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.