Research Profile

The last unwanted guests:
Diet analysis of wild house mouse in the Karori Wildlife Sanctuary

Carlos R. Ochoa, B.Sc.
Tim Markwell, Ph.D. (Supervisor),
School of Biological Sciences
Victoria University of Wellington

Mouse stomach specimen

Although it is almost free of pest mammals, one unwanted rodent still remains within the boundaries of the Karori Wildlife Sanctuary. A mouse (Mus musculus) re-invasion was first detected in February 2000. Because of the lack of mammalian predators within the Sanctuary, the number of mice increased and they are now an obvious and incongruous presence in the Valley.

Mice are known to feed on invertebrates, lizards and plant material; however, birds can occasionally be preyed upon. As well as direct effects, mice can influence ecosystem dynamics by competing with native species for food resources (e.g. native insectivorous birds).

As part of the monitoring regime carried out by KWS staff and volunteers, mice were caught in three different habitats in each of the four seasons. The stomach of each mouse killed was tag-labelled and preserved in 70% ethanol. The contents of these stomachs were analysed to determine the presence or absence of different elements in the mice diet, such as seeds, leaves, invertebrate parts, etc.

Profile Carlos Ochoa, B.Sc. Carlos has a BSc. from the University of Antioquia in Colombia. He has worked on a range of projects in South America including a survey of leatherback turtles, an investigation into herpeto-faunal diversity in rainforest patches and a five-month study of the prey of the jaguar (Panthera onca) in the Colombian Amazonian Trapezium. Carlos is currently working towards a B,Sc. Hons degree at Victoria University of Wellington. On completion of this, he hopes to continue his studies, working on interesting animals in other parts of the world.

A selection of 240 mice kill-trapped during 4 consecutive seasons and 3 different habitats was used in the analysis. Overall, results showed a diet consisting mainly of arthropods (present in 92.9% of the samples), predominantly caterpillars (Lepidoptera larvae, 43.3%), spiders (Araneae, 40%), and maggots (Diptera larvae, 25.7%). Plant material was less important. Although vegetable matter was identified in 47.9% of samples, this was almost exclusively fern scales and sporangia (capsules). Parasitic nematodes were recorded in 14.6% of the stomachs. No remains of vertebrates were found.

The relative rarity of plant material indicates that plants are not an important food resource for KWS mice. Possibly the special conditions in the sanctuary (in particular the lack of other mammalian insectivores, the high abundance of arthropods and the scarcity of seedy grasses and herbs) have allowed mice to specialise on an energy-rich arthropod-based diet. This could impact upon invertebrate numbers, especially on spiders, which have been shown (Berndt, 1998) to occur at low densities in the Valley. Although the mice do not appear to prey directly upon native vertebrates, this specialisation on invertebrate prey may lead to competition with insectivorous birds or lizards when mouse numbers are high.

We know that, in the absence of other mammalian predators, mouse numbers have increased rapidly in the Valley. This study provided a valuable opportunity to investigate the dietary implications of this competitive release. The work will also help clarify the effects the mice are having on the invertebrates, plants and birds of the KWS and potentially aid in any control efforts by increasing our knowledge of mouse ecology.

References

Berndt, L. A. 1998: Aspects of the ecology of ground-active spiders (Araneae) of the Karori Wildlife Sanctuary, Wellington. Unpublished Master of Science in Zoology thesis. Wellington, Victoria University of Wellington.