Assessing and comparing population densities and indices of skinks under three predator management regimes

Abstract

Introduced mammalian predators threaten populations of endemic New Zealand skinks. Their effects on skink populations have been not often quantified on the mainland and are known primarily from skink population increases on islands from which mammals have been eradicated. Estimating skink population density with capture-recapture trapping is time-consuming and costly. Counting skinks in artificial retreats in specific weather conditions may be a useful and relatively quick way to index population density, but needs calibration for different habitats and species. In 2007 and 2009, we estimated the population density of small terrestrial skinks (McCann's skink Oligosoma maccanni, southern grass skink O. aff. polychroma clade 5 and cryptic skink O. inconspicuum), based on spatially explicit capture-recapture (SECR) in pitfall traps in three mammal-management treatments at Macraes Flat, Otago. The treatments were eradication of large predators and near-eradication of rodents inside a mammal-resistant fence, suppression of mammalian predator populations through continuous trapping (two locations within an extensive area), and no mammal management. We tested for relationships between the estimated population densities and dawn and late-morning counts of skinks in artificial retreats. Skink density (three species combined) ranged from c. 1200 per ha at the experimental control site to c. 4000 per ha at the fenced site. These treatment differences in skink density may be the combined effect of predator management and pre-existing differences due to habitat characteristics and farming practices. Skink counts done in late morning (2009) were related to estimated skink densities but did not differ significantly between treatments. Skink counts done at dawn (both years) were not related to densities. Counts, but not densities, were significantly higher at locations with a more northerly aspect. We recommend further investigation of the utility of skink counts in artificial retreats for monitoring skink density at this location, with careful control of ambient temperature during sampling, and of aspect, habitat and device placement.

Population dynamics of house mice without mammalian predators and competitors

“…Models in which g 0 was also an additive function of mouse weight and/or a behavioural response to capture were also tested. Two alternative types of behavioural responses to capture (Efford 2016) were considered: b, a permanent response in which an animal's probability of capture increased (a trap-happy response) or decreased (trap-shy) after its first capture; and bk, a trap-specific permanent response in which an animal became trap-happy or trap-shy in relation to a particular trap (Royle et al 2011;Wilson et al 2017). Table 2.…”

Evaluating mitigation translocations for better biodiversity outcomes

“…A suite of introduced mammals such as rats (kiore, Rattus exulans, R. norvegicus, and R. rattus), mustelids (stoats (Mustela erminea), ferrets (M. furo), and weasels (M. nivalis), mice (Mus musculus), along with some domesticated animals (e.g. dogs (Canis familiaris), cats (Felis catus)) prey on native reptiles (Hoare et al, 2007;Salo et al, 2007;Wilson et al, 2017). Predator control has increased in the country since studies reported a positive relationship between the eradication of introduced predators on offshore islands and improved survivability and reproduction of native species (Whitaker, 1973;Towns, 1991;Towns, 1995;Monks et al, 2014;Parkes et al, 2014;Jones et al, 2016).…”

“…Individual dispersal from release site is reported strongest in the first weeks to months following release and is a key determinant of success (Tuberville et al, 2005 (Eifler & Eifler, 1999;Wilson et al, 2017)).…”

“…Management of predators should begin at receptor sites prior to release to effectively reduce predator presence at receptor sites (Parkes et al, 2014;Pech & Maitland, 2016) and optimize success of translocated lizards (Norbury et al, 2014;Wilson et al, 2017).…”

“…Habitat enhancement often includes vegetation projects which may benefit species (such as those in Herbert, 2020), but recent studies investigated the use of rock piles to benefit lizards and exclude predators (Lennon, 2019). Enhancements such as soft-release pens (Ebrahimi & Bull, 2013;Knox & Monks, 2014;DeGregorio et al, 2020) or predator-proof fences (Burns et al, 2012;Nelson et al, 2016;Wilson et al, 2017) should also be considered, particularly for translocations to mainland sites (which often have a suite of predators, Romijn & Hartley, 2016;Lennon, 2019).…”