Selectivity and recovery of seagrass from herbivory
In addition to direct impacts of temperature on P. oceanica, high rates of grazing by herbivores on cool-warm transplants highlights the importance of indirect impacts of warming on ecosystem function. The selective patterns of grazing at warm-edge sites were consistent with the contrasting patterns of nutritional quality of transplants from different locations. These results are consistent with previous studies that have demonstrated higher rates of herbivory on nutrient enriched plants (Prado & Heck Jr 2011; Campbell et al. 2018). Interestingly, increased grazing on nutrient-rich seagrass was only observed in warm-edge, not central sites, where low nutrient meadows were also present. This pattern, in conjunction with feeding scars suggests that warm-affiliated fishes and sea turtles were likely responsible for the observed over-grazing. Tropical range extending fishes such as the rabbitfish Siganus rivulatus and S. luridus are the dominant herbivores in Cyprus (Santana-Garcon et al. personal communication) and while currently absent from Spain, are expected to establish in the coming decades (Daniel et al. 2009). Herbivore pressure by native cool-affiliated species is already high in the western Mediterranean (Pagès et al. 2012) and therefore the cumulative effects of cool and warm water herbivores on nutrient rich seagrass in the western Mediterranean needs to be assessed and monitored carefully.