Proportional top-down effects of grapsoid crabs on growth of Spartina maritima cordgrass in southern African salt marshes

Abstract

Salt marsh productivity has historically been viewed as being driven primarily by bottom-up processes, but recent studies in North America, Europe, Asia and South America have shown that top-down forces by grazers also structure marsh vegetation. This generality of grazing pressure has not been tested to date in African salt marshes. Here, we investigated whether dominant crabs in South Africa’s estuarine marshes consume live plants and whether that interaction has direct effects on the foundational species Spartina maritima. We employed natural surveys, lab feeding trials, diet analysis and field experiments. Although we found no significant relationships between crabs and marsh plant structure in surveys, gut contents and stable isotope analysis showed that S. maritima is present but not prominent in their diet. All S. maritima components were consumed. Manipulation of crab density and size structure in the field (crabs >5 mm excluded) revealed small effects on S. maritima stem density and aboveground biomass compared to controls. Combined, this research demonstrates that crabs in these South African marshes do indeed eat live cordgrass, and their effects appear to be density dependent. Top-down impacts on marsh plants were not detected by natural density correlations, probably due to the different scales at which data were collected compared to field experiments. These results establish that grazing of live foundational marsh plants is globally common. Future studies in these systems should manipulate crab density through addition experiments or predator exclusions to understand the impact of crabs at high densities and what forces regulate their populations.