The importance of an underestimated grazer under climate change: how crab density, consumer competition, and physical stress affect salt marsh resilience
Climate change and consumer outbreaks are driving ecosystem collapse worldwide. Although much research has demonstrated that these factors can interact, how heterogeneity in top–down control intensity and physical forcing modulates ecosystem resilience to climate stress remains poorly understood. Here, we explore whether the nocturnal herbivorous crab Sesarma reticulatum can control spatially dominant cordgrass (Spartina alterniflora) growth and how its top–down effects vary with crab density, drought stress, and large-scale disturbance in southeastern US salt marshes. In multiple field experiments and surveys, we show that Sesarma depresses cordgrass growth and that its effects increase in a saturating manner with increasing crab density, such that the highest naturally occurring densities of this consumer can trigger local cordgrass die-off. This top–down effect of Sesarma is similar in magnitude to what is thought to be the dominant grazer in the system, the marsh periwinkle snail Littoraria irrorata. In a drought stress by Sesarma density experiment, we further show that salinity stress and intensive crab herbivory additively suppress cordgrass drought resistance. After drought subsides, surveys and experiments reveal that Sesarma also stifles cordgrass re-growth into existing die-off areas. Together, these results show that multiple grazers powerfully regulate the productivity and drought resilience of these intertidal grasslands and that heterogeneity in physical stress and consumer density can dictate when and where top–down forcing is important. More generally, this work provides a rare, experimental demonstration of the critical role top–down control can play across the initiation and recovery stages of ecosystem die-off.
KeywordsDrought Density dependence Foundation species Spartina alterniflora Top−down control
We thank the Sapelo Island and Guana Tolomato Matanazas National Estuarine Research Reserves and Georgia Coastal Ecosystems LTER for field support and access to study sites, and A. Beyerlain and J. McIntosh for field assistance. Funding was provided by National Science Foundation Division of Environmental Biology EAGER-1546638 award to CA; National Oceanic Atmospheric Administration National Estuarine Research Reserve Graduate Fellowship award to SvM; and National Science Foundation CAREER-1445834 and National Science Foundation OCE-0620959 awards to BRS.
Author contribution statement
CA, SGVM, and BRS conceived and designed the experiments. CA, SGVM, and MJSH performed the experiments. CA and QH analyzed the data. CA wrote the manuscript; other authors provided editorial advice.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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