Data from: Natural selection by pulsed predation: survival of the thickest

Selective predation can lead to natural selection in prey populations and
may alleviate competition among surviving individuals. The processes of
selection and competition can have substantial effects on prey population
dynamics, but are rarely studied simultaneously. Moreover, field studies
of predator-induced short-term selection pressures on prey populations are
scarce. Here we report measurements of density dependence in body
composition in a bivalve prey (edible cockle, Cerastoderma edule) during
bouts of intense predation by an avian predator (red knot, Calidris
canutus). We measured densities, patchiness, morphology, and body
composition (shell and flesh mass) of cockles in a quasi-experimental
setting, i.e. before and after predation in three similar plots of 1 ha
each, two of which experienced predation, and one of which remained
unvisited in the course of the short study period and served as a
reference. An individual's shell and flesh mass declined with cockle
density (negative density dependence). Before predation, cockles were
patchily distributed. After predation, during which densities were reduced
by 78% (from 232 m-2 to 50 m-2), the patchiness was substantially reduced,
i.e. the spatial distribution was homogenized. Red knots selected juvenile
cockles with an average length of 6.9 mm (SD 1.0). Cockles surviving
predation had heavier shells than before predation (an increase of 21.5
percentage points), but similar flesh masses. By contrast, in the
reference plot shell mass did not differ statistically between initial and
final sampling occasions, while flesh mass was larger (an increase of 13.2
percentage points). In this field-study, we show that red knots imposed a
strong selection pressure on cockles to grow fast with thick shells and
little flesh mass, with selection gradients among the highest reported in
the literature.