Survival rates for short-tailed bats at two managed sites

This measure relates to indicator 1.4.2 – Security of threatened and at risk taxa.

Background

The lesser short-tailed bat belongs to a family that is only found in Aotearoa New Zealand. The northern subspecies is classified as ‘Nationally Vulnerable’, the central subspecies is classified as ‘Declining’ and the southern subspecies is classified as ‘Recovering’ under the New Zealand Threat Classification System. Threats to these bats include not only predation, forest clearance and land development, but potentially also toxins used to manage predators in the bats’ habitat (because they feed on insects and fruit in the leaf litter). DOC has been monitoring short-tailed bats in the Eglinton Valley, Fiordland and at Pureora in the 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 estimated 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 composition and patterns of predator abundance. The Eglinton Valley is mostly beech forest, and predator numbers are driven by periodic pulses of high food supply related to beech mast seeding. Pureora Forest has a diverse mixture of podocarp and hardwood species and consistently high predator numbers.

Predator management also differs between the two sites. In the Eglinton Valley, aerial 1080 has been used to control predator irruptions in mast events since 2015, with pindone in bait stations also used in the last 2 years. In Pureora Forest, diphacinone cereal pellets were used in 2013 and pindone pellets have been used since, at first for 9 months per year, then dropping to 3 months and recently (2017-2020) only in September, just before the bat breeding season. There was also an aerial 1080 operation in Pureora Forest in 2016/17.

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 maternity 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 higher than 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 in all monitored years, except 2007 when beech tree masting in 2006/07 caused a rat irruption that was not adequately controlled by intensive predator control over 900 ha. In subsequent mast events, predator numbers were controlled over much larger areas (>3,500 ha) and survival rates were above 0.79 for four of the five events (Figure 2).
  
• 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.
• Survival rates for adult females in Pureora Forest have remained high for the last three years suggesting that one month of pindone, with periodic aerial 1080, may be an effective and efficient combination for future management of this population.

Maps

Figure 1: Locations of the two monitored populations of lesser short-tailed bats.

Figures

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.

Another season of monitoring is required to calculate bat survival after the 2019/20 beech mast in the Eglinton Valley.

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.