Invasive mammals disrupt food webs in many natural habitats by destroying vegetation, preying on native species, and changing species interactions. Many ecosystems that depend on species interactions to maintain diversity, functioning, and ecosystem resilience in the face of stressors could be especially vulnerable to the pervasive disturbance of these invaders. Here, we use drone surveys and a series of manipulative field experiments to determine how a well-known large invasive consumer, the feral hog Sus scrofa, affects large scale patterns within southeastern US salt marsh ecosystems dependent on positive species interactions (i.e. a cordgrass-mussel mutualism, positive cordgrass density dependence) for resilience and recovery from semi-regular drought events that convert grassy marshes to bare mud flats.
Results/Conclusions
A drone survey of 10 marshes in Georgia and Florida found that marsh landscapes associated with high hog activity were characterized by distinctive patchiness. Marshes with hogs had about twice as many, and half as large, recovering patches of grass than marshes without hogs. To test if hog activity can cause this pattern, we conducted a 4-year hog exclusion on 40 post-drought recovering cordgrass patches in two coastal Georgia marshes. We found that uncaged patches recovered into surrounding mudflat six times slower than caged patches as hogs tended to trample grass on the edge of patches as they travel through the marsh. As recovery in Southeastern marshes is often dependent on a cordgrass-mussel mutualism, we evaluated whether hogs affected this key mechanism by conducting a 4-year hog exclusion by mussel addition experiment. In the presence of hogs, positive interactions between grass and mussels were reversed: patches of grass associated with mussels were preferentially targeted and completely destroyed by hogs while the density of mussel mound community residents (i.e. bioturbating crabs) totally collapsed. Additionally, a mussel mound transplant experiment revealed that the presence of cordgrass increases mortality rate of mussels when hogs are present, as hogs are much less likely to access mussel mounds without the sediment stability provided by live cordgrass root structures. Our results show that invasive megaconsumers can reshape whole coastal ecosystems by disrupting and reversing key positive interactions in habitats subjected to regular stressors. Ecosystems like salt marshes that depend on positive interactions for recovery from increasingly regular large scale stressors may expect resilience to weaken faster and recovery times to be slower without management of invasive species.