How the litter-feeding bioturbator Orchestia gammarellus promotes late-successional saltmarsh vegetation

Traditionally, studies on vegetation succession have focused either on plant-plant interactions or on interactions between plants and their physical environment, for example through organic matter build-up and increased nutrient cycling. These interactions can change conditions for macrodetritivores that feed on plant litter, but their role in vegetation succession is rarely studied. In this paper, we explore whether the bioturbating crustacean macrodetritivore Orchestia gammarellus alters soil conditions in a saltmarsh ecosystem in such a way that it promotes late-successional, less stress-tolerant plant species at the expense of early successional species. To answer this, we performed a field and a laboratory experiment in which we manipulated abundances of O. gammarellus, and studied the consequences for soil physical and chemical parameters and for vegetation community composition. Our field experiment showed that O. gammarellus stimulated nitrogen mineralization, likely resulting from the positive effect of this macrodetritivore on soil aeration and litter decomposition. Moreover, results from the laboratory experiment showed that O. gammarellus negatively affected dicot seedling survival of mainly early successional plant species, likely through grazing, thus affecting plant community composition. The experiments together provided evidence that O. gammarellus promotes late-successional plant species in multiple ways: by alleviation of anoxic conditions, by promoting nutrient cycling and by selective herbivory on early successional species. Synthesis. By demonstrating that a species traditionally considered as part of the detrital ('brown') food web is thus an important accelerator of vegetation succession, this study documents an important but often overlooked link in food web and ecosystem ecology. By demonstrating that a species traditionally considered as part of the detrital ('brown') food web is thus an important accelerator of vegetation succession, this study documents an important but often overlooked link in food web and ecosystem ecology.