Key findings

Most of the monitored groups of long-tailed bats in the Eglinton Valley continue to have a high level of survival, with inter-annual declines in survival being ameliorated by intensive pest control, which generally enables a swift recovery to previous levels. Although the reasons for the lower and more variable survival at Knobs Flat are unclear, the overall picture is one of continued sustained increases for these populations.

The population trends of three social groups of adult female long-tailed bats in the Eglinton Valley (Fig. 1) have been followed since 1995, using inter-annual survival as a proxy for abundance.
Two of the three social groups (Walker Creek and Mackay Creek) have exhibited a positive trend in survival (and presumably abundance) since monitoring commenced in 1995 and 1997, respectively (Fig. 2). Obvious declines have correlated well with beech mast seeding-induced predator irruptions, but the early initiation of pest control in these two areas appears to have reduced the long-term impacts of these events, enabling sustained positive trends in survival and population growth.
Monitoring of the Knobs Flat group of long-tailed bats commenced much later (2006) and pest control here was only implemented in 2011 (initially covering half the roosting range of the bats). Survival estimates for this group have remained lower and the overall trend is one of continued slow decline (Fig. 2).
Reasons for the variable response of long-tailed bats at Knobs Flat compared with Walker Creek and Mackay Creek remain unclear. However, the later and partial implementation of pest control, and continued habitat degradation resulting from severe wind damage to forests in the area appear to be the most likely reasons for this.
Despite the poorer survival estimates for bats at Knobs Flat, the valley-wide index of relative abundance of bat presence that has been conducted since 1992 using sixty 1-km-long transects suggests that the overall number of long-tailed bats throughout the Eglinton Valley is continuing to increase (Fig. 3).

Figure 1. Location of the Eglinton Valley.

Figure 2. Survival estimates for three studied long-tailed bat populations in Eglinton Valley, Fiordland.

Figure 3. Percentage of bat monitoring transects with bats detected in Eglinton Valley, Fiordland.

Definitions and methodology

The survival of long-tailed bats in the Eglinton Valley has been estimated annually since 1993 using mark-recapture field data derived from banded individuals. In November each year, a 50-km stretch of the Eglinton-Milford highway is walked at night by a team of observers equipped with bat detectors that convert ultrasonic bat calls into signals that are audible to humans. The percentage of 1-km segments on which bats are detected is then graphed for each year. Program MARK was used to compute inter-annual survival estimates for juvenile and adult female bats, and a simple linear model was used to fit a trend line. Increased bat survival is assumed to equate to an increasing population size.

Data quality

This measures is classified as a case study. Although the general trends may well apply to long-tailed bat populations elsewhere with similar habitats, caution should be used if attempting to extrapolate these results.

Relevance

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

Accuracy

These survival estimates are considered robust given the extended capture histories that have been collected for many bats over an extended period of time. Although the percentage of transects on which bats are detected is a very simple index of abundance, power analysis suggests that the general trend is robust after at least 15 years of surveys.

This measure complies with the data quality guidelines used in New Zealand’s Environmental Reporting framework.

Outcomes Monitoring Framework

The Department of Conservation’s (DOC’s) Outcomes Monitoring Framework provides a platform on which DOC and others can assess outcomes in a clear, structured and transparent way (Lee et al., 2005). It has been developed as a logical hierarchy that is based on broad, overarching Outcomes, beneath which are nested Outcome Objectives, Indicators, Measures and Data Elements to provide ever increasing levels of detail. The framework is scalable, as the indicators and measures remain compatible and consistent whether applied locally, regionally or nationally. The recently updated framework provides a roadmap for gathering information to meet the specific objectives of DOC and other agencies (McGlone and Dalley, 2015). The provision of a national framework with agreed outcomes, indicators and measures supports collaboration with land management and regulatory agencies, allowing for more integrated environmental policy and ‘State of the Environment’ reporting. DOC has partially implemented a national monitoring and reporting system, whereby priority indicators and measures are routinely used to report on progress against the objectives and outcomes. This factsheet reports on a measure for the 2015/16 year.

Glossary of terms

Mast seeding is the synchronous production of large quantities of seeds within a population of plants at irregular intervals, and occurs in a number of New Zealand forest tree and tussock grass species.

References

Lee, W., McGlone, M., Wright, E., 2005. Biodiversity inventory and monitoring: A review of national and international systems and a proposed framework for future biodiversity monitoring by the Department of Conservation. Landcare Research Contract Report LC0405/122 (unpublished) for the Department of Conservation, Wellington.

McGlone, M., Dalley, J., 2015. A framework for Department of Conservation inventory and monitoring: Intermediate outcomes 1-5. Landcare Research Contract Report LC2427 (unpublished) for the Department of Conservation, Wellington.

Taxon under management

South Island long-tailed bats (Chalinolobus tuberculatus), Eglinton Valley, Fiordland