Trends in survival of short-tailed bats at Eglinton Valley and Pureora Forest Park
Background
This measure relates to indicator 1.4.2 – Security of threatened and at risk taxa.
The lesser short-tailed bat belongs to a family found only in Aotearoa New Zealand. 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. These bats are threatened by predation, forest clearance and land development, and potentially also by toxins used to manage predators in their habitat (because they feed on insects and fruit on the forest floor). DOC has been monitoring short-tailed bats in the Eglinton Valley, Fiordland and in Pureora Forest, King Country to measure changes in their survival in response to predator management.
Short-tailed bats have high survival with predator management.
What did we measure?
DOC has monitored the annual survival of southern lesser short-tailed bats in the Eglinton Valley since 2006 and central lesser short-tailed bats in Pureora Forest since 2012 (see Figure 1 - Maps). The two sites have different forest types and patterns of predator abundance. 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, mixed podocarp and hardwood forest and predator numbers are consistently high.
Predator management also differs between the two sites. In the Eglinton Valley, a combination of racumin and diphacinone was used in bait stations over a small area (900 ha) in 2007. Pindone in bait stations was applied in 2010 and 2012, aerial 1080 in 2015 and 2017, and bait stations again in 2018 and 2019 to control increasing rat numbers. The bait stations were used for 2 months in spring. In Pureora Forest, diphacinone cereal pellets were used in 2013 over three months. Since then, pindone pellets have been used; at first for 9 months per year, then dropping to 2 months and, since 2017, only for 5 weeks in spring, just before the bat breeding season. There was also an aerial 1080 operation in Pureora Forest in 2016
DOC staff catch adult female short-tailed bats in mist nets and attach transmitters so they can be tracked to maternity roosts. Short-tailed bats use several maternity roosts in a season. A harp trap is set outside the roosts to capture and mark a sample of the colony, approximately 200 bats a year at each site, using passive integrated transponders (PIT). Antennae are placed around the maternity roost entrances to detect and log marked bats as they enter and leave. Annual survival can be estimated from the number of marked individuals that are re-detected in following years. Population modelling shows that an average annual adult female survival rate above 0.79 results in a growing population (O’Donnell et al., 2017; Pryde et al., 2005).
What did we find?
- Survival of southern lesser short-tailed bats has been high in the Eglinton Valley since monitoring began, except for 2007 when beech tree masting caused a rat irruption that was not adequately controlled by predator control over a small area. In subsequent mast events, predator numbers were controlled over larger areas (up to 3,500 ha) and adult female survival rates were above 0.79 for three of four events (Figure 2). Juvenile female survival was above 60% for all but three years. Rates of survival after the 2020 mast will be reported next year.
- Survival of central lesser short-tailed bats in Pureora Forest was also high, with the adult female survival rate above 0.81 in all years except 2013 (Figure 3), which suggests an increasing population.Juvenile female survival was above 60% except in 2016 and 2018.
- Diphacinone and pindone were found in guano (bat poo) from Pureora Forest when toxins were used for nine or three months. No toxins have been found in guano since reducing the treatment to five weeks. This has to be balanced with how effectively the treatment reduces rat numbers.
- The high survival rates for adult females in Pureora Forest since 2017, suggest that five weeks of pindone, with periodic aerial 1080, may be an effective combination for future management of this population.
Figure 1: Locations of two monitored populations of lesser short-tailed bats.
Figure 2: Survival of adult and juvenile female southern lesser short-tailed bats in the Eglinton Valley calculated using RMark. Bars indicate the beech mast and management response in the preceding season. Values are means ± 95% confidence intervals.
Figure 3: Survival of adult and juvenile female central lesser short-tailed bats in Pureora Forest calculated using RMark. Bars indicate the management method used each year. Values are means ± 95% confidence intervals.
Data quality
This measure is classified as a case study and complies with the data quality guidelines used in New Zealand’s Environmental Reporting framework.
These survival estimates are considered robust because the capture histories have been collected for many bats over a long period and recapture rates are high.
Survival for the 2020 / 2021 season cannot be verified until 2022.
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 forest 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. It 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
Laake, J.L., 2013. RMark: An R interface for analysis of capture-recapture data with MARK (AFSC Processed Rep. No. 2013-01). Alaska Fisheries Science Centre, NOAA, US Department of Commerce., Seattle, WA.
McGlone, M.S., McNutt, K., Richardson, S.J., Bellingham, P.J., Wright, E.F., 2020. Biodiversity monitoring, ecological integrity, and the design of the New Zealand biodiversity assessment framework. New Zealand Journal of Ecology 44, 3411.
O’Donnell, C.F., Pryde, M.A., van Dam-Bates, P., Elliott, G.P., 2017. Controlling invasive predators enhances the long-term survival of endangered New Zealand long-tailed bats (Chalinolobus tuberculatus): implications for conservation of bats on oceanic islands. Biological Conservation 214, 156–167.
Pryde, M.A., O’Donnell, C.F., Barker, R.J., 2005. Factors influencing survival and long-term population viability of New Zealand long-tailed bats (Chalinolobus tuberculatus): implications for conservation. Biological Conservation 126, 175–185.
Walker, S., Kemp, J.R., Elliott, G.P., Mosen, C.C., Innes, J.G., 2019. Spatial patterns and drivers of invasive rodent dynamics in New Zealand forests. Biological Invasions 21, 1627–1642.