Safety in numbers for secondary prey populations: an experimental test using egg predation by small mammals in New Zealand

New Zealand's native avifauna is threatened by introduced mammalian predators. Native species are often not the primary prey of these predators, which depend on introduced mice and rabbits as their primary food source. Theoretical models predict that predation risk for a subsidiary, or “secondary” prey species is inversely proportional to its population size. This prediction was tested by a quasi‐natural experiment in which four different sized prey “colonies” were constructed at four existing sooty shearwater breeding sites. Domestic hens' eggs were placed in shearwater burrows immediately following the shearwater breeding season and egg predation rates monitored at five, ten and fifteen days. Treatments were switched between sites and the experiment run for a second time after a two‐week stand‐down period. The net effect of increasing colony size was to lower individual risk of predation. The larger number of individuals present served to effectively “buffer,” or dilute, per‐capita predation risk from predators whose numbers are fixed by extraneous factors: chiefly the abundance of their primary prey. Although eggs were removed more slowly from smaller colonies than from larger ones, each loss had a greater per‐capita effect on individual mortality risk. The inverse density dependent relationship found between colony size and predation risk implies that predator population dynamics are largely independent of secondary prey numbers. Abundant introduced predators can therefore easily drive a small secondary prey population to extinction. Control of primary prey populations may be an important management tool in these circumstances.