The Demography of an Island and Mainland Population of House Mice in the Marlborough Sounds, New Zealand

Abstract

The main aim of this study was to compare the demography of an island and mainland population of mice in New Zealand forests by both live- and snap-trapping. The island was free of mammalian predators of mice, the mainland had both feral cats and mustelids. In the first year of study, the mainland population was at a low density (about 5 mice/ha) which was maintained year round. The density of mice on the island was higher, peaking at 17/ha. The island population was not stable year round, numbers being lowest in winter/spring and highest in autumn. Mice on the mainland had a longer breeding season and became sexually mature earlier than island mice but both had similar litter sizes. There was an older age structure of mice on the island than on the mainland. Unlike island mice, most mainland females bred in the season of their birth. Juvenile survival was low in the early part of the breeding season in both island and mainland areas. Out of the breeding season there was a delay in sexual maturation for both island and mainland mice; island mice also showed a decrease in body measurements for a given age and high mortality. The second main aim of the study was to determine what part the availability of food played in regulating mouse populations in the wild. On the island, supplementary feeding with wheat led to increased density, enhanced juvenile survival at the start of the breeding season, decreased mortality over winter and allowed some breeding in winter. On the mainland, supplementary feeding did not lead to an increased density of mice. Subsequent experiments in the laboratory showed that although mice could survive on a wheat-only diet, it was not of sufficient quality to breed on. Analysis of the wheat suggested that it did not contain sufficient protein or calcium to allow breeding. Changes in density and breeding of mice in a beech forest on the mainland were monitored following the natural increase in food availability during a beech mast year. Mice bred during winter and increased in density. Beech seed was analysed and found to be a very nutritious food source. Following the conclusion of the field experiments wild-caught mice were bred in the laboratory to compare breeding by island and mainland mice under controlled conditions. These experiments were inconclusive as island mice failed to breed. One possible explanation for this was the presence of a parasite, the bile duct tapeworm Vampirolepis straminea (= Hymenolepis microstoma), in most of the mice from the island. This possibility was investigated by infecting mice and comparing breeding with uninfected controls. Infected mice showed a significant delay between first and second litters compared to controls. Finally an attempt is made to integrate the results of this study with those of others; factors affecting population density (such as reproduction, food supply, predation, dispersal and parasitism) are reviewed and discussed.