Selective herbivory in transplants with higher nutritional quality
Ten days after initial deployment, transplants were surveyed revealing selective overgrazing on cool-warm P. oceanica (Fig. 2). Shoot lengths of cool-warm transplants were reduced by 74.5 ± 4% across both transplant sites in Cyprus, from 22.3 ± 3 cm to 5.7 ± 9 cm. In contrast, centre-warm and warm-warm plants that were interspersed with cool-warm plants, lost significantly less biomass, from 25 ± 3% and 27.9 ± 5 % of initial length, respectively (ANOVA, F(2,194) = 37.64, p<0.001, Fig. 2A). Selective grazing patterns were consistent between two warm-edge sites separated by 1.6 km. No other transplants in cool-edge or central locations were over-grazed by herbivores. Selectivity patterns in grazing rates reflected differences in nutritional quality between seagrass populations and regional differences in herbivore assemblages. C:N of cool-edge P. oceanica leaves were 18.1 ± 1.4 (mean ± SE), significantly lower (i.e. higher nutritional quality) at the start of the experiment than C:N ratios for central 34.7 ± 1.4 or warm-edge 52.3 ± 1.8 populations (ANOVA, F(2,8) = 97.76, p <0.001, Fig. 2B).
By week six, significant differences in shoot length were still observed between treatments (ANOVA, F(2,245) = 87.69, p<0.001), but difference between treatments was reduced (Fig. 2A). Similarly, C:N in leaf tissue of the transplants started to equilibrate with local conditions. By the recovery period, no significant differences in C:N (p = 0.83) and no differences in herbivory impacts (i.e. shoot length, p = 0.79) were detectable between source populations in warm-edge sites (Fig. 2). The morphology of the bite scars observed on the consumed seagrass in Cyprus suggested that herbivorous fishes (Siganus rivulatus, S. luridus, Sparisoma cretense ) and potentially sea turtles (Chelonia mydas ) were responsible for the intense grazing rates, although relative contributions of each species could not be quantified (Fig. S2).