Mice in ecosanctuaries take a toll on caterpillars, spiders, wētā, beetles and earthworms

Monday, 31 January 2022

The forest can work its magic on your mental state, says Rosie Cahalane, one of the creators of the Nature of Work programme at Waikato's Sanctuary Mountain Maungatautari.

Evidence from a Waikato ecosanctuary indicates mice reduce the numbers of ground-dwelling invertebrates, in particular caterpillars, spiders, wētā and beetles, researchers say.

Their work also showed earthworms could become more abundant if there were no mice in an area.

The researchers, mostly from Manaaki Whenua – Landcare Research, said it was difficult to keep house mice out of mainland ecosanctuaries. Mice were the only exotic mammals living in some fenced reserves, and reached a high population density.

While biodiversity gained substantially when all pest mammals, apart from mice, were removed from an area, mice may be catastrophic in ecosanctuaries that focused on the recovery of invertebrates or lizards, they said in a report published in the New Zealand Journal of Ecology.

Mice are less of a problem than other mammalian predators, but they still have an impact on ecosanctuaries.

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It was expected that ways to control mice would steadily improve, so that in future they could be eradicated and excluded from forest reserves.

The pest-proof fence at Sanctuary Mountain Maungatautari ecosanctuary.

The researchers studied the impact of mice on biodiversity over five years at Sanctuary Mountain – Maungatautari, south of Cambridge.

They managed two independently fenced sites. When the study started, one 17-hectare site had high mouse numbers of up to 46 per hectare, while mice were undetectable at the other, 24ha, site.

Half-way through, the arrangement was reversed, with mice eradicated from the first site, while being allowed to increase at the second.

Previous studies in New Zealand had shown that mice ate a range of small invertebrates (3-12mm long) and plant material, the study said. 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 caught in pitfall traps. There was strong statistical evidence that the number of invertebrates that were part of the preferred diet of mice were lower at both sites when mouse numbers were higher, the study said.

Wētā may be particularly attractive as prey to mic because it’s easy to see them moving.

When mouse numbers were higher, strong statistical evidence showed lower numbers of invertebrates that were part of the preferred mouse diet, the study said.

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 Auckland tree wētā and other wētā were found to be higher when mouse density was lower.

“Wētā are not only within the preferred prey size range of mice but are also mobile and may be particularly attractive to mice because their movements are readily detectable,” the study said.

But while wētā might become more abundant after mammals were eradicated, their behaviour could also change. For example, wētā could start spending more time on the ground.

In the block that started with high mouse numbers, earthworm biomass, abundance and diversity increased once mouse density was reduced, but there was no significant change in the block that started with low mouse density.

That suggested earthworm populations were in the early stages of recovery after years of sustained mouse predation, the study said.

An unexpected and unwelcome observation in the block that started with high mouse numbers was the apparently faster recovery in 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 to be small compared with the combined impacts of the larger mammalian predators and browsers/tramplers that had been successfully removed.

Those included stoats, ship rats and Norway rats, brushtail possums, hedgehogs, feral cats, feral goats, red deer and pigs.

“Abundant mice may be more damaging to lizards that routinely use small crevices as refuges, because the smaller mice can access spaces unavailable to larger rodents, mustelids and cats,” the study said.

It also noted that if mice could be eradicated, and if populations of birds and lizards that had been destroyed could be at least partially restored, pressure on invertebrates could actually increase.

“The primary objective of restoration in New Zealand sanctuaries is to restore prehuman ecological interactions and processes as much as possible, and not to increase the abundances of all taxa,” the study said.