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).