Abundance and distribution of possums

This measure relates to indicator 1.3.2 – Invasive species dominance.

Background

Possums are widespread across Public Conservation Land (PCL) in New Zealand, and can have significant impacts on native flora and fauna though competition, predation and herbivory (see pests and threats for more information). Therefore, understanding the national distribution and abundance of possums can help to direct management efforts by the Department of Conservation (DOC) and others to protect New Zealand’s biodiversity. These data also provide baseline information against which future trends and/or management results can be compared.

Possum numbers are stable in forest and scrub but slightly increasing in non-woody habitats.

What did we measure?

DOC has developed a national monitoring programme to assess the status and trends of biodiversity using approximately 1,400 sites that are spaced evenly across PCL. A random sample of approximately 280 of these sites are measured each year, so that every site is measured once on a five-year rotation. Possum signs are measured at each site, and data are also collected about vegetation, birds, and pellets of other introduced mammals.

Abundance estimates of possums at each site are based on 4 transects where chew cards are left overnight. Older data was collected using traps (TCI) or wax tags (WTI) instead of chew cards. These have been calibrated to chew cards indices.

Occupancy is the proportion of sites where possums are present, taking into account the likelihood of detecting a possum if it is present. Data are chew cards indices and records from four faecal pellet lines. Estimates were modelled using a Bayesian zero-inflated negative binomial with zero inflation being informed by occupancy at a site.

What did we find?

Figure

Figure 1: Possum abundance based on chew card indices (CCIs) on public conservation land (PCL) over the last eight seasons from 2012/13 to 2019/20. ‘Occupied CCI’ represents the trend in possum-occupied areas, whereas ‘PCL CCI’ represents the overall trend on PCL. Different ecosystem types (woody, non-woody) and conservation status (national park, non-national park) are represented by different line types and colours, respectively.

Figure 2: Possum occupancy on PCL over eight seasons from 2012/13 to 2019/20. Different ecosystem types (woody, won-woody) and conservation status (national park, non-national park) are represented by different line types and colours, respectively

Figure 3: Observed average possum abundance indices at each park. Choose one or more parks in the selection box at the top (selected parks turn red) and hover over an individual point to see the details. Values are mean ± 1 standard error for each index (chew card index (CCI), wax tag index (WTI), and trap catch index (TCI)). Several outliers are not visible but can be seen using the tools in the top right of the figure. WTI data are for 2014 and TCI data for 20120-2014: since the 2015/2016 season, only CCI has been used

Table

Table 1: Number of plots observed annually for each combination of ecosystems type and conservation status included in this factsheet.
Ecosystem Conservation Status 2012 2013 2014 2015 2016 2017 2018 2019 Total
non-woody national park 21 15 19 22 17 25 22 19 119
non-woody non-national park 43 56 38 56 59 59 59 40 311
woody national park 8 52 54 72 60 64 52 54 310
woody non-national park 22 138 112 115 119 123 140 113 629
total total 94 261 223 265 255 271 273 226 1369

Map

Figure 4: Index of relative abundance on PCL over eight seasons from 2012/13 to 2019/20 - only the most recent measurement is shown. Only sites with chew card index (CCI) data are shown as there were only 8 sites for which the most recent measurement was based on trap catch index (TCI) or wax tag index (WTI). Tick ‘Present/Absent’ to get presence of possum (from detection of faecal pellets and/or one of the indices) indicated by a black edge around the circles. The mean index of relative abundance gives a sense of distribution/abundance of possums across PCL. Switch between satellite and terrain views by clicking on the tile in the upper right of the map. True locations have been randomly jittered.

Data quality

This measure is classified as a partial measure of high accuracy and complies with the data quality guidelines used in the Environmental Reporting framework.

Glossary of terms

Chew cards are small rectangular plastic sheets that have attractant paste pressed into their internal channels. A chew card index (CCI) is calculated by deploying up to 40 chew cards along four evenly spaced 200-m long transect lines (10 chew cards per line at 20-m intervals). An estimate of possum abundance is then generated by calculating the percentage of chew card interference (tooth impressions or scratches).

Credible interval (CI): A 95% CI indicates that the true mean lies inside the interval with 95% probability given the posterior probability distribution.

Trap catch indices (TCIs) are used to measure possum abundance. A TCI is calculated by deploying up to 40 leg-hold possum traps along four evenly spaced 200-m transect lines (10 traps per line at 20-m intervals). The TCI is defined as the percentage of trap-nights on which a possum was captured. This information can be used to infer the presence of possums and their approximate abundance within an area.

Occupancy indicates whether or not a site is being used by a species. A model is used to estmate the proportion of sites at which the species occurs, adjusted for the probability of detection.

Wax tags are scented, ice-cube sized wax blocks and are used to estimate possum abundance. A wax tag index (WTI) is calculated by deploying up to 40 wax tags along four evenly spaced 200-m transect lines (10 wax tags per line at 20-m intervals). The percentage of wax tag interference (tooth impressions or scratches) is then used to estimate possum abundance.

Additional resources

DOC’s Outcome Monitoring Framework: https://www.doc.govt.nz/our-work/outcome-monitoring-framework

Lee, William and McGlone, Matt and Wright, Elaine: Biodiversity inventory and monitoring: a review of national and international systems and a proposed framework for future biodiversity monitoring by the Department of Conservation (2005). https://www.landcareresearch.co.nz/publications/researchpubs/biodiv_inventory_system_review_framework.pdf

Gormley, Andrew M et al.: Cost-Effective Large-Scale Occupancy-Abundance Monitoring of Invasive Brushtail Possums (Trichosurus vulpecula) on New Zealand’s Public Conservation Land. (2015) PloS One (10:6)