Natural selection by pulsed predation: survival of the thickest

Abstract

Abstract. 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 quasiexperimental 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 to 50 cockles/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 6 1.0 mm (mean 6 SD). 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.

Understanding spatial distributions: negative density-dependence in prey causes predators to trade-off prey quantity with quality

“…1.7) which compares well with earlier findings [34]. To calculate e(N ), we assumed that knots fed on cockles of 7 mm long, which is the size that knots preferentially selected in this area the previous year [24]. We multiplied the spatially interpolated measurements of relative AFDM flesh by 1.7 mg (the average AFDM flesh of 7 mm cockles, electronic supplementary material, figure S5a).…”

“…Even though it is highly speculative, knots may optimize energy intake rates by 'gardening' their cockle prey. However, opposite to grazers, predators kill their prey and reduce their density, which thereby become difficult to find [24], which in turn reduces the benefit from such 'gardening'. One way to investigate this 'gardening hypothesis' is to determine whether knots, after thinning cockle densities, allow time for their prey to increase in flesh mass before revisiting these locations [52].…”

Benthic primary producers are key to sustain the Wadden Sea food web: stable carbon isotope analysis at landscape scale

“…, Bijleveld et al. ). Examples of consumers that depend on benthic production and might also exhibit size selectivity include Limosa lapponica (Duijns et al.…”

Food web consequences of an evolutionary arms race: Molluscs subject to crab predation on intertidal mudflats in Oman are unavailable to shorebirds

“…It can be expected that the molluscs at Barr Al Hikman have been and are subject to strong predation pressure, as molluscs will only show costly morphological and behavioural defences when they are exposed to strong predation pressure. This is the case both on an evolutionary time‐scale (Bijleveld, Twietmeyer, Piechocki, van Gils, & Piersma, ; Dietl & Kelley, ) and on the level of individual development (Appleton & Palmer, ; Griffiths & Richardson, ; Zaklan & Ydenberg, ). Several durophagous predators occur in Oman, including crabs, fishes, lobsters, stomatopods, starfish, sea anemones, gastropods and birds (de Fouw et al., ; Khorov, ; Randall, ).…”

“…Depth distributions for P. ceylonica are based on samples collected in 2008 and for P. fischeriana based on samples collected in 2010 (see Methods) exposed to strong predation pressure. This is the case both on an evolutionary time-scale (Bijleveld, Twietmeyer, Piechocki, van Gils, & Piersma, 2015;Dietl & Kelley, 2002) and on the level of individual development (Appleton & Palmer, 1988;Griffiths & Richardson, 2006;Zaklan & Ydenberg, 1997). Several durophagous predators occur in Oman, including crabs, fishes, lobsters, stomatopods, starfish, sea anemones, gastropods and birds (de Fouw et al, 2017;Khorov, 2012;Randall, 1995).…”