Background
This measure relates to indicator 1.4.2 – Security of threatened and at risk taxa.
The lesser short-tailed bat is in a family unique to Aotearoa New Zealand. These pekapeka / bats are under threat from predation, forest degradation and loss, and potentially (because they eat insects and fruit on the forest floor) by toxins used to manage predators in their habitat. The northern subspecies is classified as ‘Nationally Vulnerable’, the central subspecies as ‘Declining’ and the southern subspecies as ‘Recovering’ under the New Zealand Threat Classification System. DOC monitors lesser short-tailed bats to measure their survival in relation to predator management.
Lesser short-tailed bats have high survival with predator management.
What did we measure?
DOC has been monitoring the annual survival of southern lesser short-tailed bats in the Eglinton Valley, Fiordland since 2006 and central lesser short-tailed bats in Pureora Forest, King Country since 2012 (Figure 1). These sites have different forest types, patterns of predator abundance, and histories of predator management1.
DOC staff catch adult lesser short-tailed bats in mist nets and attach transmitters to track females to maternity roosts. Lesser short-tailed bats use several maternity roosts each year. When a roost is located, a harp trap is set outside, so that staff can catch and mark some of the colony (approximately 200 pekapeka / bats each year) with passive integrated transponders (PIT). Antennae are placed around the maternity roost entrances to detect marked pekapeka / bats as they come and go. Annual survival is estimated based on the number of marked individuals that are re-detected in following years. Study of other pekapeka / bat species show average adult female survival rates above 0.79 per year result in population growth (Pryde et al., 2005).
What did we find?
- Survival of southern lesser short-tailed bats in the Eglinton Valley has been high since monitoring began, except for 2007 when beech tree masting caused a rat irruption that was not adequately reduced by predator control over a small area. Adult female survival rates were above 0.79 for four of five subsequent mast events (Figure 2).
- Survival of central lesser short-tailed bats in Pureora Forest was also high, with adult female survival rates estimated to be over 0.79 in all years except 2020 (Figure 3).
- Diphacinone and pindone were found in guano (bat poo) from Pureora Forest when toxins were used for nine or three months. No toxins were found in guano in 2018 or 2019, although pindone residue was found in a dead pekapeka / bat in 2018 and another in 2019. There was no evidence pindone was the cause of death.
- The risk of toxin exposure should be balanced with how effectively our management tools reduce rat numbers and allow pekapeka / bat populations to recover. Results in Pureora Forest suggest that low use of pindone, with periodic aerial 1080 operations, may be an effective combination for managing this population.
Figure 1: Locations of two monitored populations of lesser short-tailed bats.The Eglinton Valley is mostly beech forest, and predator numbers are driven by periodic pulses of abundant food from beech mast seeding. Pureora is a diverse podocarp and hardwood forest and predator numbers are consistently high.
Figure 2: Survival of adult female southern lesser short-tailed bats in the Eglinton Valley calculated using RMark. Bars indicate the occurence of beech mast and predator management in each year. Values are means ± 95% confidence intervals.
Figure 3: Survival of adult female central lesser short-tailed bats in Pureora Forest calculated using RMark. Bars indicate the predator management tools (toxin and delivery method) used each year. Values are means ± 95% confidence intervals.
Data quality
This measure complies with the data quality guidelines used in New Zealand’s Environmental Reporting series.
Adult survival estimates are accurate and reliable because capture histories have been collected for many individual pekapeka / bats over a long period and recapture rates are high. Methods have been published in peer reviewed journals and use standard analyses for this type of data. However, because it reports on only two populations, this factsheet is only partly related to the national indicator.
Survival for the most recent season cannot be verified until next year.
Glossary of terms
95% confidence interval is the range of values that have a 95% likelihood of containing the true value.
Mast seeding is the synchronous production of large quantities of seeds within a population of plants at irregular intervals. This occurs in a number of New Zealand tree and tussock grass species.
RMark is an interface to the software package MARK developed by Laake (2013). MARK was developed by Gary C. White to derive parameter estimates from animals that are marked and then re-encountered at a later time.
Survival is the proportion of a population that remains alive over time. Female survival is a fundamental demographic parameter and, together with estimates of reproduction and dispersal, shows whether a population is increasing, decreasing or stable. Due to natural mortality, even a healthy population will not have 100% survival, but this will be balanced by recruitment.
Additional resources
In the Eglinton Valley, a combination of racumin and diphacinone in bait stations was used over a small area (900 ha) in 2007. Pindone in bait stations was applied in 2010 and 2012, with aerial 1080 operations over areas up to 26,000 ha in 2015, 2017, 2019, and 2020. Since 2018, rat populations have not been suppressed as effectively as in earlier 1080 operations, so bait stations were also used for two months in spring. In Pureora Forest, diphacinone cereal pellets were used in 2013 over three months. Since then, pindone pellets have been used; initially for nine months per year, then dropping to two months and, since 2017, only for five weeks in spring, just before pekapeka / bats give birth. Aerial 1080 operations occurred in Pureora Forest in 2016 and 2022.↩︎