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 declined 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. Predators, like rats, are a major threat to Long-tailed bats. Since 1994, DOC has monitored pekapeka/bats in the Eglinton Valley, Fiordland, to track population changes in relation 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. Monitoring takes place in December, and shows changes to the population over the last year.
DOC staff catch long-tailed bats using harp traps and mark them with identifying bands before releasing them. 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 for a time, but re-caught in later years, it is included in the minimum number alive (MNA) for all intervening years. Results are modeled 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?
- Long-tailed bat numbers have increased since the 1990s, but the rate of increase has slowed in recent years (Figure 2).
- Pekapeka / bat survival has varied from year to year.
- Predator irruptions have a lasting effect. Pekapeka / bat survival can be partly explained by whether there was beech mast seeding, and whether rats were managed during the mast, in both the year of monitoring and the previous year.
- Management can make a difference. Before 2009, beech mast seeding was associated with low pekapeka / bat survival, but this was not seen in the 2009, 2011 or 2014 masts, suggesting that predator management effectively reduced the impact of rat irruptions.
- However, in the 2016 and 2019 beech masts, rat populations were not reduced enough, or recovered too quickly, and survival declined, especially for the Walker Creek group after the 2016 beech mast (Figure 3).
- DOC is making changes to predator management, with the aim that rats are reduced and pekapeka / bat populations continue to grow. Since 2017, DOC has done more predator management with bait stations.
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 series.
These survival estimates are considered highly accurate because capture histories have been collected for many pekapeka / 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 estimated with the index of MNA and recapture rates. However, because data comes from a single site, it can only give partial information 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.