Context

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. 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. See pests and threats for more information.

Key findings

Occupancy

Occupancy refers to the presence/absence of a possum at a site, as indicated by the detection of faecal pellets or trapping.¹ As can be seen in Figures 1 and 2, possum occupancy on PCL was:

• 65.7% (95% credible interval (CI) = 63.3 - 68.1%).
• a significant growing trend in non-woody areas over the five seasons from 2013 to 2017.
• higher in woody ecosystems.
• lower in national parks.

Figure 1: Possum occupancy on PCL over five seasons from 2013 to 2017. Different ecosystem types (woody, non-woody) and conservation status (national park, non-national park) are represented by different line types and colours, respectively. Click on the legend to select different combinations and hover over an individual point to show the value and 95% credible interval.

+−

Simple

Terrain

Satellite

TCI

WTI

CCI

Present/Absent

Mean Index

Leaflet | Tiles © Esri — Esri, DeLorme, NAVTEQ

Figure 2: Observed trap catch index (TCI), wax tag index (WTI) and chew card index (CCI) on PCL over five seasons from 2013 to 2017. A black line around a point indicates that a possum was present as shown by the detection of either faecal pellets or one of the indices. 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, as well as between indices and presence/absence. True locations have been randomly jittered to protect the integrity of the plot.

Definitions and methodology

DOC has developed a national monitoring programme to assess status and trend of biodiversity across of all of the land it manages with a particular emphasis on terrestrial monitoring. The programme collects data on indicators and measures of ecological integrity outlined in the Department’s Biodiversity Outcomes Assessment Framework (PDF, 1.07 MB).

The terrestrial monitoring programme has been established at approximately 1400 plot locations spaced evenly across PCL. Each year, approximately 280 plots are measured with every plot being measured once over a 5-year rotation cycle. This spatially extensive monitoring programme has been designed to provide unbiased, repeatable, national-scale estimates of priority ecological integrity indicators and measures. See Table 1 for the sample sizes in different ecosystems during the five seasons from 2013-2017.

Monitoring of possums consists of 4 transects leaving traps, chew cards or wax tags over night to give an index of abundance and an estimate of occupancy. Four faecal pellet lines also contribute to the estimate of occupancy at a site but not abundance.

Data were modelled using a Bayesian zero-inflated negative binomial with zero inflation being informed by occupancy at a site (Gormley et al., 2015). Both occupancy $(z_{i,j})$ and the index of abundance $(y_{i,j})$ depended on cover (woody, non-woody), park status (national park, not national park), and time at site $i$ and transect $j$. Different devices are referenced by $k$. The model was specified as follows $$\begin{array}{rcl} y_{i,j}|z_{i} = 1 & \sim & NB(\mu_i, r) \\ x_{i,j}|z_{i} & \sim & bernoulli(p \times z_{i}) \\ z_{i} & \sim & bernoulli(\psi_{i})\\ \mu_{i} & = & \frac{r}{r + \lambda \times z_{i}}\\ logit(\lambda_{i,j}) & = & \alpha_{0} + \alpha_{1}woody_{i} + \alpha_{2}time_{i} + \alpha_{3}time_{i}woody_{i} + \alpha_{4}park_{i} \\ log(\psi_{i,j}) & = & \beta_{0} + \beta_{1}woody_{i} + \beta_{2}time_{i} + \beta_{3}time_{i}woody_{i} + \beta_{4}park_{i} \\ \end{array}$$ with priors $$\begin{array}{rcl} \alpha & \sim & \mathcal{N}(0, 1000) \\ \beta & \sim & \mathcal{N}(0, 1000) \\ r & \sim & \Gamma(0.01, 0.01)\\ p_k & \sim & B(1, 1)\\ \end{array}$$

The CCI, TCI, and WTI were adjusted to reflect only CCI so that we could estimate trend in abundance. This was done by

where $\mu$ and $\sigma$ are defined in Table 2.

Table 1: Number of plots observed annually for each combination of ecosystems= type and conservation status included in this factsheet.

Table 2: Prior and posterior mean $\mu$ and standard error (SE = $\sigma$) used to convert trap catch and wax tag indices to chew card indices.

Data quality

This measure is classified as a national indicator.

Relevance

This measure relates to indicator 1.3.2 - Invasive species dominance.

Accuracy

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

Outcomes Monitoring Framework

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 2017/18 year.

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: Whether or not a site is being used by a species so that it is available to be observed in the sampling period.
• 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.