Evidence from a Waikato ecosanctuary indicates that mice reduce the number of ground-dwelling invertebrates, particularly caterpillars, spiders, wētā and beetles, researchers say.
Their work also showed that earthworms could become more abundant if there were no mice in an area.
The researchers, mainly from Manaaki Whenua – Landcare Research, said it was difficult to prevent house mice from entering ecosanctuaries on the mainland. Mice were the only exotic mammals living in some fenced reserves and reached high population densities.
While biodiversity has increased dramatically when all mammal pests except mice have been removed from an area, mice can be disastrous in ecosanctuaries focused on invertebrate or lizard recovery, they said. in a report published in the New Zealand Journal of Ecology.
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It was expected that the means of controlling mice would steadily improve, so that in the future they might be eradicated and excluded from forest reserves.
Researchers studied the impact of mice on biodiversity for five years at Sanctuary Mountain – Maungatautari, south of Cambridge.
They managed two fenced sites independently. When the study began, one 17-hectare site had high numbers of mice of up to 46 per hectare, while mice were undetectable at the other 24-ha site.
Halfway through, the arrangement was reversed, with mice being eradicated from the first site, while being allowed to increase to the second.
Previous studies in New Zealand had shown that mice ate a range of small invertebrates (3-12mm long) and plant material, according to the study. Caterpillars were often the most common invertebrate group eaten by mice in forests, followed by spiders, beetles, and wētā.
During the project, 42,639 invertebrates were captured in pitfall traps. There was strong statistical evidence that the number of invertebrates that were part of the mice’s preferred diet was lower at both sites when the number of mice was higher, according to the study.
When the number of mice was higher, strong statistical evidence showed a lower number of invertebrates being part of the mice’s preferred diet, according to the study.
The pattern was similar for total invertebrates and for spiders, beetles, wētā, and beetle species.
The study also used tracking tunnels to get an indication of wētā density. Tracking rates of adult wētā trees from Auckland and other wētā were found to be higher when mouse density was lower.
“Wētā are not only in the preferred prey size range of mice, but are also mobile and may be particularly attractive to mice because their movements are easily detected,” the study states.
But while wētā may become more abundant after the eradication of mammals, their behavior may also change. For example, wētā might start spending more time on the ground.
In the block that started with a high number of mice, earthworm biomass, abundance, and diversity increased once the mouse density was reduced, but there was no significant change in the block. which started with a low density of mice.
This suggests that earthworm populations are in the early stages of recovery after years of sustained predation by mice, according to the study.
An unexpected and unwelcome observation in the block that started with high numbers of mice was the apparently faster recovery of abundance and biomass by invasive earthworm species, compared to native species.
The researchers said the impacts of mice on invertebrates and birds at Maungatautari were likely small compared to the combined impacts of large predatory mammals and browsers/tramplers that had been successfully eliminated.
These included stoats, sea rats and Norway rats, brush-tailed opossums, hedgehogs, wild cats, feral goats, red deer and pigs.
“Abundant mice may be more damaging to lizards that regularly use small crevices as refuges, because smaller mice can access spaces inaccessible to larger rodents, mustelids and cats,” the study says.
He also noted that if mice could be eradicated, and if bird and lizard populations that had been destroyed could be at least partially restored, the pressure on invertebrates might actually increase.
“The primary goal of restoration in New Zealand sanctuaries is to restore pre-human ecological interactions and processes as much as possible, not to increase the abundance of all taxa,” the study states.