Macrophyte species richness improves resilience to grazing

Abstract

Grazing pressure is increasing in the northern Gulf of Mexico due to tropicalization (i.e. range expansion of tropical species) and conservation efforts. Populations of herbivorous parrotfish, green turtles and manatees are all increasing in this region, and overgrazing can lead to loss of foundation species and their associated ecosystem services. With environmental conditions changing rapidly and seagrass abundance declining in many regions globally, understanding mechanisms that promote seagrass resilience to stressors (e.g. grazing) will be crucial for managing and restoring seagrass meadows and the valuable ecosystem services they provide. Diverse communities often exhibit higher resilience due to positive interactions and increased response diversity (i.e. differing reactions to environmental change), indicating that diversity may provide a tool for enhancing seagrass resilience. To investigate how macrophyte species richness influences resilience to increased grazing pressure, we simulated green turtle grazing events every 2 weeks for 8 weeks in plots that naturally varied in species richness. We recorded macrophyte metrics between the simulated grazing events and after a longer 4‐week recovery period. Both species identity and species richness impacted response to simulated herbivory. Four weeks after the last simulated grazing event, plots with higher species richness had recovered macrophyte shoot density better than plots with lower species richness, indicating that species richness may increase resilience. The grazing events had the strongest negative impact on the persistent species (i.e. longer‐lived species with slower shoot turnover), Thalassia testudinum, reducing T. testudinum density and dominance and indicating that areas dominated by this species may be the most negatively impacted by increased grazing pressure. Synthesis: Our results showed a positive impact of species richness on resilience and indicated that persistent seagrass species may have reduced dominance following repeated disturbances. Diverse seagrass beds can, therefore, improve ecosystem stability by allowing opportunistic species to replace species that are most negatively impacted by disturbances. These findings can be used to incorporate resilience into ecosystem management and identify seagrass beds that may be less resilient to changing environmental conditions.