Predators mediate above‐ vs. belowground herbivory in a salt marsh crab

Abstract

Predators can significantly affect prey by removing prey individuals and by changing prey behavior. The tradeoff between foraging behavior and predation risk may result in a trophic cascade that can have important effects on ecosystem processes. For herbivores that can feed both above‐ and belowground, it is likely that predation risk affects the location of feeding. We tested whether two species of predatory marsh crabs affected feeding behavior of the herbivorous crab, Sesarma reticulatum. We found that predatory crabs could kill or injure Sesarma and that Sesarma did less damage to its food plant Spartina alterniflora in the presence of the more dangerous predator. Sesarma prefers to feed on and grows better on belowground rhizomes than aboveground leaves; however, the costs of digging burrows to access rhizomes lead to higher mortality if it is the only diet option. The location of feeding did not affect total biomass of S. alterniflora. For Sesarma, a choice in feeding location allows the crabs the behavioral flexibility to balance the risks of predation, the nutritional benefit of feeding belowground, and the survival costs of belowground feeding. Similar tradeoffs are likely to increase the success of other herbivores that can feed both above‐ and belowground.

Intraspecific diversity at two trophic levels influences plant–herbivore interactions

“…The larger impact of big crabs may result from differences in grazing patterns among size classes. Crabs consume rhizomes opportunistically when they encounter them while burrowing, and belowground biomass in burrows is almost completely consumed (Coverdale et al 2012, Vu and Pennings 2018). Because big crabs have more extensive burrows (A. Noto, personal observation ), they are more likely to encounter and consume rhizomes.…”

“…Because big crabs have more extensive burrows (A. Noto, personal observation ), they are more likely to encounter and consume rhizomes. In addition, feeding belowground uses more energy than feeding aboveground as it requires crabs to dig burrows (Vu and Pennings 2018), and big crabs have greater energy reserves than small crabs. Belowground disturbance can also lead to an increase in sexual reproduction in clonal plants (Xiao et al 2015), consistent with the observed effects of big crabs on Spartina in our field study.…”

Consumer impacts on ecosystem functions in coastal wetlands: The data gap

“…, Vu and Pennings ). Experimentation in salt marshes of the southern United States has shown that predators alter the feeding behavior of the herbivorous purple marsh crab ( Sesarma reticulatum ) with implications for S. reticulatum fitness and S. alterniflora productivity (Vu and Pennings ). In the absence of predators, S. reticulatum reduced aboveground biomass by 66–81% and belowground biomass by 35–67%.…”

“…Further, studies that evaluate consumer effects often only consider one trophic level (He and Silliman 2016), yet emerging evidence suggests that predator-prey dynamics can have both consumptive and nonconsumptive effects on coastal wetland vegetation growth (Coverdale et al 2013, Vu andPennings 2018). Experimentation in salt marshes of the southern United States has shown that predators alter the feeding behavior of the herbivorous purple marsh crab (Sesarma reticulatum) with implications for S. reticulatum fitness and S. alterniflora productivity (Vu and Pennings 2018). In the absence of predators, S. reticulatum reduced aboveground biomass by 66-81% and belowground biomass by 35-67%.…”

“…In the absence of predators, S. reticulatum reduced aboveground biomass by 66–81% and belowground biomass by 35–67%. However, when predators were present, there was a threefold increase in aboveground biomass due to predator avoidance behavior and a decrease in S. reticulatum functional density (Vu and Pennings ). Studies such as these highlight the importance of considering trophic interactions in the maintenance of wetland ecosystem properties and functions, but such evaluations are currently few and far between.…”