Background
This measure relates to indicator 1.4.2 – Security of threatened and at risk taxa.
Long-tailed bats were common throughout Aotearoa New Zealand in the 1800s but have been declining in many regions since the early 1900s as a result of forest clearance, land development, and predation. Consequently, they are now listed as Nationally Critical under the New Zealand Threat Classification System. DOC has been monitoring bats in the Eglinton Valley, Fiordland, since 1994 to detect changes in their survival and abundance, and relate this to predator management.
Survival of long-tailed bats at Eglinton has changed over time.
What did we measure?
DOC has been monitoring the annual survival of long-tailed bats in two social groups in the Eglinton Valley (Figure 1) for over 20 years. This site is prone to predator irruptions driven by periodic mast seeding of beech forest.
DOC staff catch long-tailed bats using harp traps and mark them with identifying bands before their release. The banded individuals that are recaptured each year are then used to estimate survival and the size of each social group. If a marked animal was not caught one year, but then caught in later years it is included in the minimum number alive (MNA) for the earlier year. Results are modelled in relation to beech mast seeding and predator management using multi-state mark-recapture models.
Predator numbers are monitored with an index of rat abundance derived from tracking tunnels.
What did we find?
- Survival of long-tailed bats in both groups has varied from year to year, but numbers of adult females have increased (Figure 2).
- Before 2010, beech mast seeding was associated with low bat survival, but this was not evident in the 2010, 2012 or 2014 mast events, suggesting that predator management in the Eglinton Valley reduced the impacts of predator irruptions.
- However, during the 2016 and 2019 beech masts, management did not supress rat numbers adequately, or for a long enough period, and survival declined for both social groups of bats, especially for the Walker Creek group after the 2016 beech mast (Figure 3).
- DOC has added more predator management with bait stations since 2017/18, because rat numbers have not been suppressed as effectively as they were by the earlier aerial 1080 operations.
- Long-tailed bat numbers continue to increase, but the rate of increase has slowed.
- DOC is making changes to predator management, with the aim that rats are reduced and bat populations continue to grow.
Figure 1: Location of the two long-tailed bat social groups that are monitored by DOC in the Eglinton Valley, Fiordland.
Figure 2: Abundances of adult female long-tailed bats at Walker Creek and Mackay Creek estimated as the minimum number alive using recapture rates. Coloured bars indicate beech mast and predator management in the preceding season. Values are means ± 95% confidence intervals.
Figure 3: Survival of adult and juvenile female long-tailed bats at Walker Creek and Mackay Creek calculated using RMark. Coloured bars indicate beech mast and predator management in the preceding season. Values are means ± 95% confidence intervals.
Data quality
This measure complies with the data quality guidelines used in New Zealand’s Environmental Reporting framework.
These survival estimates are considered highly accurate because capture histories have been collected for many bats over a long period of time and recapture probabilities are consistently high. Analysis follows widely used methods for this type of data, and the general trend is similar to that modelled using the index of MNA and the recapture rates. However, because data comes from one site, it is only partially relevant to the national indicator.
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.
Minimum number alive (MNA) is the number of individuals caught in a capture session plus those any previously marked that were not caught but were caught in subsequent capture sessions (Krebs, 1966). Here, we model MNA taking into account the estimated recapture probability in each year.
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.
Additional resources
Krebs, C.J., 1966. Demographic changes in fluctuating populations of Microtus californicus. Ecological monographs 36, 239–273.
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.
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.