Carnivore-Mouse Predation: An Example of its Intensity and Bioenergetics

Abstract

Populations of Microtus, Reithrodontomys and Mus reached a peak in Tilden Park, California, in June 1961, and reproduction almost ceased until the following spring. The impact of carnivores (feral cats, gray foxes, raccoons and skunks) on this standing crop of mice was measured by analysis of carnivore droppings systematically recovered from a 35-acre study area. Eighty-eight per cent of the 4,400 Microtus, 33% of the 1,200 Reithrodontomys and 7% of the 7,000 Mus were eaten by the carnivores before the next spring. As Microtus became scarce, the carnivore diet included more Reithrodontomys, Mus, gophers and finally wood rats. Numerous carnivores continued to hunt mice on the area even after the mice had been practically exterminated. At the beginning of the study the standing crop per acre, in kilocalories, of various components of the food chain was: roots, 7.3 million; hay (excluding seeds), 8.1 million; seeds, 1.9 million; Microtus, 6,402; Mus, 4,543; Reithrodontomys, 434; and carnivores, 650. The annual caloric requirements of the peak populations (per acre) would have been: Microtus, 1.4 million; Mus, 876,000; Reithrodontomys, 82,000; and carnivores, 11,700. The peak population of mice could not have survived for one year on the available seed crop. The carnivores ate 55% of the calories available as mice and could have survived a full year if all mice had remained available to them, but by the end of December so many mice had been lost to other agents of mortality and by emigration that only a 12-day supply remained. In spite of the initial high density of mice, 7% of the seed crop escaped destruction and grew to form a rich vegetation in the following season.