UC San Diego

Disturbance plays a significant role in structuring a variety of habitats, but in seagrass beds disturbance by animals may be more ecologically important than disturbance by abiotic factors. However, not all animal disturbances have identical impacts on seagrass beds and humans have increased the frequency of some animal impacts and decreased the frequency of others. This research investigated the impacts of disturbance by sea turtles (leaf-eating megaherbivores), manatees (leaf-and-root-eating megaherbivores), stingrays (bioturbators), and fish and urchins (macroherbivores) on seagrass beds in Bocas del Toro, Panamá. Repeated simulated sea turtle grazing was found to reduce percent cover and productivity of the late successional seagrass Thalassia testudinum, suggesting that seagrass beds in Bocas del Toro can no longer tolerate pre-Colombian levels of turtle grazing, likely due to increased sedimentation as a result of terrestrial deforestation. Artificially grazed plots were also vulnerable to subsequent bioturbation by stingrays. One-time simulated manatee grazing was not as attractive to stingrays although still did facilitate stingray bioturbation in comparison to undisturbed seagrass. T. testudinum in this study recovered quickly and with no colonization of experimentally created bare patches by species other than T. testudinum. High nutrient availability in Bocas del Toro may allow T. testudinum to colonize sediment gaps without prior facilitation by other organisms. Other organisms’ failure to colonize bare patches created due to simulated turtle or manatee grazing suggests they are not primarily limited by competition with T. testudinum. A third experiment was conducted to investigate whether the early successional seagrass, Syringodium filiforme, is excluded from monospecific beds of T. testudinum by runoff regimes or macroherbivores. Results indicated that S. filiforme only thrives in Bocas del Toro where the impact of runoff is limited and urchins in the genus Echinometra are not abundant. Echinometra is not usually thought to be a seagrass-associated species and may be colonizing this habitat as a result of overfishing of its predators. Human impacts in Bocas del Toro have reduced the ecological impacts of disturbances by megaherbivores, increased the impacts of one genus of macroherbivore, and made seagrass less resilient to repeated disturbance. Human impacts on disturbance by bioturbators are less clear and require further study.