Old trees for bats

Bats are New Zealand’s only terrestrial native mammals. There are three species. Greater short-tailed bats are thought to be extinct, while long-tailed bats and the lesser short-tailed bats are classed as threatened. They are in danger of extinction in the medium term if nothing is done to reverse their population declines. Therefore, these species are a high priority for conservation.

Bats were common throughout New Zealand in the 1800s when they were recorded in colonies of “scores”, “hundreds”, and “thousands”, but by 1900-1930 they were becoming scarce in many districts. Bats still occur widely, from Northland south to Stewart Island, but their distribution is patchy, and their numbers very low in many areas.

Multiple threats

New Zealand bats roost in tree hollows and their decline has been attributed to clearance and logging of lowland forests, and cutting of old-age trees for firewood. Bats are also vulnerable to human interference and to predation by introduced animals such as cats, possums, rats, and stoats, and exclusion from roosts by introduced mammals, birds and wasps.

Importance of old-aged forest trees for bats

The majority of New Zealand bats live within areas of native forest. Recent research by the Department of Conservation has revealed that bats actively select the largest and oldest trees in the forest for roosting and breeding. In Fiordland, long-tailed bats shelter and breed in cavities in old-aged trees. 95% of roosts are in mature lowland forest on the valley floor. Individual bats range over 50 square kilometres when feeding at night, but always return to traditional areas within the forest during the day. Within the roosting areas they use clumps of 3-6 trees over a few days, then move to a new clump of trees close by.

Short-tailed bats have similar requirements but usually live deeper within the forest.

Implications of logging

Studies indicate that logging would be detrimental to bats if roosting trees are felled. Large diameter trees, such as those preferred by bats, have been a target for logging in the past. The Resource Management Act (1991), Forests Amendment Act (1993) and Rules of some District Plans require that wildlife populations, particularly populations of threatened species, be sustained at their current levels through logging operations. Any sustainable management systems proposed for use in the future need to leave sufficient trees to ensure bat populations survive. Sustainable Management Plans need to demonstrate that safeguards are in place so that bat populations are not threatened.

Roost trees for short-tailed bats

Short-tailed bats living in mixed beech forest roost in large splits and hollows mainly in large diameter red beech trees 40-160 cm in diameter. Bats in podocarp-hardwood forest show a similar dependence on large diameter trees, including Hall’s totara, rimu, southern rata and miro. Most roosts are in trees greater than 80 cm in diameter, but some are also in smaller trees. These are most often used by solitary bats and groups that require smaller cavities for hibernation. Short-tailed bats may use a particular roost for just a day, or continuously up to 6 weeks, before moving to another tree roost.

back to top

Roost trees for long-tailed bats

Long-tailed bats roost in smaller cavities than short-tailed bats, high up trees (usually 15-20 metres high). Roost trees also have large trunk diameters. 80% of roosts are in trees greater than 80 cm, and up to 250 cm, in diameter. Such trees are usually 200-650 years old. In Fiordland, roosts were in red beech (74%), standing dead trees (21%), silver beech (4%) and mountain beech (1%). Long-tailed bats move to a new roost tree virtually every day and one group can use over 100 different roosting trees. In Northland, long-tailed bats roost in the large kauri trees. In podocarp-hardwood forests roosts have been found in rimu, miro, kahikatea, matai, and totara, from 50-180 cm in diameter.

How to protect roosting trees for bats

1. Identify and protect bat roosting trees. Logging generally targets a significant proportion of trees preferred by bats. Unless roosting patches are identified and protected, there is a high risk of localised tree selection wiping out a population. Bat detectors can be used to determine the presence of bats. The Department of Conservation can help by letting you know if there are bat records in your region.
    
2. Where bats are found, do not disturb them It is safe to watch bats as they emerge from roosts or feed, but sit quietly so they will not be frightened away.
    
3. Protect standing dead trees and old-age trees with cavities Dead trees and old trees with hollows and cavities are still valuable for wildlife. Bats rest by day and breed in cavities in old-aged trees. Bats move to a new roost tree regularly so are not always present at a site.
    
4. Demonstrate in Sustainable Management Plans that sufficient roosting, breeding and feeding habitat remains to sustain bat populations following timber extraction. In addition to protecting existing roosts, remember to provide for the next generations of roost trees to grow old enough to replace existing ones.

back to top