Joint control by rodent herbivory and nutrient availability of plant diversity in a salt marsh-salty steppe transition zone

Questions: Do current models that predict shifting effects of herbivores on plant diversity with varying nutrient conditions apply to stressful systems like salt marshes? Do herbivores affect different components of the diversity as nutrient availability varies? Location: Salt marsh–salty steppe transition zone at the SW Atlantic Mar Chiquita coastal lagoon (37°44′52″S, 57°26′6″W), Argentina. Methods: We experimentally evaluated the separate and interactive effect of nutrients and rodent (Cavia aperea) herbivory, using exclosures and applying fertilizer (mostly nitrogen), following a factorial design in 50 cm × 50 cm plots. Results: We found a negative effect of herbivory on diversity in the resource-poor scenario (due to a reduction in species richness), but a positive effect when nutrients were added, by reducing the abundance of the dominant plant (and hence increasing evenness). Conclusions: Our experimental results contribute to the limited factorial evidence evaluating the role of nutrients and herbivory on the diversity of terrestrial plant communities, even in highly stressful environments like salt marsh–salty steppe transition zones. Our results also support the model that predicts negative effects of herbivores on plant diversity in low-nutrient conditions and positive effects in nutrient-enriched scenarios, and also support the mechanism assumed to act in these situations.