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

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 http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12908/suppinfo is available for this article.

Climate change alterations to ecosystem dominance: how might sponge‐dominated reefs function?

“…, McMurray et al. ), shifts toward sponge‐dominated ecosystem states would have a number of direct impacts on ecosystem function (Table ). Many of these direct impacts, such as bioerosion (excavating and boring sponges, e.g., Murphy et al.…”

“…Increased sponge abundance would also require greater quantities of DOC (McMurray et al. ), although reliance of sponges on DOC varies between species (Hoer et al. ), so we may see major changes to sponge population structures.…”

“…In recent years there have been numerous reports of transitions toward sponge-dominated states in reefs spanning the Caribbean, Atlantic, Indo-Pacific, and Pacific regions (reviewed in Bell et al 2013). Given that sponges have many important functional roles on coral reefs (Bell 2008), particularly through their trophic interactions with the water column (Reiswig 1971, 1975, Maldonado et al 2012, McMurray et al 2017, shifts toward sponge-dominated ecosystem states would have a number of direct impacts on ecosystem function (Table 1). Many of these direct impacts, such as bioerosion (excavating and boring sponges, e.g., Murphy et al 2016), picoplankton removal (Morganti et al 2016), and net primary production (Wilkinson 1983) are relatively easy to quantify, but little is known about the subsequent ecosystem-level consequences of these changes.…”

“…, McMurray et al. ), and one model has considered sponge spatial competition with macroalgae and corals (González‐Rivero et al. ), no studies have yet explored ecosystem‐wide consequences of increased sponge abundance and dominance (Bell et al.…”

“…Here we use qualitative models to explore broad ecosystem-level outcomes from a change to reefs dominated by sponges and identify research gaps to refine our understanding of how these reefs would function. While a number of conceptual and parameterized models have considered current-day sponge trophic interactions (Pawlik 2011, de Goeij et al 2013, Pawlik et al 2016, McMurray et al 2017), and one model has considered sponge spatial competition with macroalgae and corals (Gonz alez-Rivero et al 2012), no studies have yet explored ecosystem-wide consequences of increased sponge abundance and dominance (Bell et al 2013).…”