Calibrating brushtail possum (Trichosurus vulpecula) occupancy and abundance index estimates from leg-hold traps, wax tags and chew cards in the Department of Conservation’s Biodiversity and Monitoring Reporting System

Abstract

Abstract:The Department of Conservation has implemented a Biodiversity and Monitoring Reporting System (BMRS) that estimates occupancy rates and relative abundances of introduced brushtail possums (Trichosurus vulpecula) at a representative sample of sites on public conservation land. Leg-hold traps have been used to monitor possums in the BMRS, but wax tags and chew cards have logistical and financial advantages over traps. If possum occupancy rate and/or abundance index estimates differ depending on which of the three methods are used, then correction of the estimates would be required for valid comparisons. We sampled possum occupancy rates and relative abundances using leg-hold traps, wax tags and chew cards in the BMRS sampling design at each of 54 forest and 54 non-forest sites. Possum occupancy rates estimated using each of the three detection methods were similar and hence do not require correction. Median possum abundance index values estimated from traps were lower than those estimated from wax tags and chew cards in forest, but were similar to those from wax tags in non-forest. Calibration is therefore required if possum abundance index values from either chew cards or wax tags are to be validly compared with trap catch abundance index values. We used a zeroinflated negative binomial (ZINB) model to calibrate the chew card and wax tag abundance indices with the trap catch abundance index. The ZINB model allowed us to account for structural zeros (i.e. possums were not present and therefore cannot be detected) and non-structural zeros (i.e. possums were present, but not detected by a particular method). The relationship between possum abundance index estimates from chew cards and leg-hold traps was better calibrated than that between wax tags and leg-hold traps, with the latter particularly poor in non-forest habitat. We therefore conclude that chew cards, rather than wax tags, should replace leg-hold traps for monitoring possum occupancy rates and relative abundances in the BMRS. Our ZINB model can be used to correct chew card abundance index values for valid comparison with previous trap catch abundance index values estimated using the BMRS design.

Habitat provision is a major driver of native bird communities in restored urban forests

“…Indices of relative abundance of invasive mammals within each site were collected using chew cards for rats and possums and camera traps for cats. Chew cards are an increasingly widely used monitoring device for mammals in New Zealand, as they are cheap, easy to use and highly sensitive to possums and rodents (Forsyth et al, 2018; Sweetapple & Nugent, 2011). Chew card indices are correlated with other indices of possum and rodent abundance, which have been shown to relate to nesting success for several native bird species (Armstrong et al, 2006; Forsyth et al, 2018; Innes et al, 1999; Innes et al, 2004; Ruffell et al, 2015; Sweetapple & Nugent, 2011).…”

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“…Individual EBVs employ widely used methods (Supporting Information S1) but never before combined simultaneously at the same sample points; each have therefore required iterative refinement (e.g. Forsyth et al., 2018a; Gormley et al., 2015). Methods provide data on 15 of 32 non‐native mammals, particularly herbivores (Figure 1) and two omnivores (brushtail possums Trichosurus vulpecula and pigs Sus scrofa ).…”

A Novel SDWSN-Based Testbed for IoT Smart Applications

“…According to [17], the initial spacing between possum traps is 20 to 40 m, but can be expanded to 100 m if possum population is low. The work in [18] used 108 brushtail possum monitoring sites to estimate possum occupancy rates in wildlife, and the transect of traps is recognized at a distance of 200 m apart. Therefore, the distance between possum traps needs to be specified.…”