐mediated carbon flux for the Caribbean giant barrel sponge Xestospongia muta on Conch Reef, Florida Keys, to investigate the demographic mechanisms that mediate changes in benthic‐pelagic coupling. Population‐mediated carbon flux increased over time with increasing sponge density and volume, with the largest individuals making the greatest contribution. Elasticity analysis revealed that the growth of sponges in all stages, the survival of sponges in the two largest stages, and the production of new recruits by the largest sponges had the greatest influence on changes in population‐mediated carbon flux. We estimate the mean carbon flux over 2000–2012 for X. muta at Conch Reef was 1575 mg C days−1 m−2, a value that exceeds that of any other single‐species rate. Projections indicated that population‐mediated carbon flux will continue to increase under present conditions; thus, X. muta is expected to play an even larger role in the transfer of carbon from the water column to the benthos on Caribbean coral reefs. Rather than being fixed, the functional role of species within a community are often dynamic and influenced by the demography of populations. The general framework developed here, a conversion of the basic unit of measurement for population models (i.e. the individual) into quantifiable metrics of species traits or activities, may be more broadly applied by further efforts to examine interrelationships between population and ecosystem processes. A plain language summary is available for this article.