Reducing the cost of DOC’s biodiversity monitoring
Robust biodiversity monitoring programmes are expensive and time-consuming, especially when conducted at large scales. As such, it is important to identify ways of monitoring more quickly and more cheaply without a loss in the quality of information collected.
The Department of Conservation (DOC) is implementing a systematic approach to Biodiversity Monitoring and Reporting (BMRS) to enable reporting on three indicators of ecological integrity: species occupancy, indigenous dominance and ecosystem representation. The BMRS is composed of three tiers of information, which operate at different scales with varying levels of detail and coverage. The Tier 1 component involves measuring a variety of taxa (i.e. native and exotic plants and birds, and pest mammals) at 1354 systematically located sites on public conservation land over a 5-year rolling period, with 271 locations sampled each field season (i.e. October–March).
An initial pilot study at 18 locations in 2008/09 determined that the two nights required for possum monitoring was a major determinant of costs, time investment and availability of adequate technical expertise. DOC is particular interested in whether reliable estimates of possum abundance could be obtained from one night of monitoring. Field teams would only need to stay 2 days (rather than 3 days) at each sampling location and so be able to cover more locations in a season. This means fewer people would need to be trained in possum monitoring protocols.
Possums are measured using the Trap-Catch Index (number of possums captured every 100 trap nights), which measures relative abundance and reflects actual possum abundance. Possum monitoring consists of 4 × 200 m trap lines, each with 10 leg-hold traps spaced at 20-m intervals, giving a total of 40 traps per sampling location (Fig. 1). Such trapping data also provide information on site occupancy by possums and are supplemented by data from surveys of 120 faecal pellet plots per sampling location.
A phased implementation of the Tier 1 programme was conducted at 85 forest locations and 79 non-forest locations in the 2011/12 and 2012/13 field seasons, during which time traps were set over two fine nights and checked daily. Andrew Gormley (Landcare Research) and Dave Forsyth (Arthur Rylah Institute (ARI)/DOC) analysed these trap-catch data to assess the consequences of reducing trapping to only one night. They found that estimates of possum occupancy were not affected, due in part to the amount of information obtained from pellet plots remaining unchanged (Fig. 2). However, estimates of possum abundance were slightly affected, with slightly more possums estimated in forest habitats and greater uncertainty. Greater uncertainty also affects the level of change that could be detected reliably, albeit very slightly. For example, if all forest locations were sampled, two nights of trapping would detect a 5.4% change, whereas one night of trapping would detect a 5.7% change.
The outcomes of Andrew and Dave’s research have led to DOC reducing possum monitoring from two nights to one night in Tier 1 of the BMRS. One night of possum monitoring has the potential to reduce the number of field days per field season from 813 (i.e. 3 days across 271 locations per year) to 542, resulting in potential savings of 6% per sampling location, due to a 20% reduction in the costs of labour, food and field allowances. However, the need to monitor other taxa and changes to the organisation of field teams means that actual savings are likely to be less than this. Another benefit of fewer field days is that it gives greater opportunity to select more suitable weather windows and therefore increased likelihood that all 271 locations will be sampled during the field season.
The research gave DOC confidence that reducing the sampling effort would compromise neither the quality of the data nor the inferences that could be made using those data.
The implementation of Tier 1 of the BMRS is a template for other large-scale biodiversity monitoring programmes: the pilot study enabled potential savings to be identified, while the phased implementation allowed key data to be collected and benefit–costs analysis to be performed before the programme was fully implemented in 2013/14.
This work was funded by the Department of Conservation.