Density‐dependent starvation in a vertebrate without significant depletion

Summary 1 It is often difficult to quantify the separate contribution of exploitation and interference competition to the starvation of poor competitors yet, in vertebrates, it seems widely supposed that depletion is normally the mechanism underlying density-dependent starvation. This paper presents arguments that density-dependent starvation occurs through interference in wintering oystercatchers Haematopus ostralegus eating mussels Mytilus edulis without prey depletion having a significant influence. 2 Exclosure experiments and mussel bed surveys showed that oystercatchers (i) depleted mussel numerical density by up to 25% in their most preferred large prey size classes, but by only 12·1% overall; (ii) reduced mean mussel length by 1·5%; but (iii) had no detectable effect on the thickness of the shells and therefore the availability of mussels to the 60% of oystercatchers that opened mussels by breaking the shell. 3 Oystercatcher intake rate did not fall until prey biomass had decreased to very low densities. Depletion was not nearly large enough for mussel biomass to be reduced to this point. 4 The flesh content of individual mussels decreased from autumn to spring by 40–50% but oystercatchers did not increase their rate of foraging to compensate. Intake rate declined over the winter because of deteriorating mussel condition; very little was because of depletion. 5 Over the current range of population size, the density-dependent functions produced by a behaviour-based individual’s model were the same whether depletion was allowed to occur or not, confirming that depletion played no role in the density-dependent starvation. 6 At current population sizes, intake rate was reduced as population density increased only through increased interference, this leading to starvation late in winter when mussel flesh content was low. It was not just the subdominant birds most susceptible to interference that starved, but the least efficient ones as well, even though the density dependence was not due to exploitation competition.

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