Rodent-mediated secondary succession in tidal marshes

Although both herbivory and environmental stress shape the trajectory of secondary succession in plant communities, these factors are generally studied individually. Here, we examine the single and interactive effects of herbivory and stress on successional patterns in tidal marsh plant assemblages. We investigated the effects of rodent herbivory on plant succession dynamics in marshes that varied in salinity (oligohaline, mesohaline, euhaline) in Narragansett Bay, RI. Herbivory reduced the pace of recovery in lower salinity marshes by maintaining greater bare space over time, whereas herbivore treatment did not significantly affect bare space in the high salinity marsh. Additionally, herbivores had a powerful effect on species composition in low salinity marshes. In rodent-accessible plots, preferred food plant species (e.g. Salicornia, Solidago, Phragmites, Typha, and Scirpus spp.) were kept in low abundance, while herbivore-resistant species (e.g. Pluchea purpurescens) were more common, benefiting from competitor removal by herbivores. We conclude that herbivores play a large role in shaping secondary successional spaces in low salinity tidal marshes, while stress is the dominant factor determining successional trajectories in high salinity marshes, supporting models that predict biotic interactions will be more important in shaping communities in less stressful environments. This work generates predictions of how communities will respond to disturbance under modified conditions of environmental stress (e.g. drought years) and consumer abundance (e.g. over-hunted conditions) dependent on the principal community-regulating factors of that habitat.