Rabbit predators in the semi-arid high country of the South Island of New Zealand

Abstract

The life history, distribution, abundance and diet of ferrets, feral cats and harriers was examined in a live-trapping study to determine these predators' impact on rabbit populations in the highly rabbit prone semi-arid high country of New Zealand's South Island, and to assess their role as regulators of the rabbit population.

In aspects of their life history (including weight, sex ratio, survival, dispersal and breeding season) cats and ferrets were essentially similar to those found in other areas of New Zealand.

The distribution of all three species was closely linked to rabbit abundance. Harrier and ferret population fluctuations lagged 1-2 months behind rabbit population fluctuations. Cats lagged rabbits by 3-4 months. These seasonal changes were due to recruitment to the predator populations in summer and autumn, and mortality in winter and spring. Multivariate regression models were built to predict predator numbers. Thirty-two per cent of the variation in ferret numbers was explained by season, rabbit density and altitude effects. Cat variation (38%) was explained by a model incorporating seasonal effects, sward and hare abundance. Seasonal effects and rabbit abundance explained 17% of the variation in harrier numbers. Ferret and harrier abundance were both significantly positively correlated with rabbit abundance, in a manner suggestive of a Type II numerical response.

Ferret diet was dominated by rabbit, with much smaller contributions from lizards, sheep carrion, birds, invertebrates, mice and hedgehogs. When young rabbits were available they were eaten in high proportions. The amount of rabbit in the diet varied seasonally, with a decrease in April and June compensated for by increased predation of lizards and other alternative prey types. The contribution of rabbit to ferret diet (91 % by weight) was similar across all rabbit densities measured.

Trapping results and ferret diet information were combined with values from the literature, and some "best guesses", to obtain an approximate model of the total predator response to changing rabbit density. The result appeared to be Type II (inversely density dependent), indicating that while predators may limit rabbits in the study area, they do not appear to regulate them. However, the only way to conclusively prove that predators regulate their prey is with experiments in which predator and prey populations are manipulated.