Predators Shape Sedimentary Organic Carbon Storage in a Coral Reef Ecosystem

Trisha B Atwood,Elizabeth M P Madin,Chris M Roelfsema,Edd Hammill,Quinn R Ollivier,Catherine E Lovelock,Peter I Macreadie,Osmar J Luiz,Alastair R Harborne

doi:10.3389/fevo.2018.00110

Abstract

Trophic cascade theory predicts that predator effects should extend to influence carbon cycling in ecosystems. Yet, there has been little empirical evidence in natural ecosystems to support this hypothesis. Here, we use a naturally-occurring trophic cascade to provide evidence that predators help protect sedimentary organic carbon stocks in coral reef ecosystems. Our results show that predation risk altered the behavior of herbivorous fish, whereby it constrained grazing to areas close to the refuge of the patch reefs. Macroalgae growing in riskier areas further away from the reef were released from grazing pressure, which subsequently promoted carbon accumulation in the sediments underlying the macroalgal beds. Here we found that carbon stocks furthest away from the reef edge were similar to 24% higher than stocks closest to the reef. Our results indicate that predators and herbivores play an important role in structuring carbon dynamics in a natural marine ecosystem, highlighting the need to conserve natural predator-prey dynamics to help maintain the crucial role of marine sediments in sequestering carbon.

Highlights

IntroductionThere is growing evidence from a wealth of ecosystems that predators, via trophic cascades, play an important and potentially irreplaceable role in carbon (C) cycling (Wilmers et al, 2012; Atwood et al, 2014a,b; Schmitz et al, 2014), and that changes to predator populations can alter CO2 concentrations and emissions from ecosystems (Schindler et al, 1997; Atwood et al, 2013; Hammill et al, 2015), C export from erosion (Coverdale et al, 2014), and C storage in plant biomass (Hawlena et al, 2012; Wilmers et al, 2012; Strickland et al, 2013; Heithaus et al, 2014)
We found that the abundance of herbivores and large bodied predators differed with distance from the reef and between tides
Trophic cascade theory predicts that predator effects should extend to influence C cycling in ecosystems

Summary

Introduction

There is growing evidence from a wealth of ecosystems that predators, via trophic cascades, play an important and potentially irreplaceable role in carbon (C) cycling (Wilmers et al, 2012; Atwood et al, 2014a,b; Schmitz et al, 2014), and that changes to predator populations can alter CO2 concentrations and emissions from ecosystems (Schindler et al, 1997; Atwood et al, 2013; Hammill et al, 2015), C export from erosion (Coverdale et al, 2014), and C storage in plant biomass (Hawlena et al, 2012; Wilmers et al, 2012; Strickland et al, 2013; Heithaus et al, 2014). The influence of predators on one of our most important long-term C storage pools, marine sediments, Marine Predators Alter Carbon Storage has rarely been explored (see Coverdale et al, 2014 on the influence of predators on C loss from salt marsh soils). Marine ecosystems dominated by benthic macrophytes (seagrass, mangroves, salt marsh, and macroalgae) are important reservoirs in the global C cycle. The capacity of macrophyte systems to naturally sequester and store C in their sediments for millennia identifies these habitats as important C sinks (Nelleman et al, 2009; Duarte et al, 2013). It is crucial that we understand the important mechanisms that influence C accumulation and preservation in the sediments of these systems

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Results

Conclusion