Establishment of a Self-Sustaining Population of a Long-Lived, Slow-Breeding Gecko Species (Diplodactylidae: Hoplodactylus duvaucelii) Evident 15 Years after Translocation
Duvaucel's Geckos (Hoplodactylus duvaucelii) are large-bodied, long-lived, and slow-breeding geckos endemic to New Zealand. Translocation is considered an important tool for conservation of this species. The first translocation of these geckos occurred in 1998 when 40 geckos were released on a pest-free island. Despite multiple survey efforts within the decade after release, captures were too sparse to determine whether the population had successfully established. We revisited the population to investigate establishment over a longer time period (11–15 yr post release). We utilized a recently developed outcome assessment framework that assesses a translocation's success as a progression through four, time-bound ‘Stages.' We recaptured 12 founder animals and all were larger than at release (Stage 1). Recruitment was evident (Stage 2) with 91.7% of all geckos captured between 2009 and 2013 being island-born. Mark–recapture results from 144 individuals estimated that 245 geckos (95% confidence interval: 179–336) were present in the population by February 2013, an approximately 6-fold increase in number since 1998 (Stage 3). Population viability analysis indicated the population has become self-sustaining, with an average population increase rate of 6.35–10.14% per year and a negligible probability of extinction within 50 yr (Stage 4). In this case, a 10–15 yr time period appeared to be a sufficient time frame for identification of translocation success by both traditional and time-bound criteria. Fulfilment of each time-bound indicator of short-term success therefore appears to have been a reliable predictor of ultimate translocation success in this long-lived, slow-breeding lizard species.Abstract
Maps of study site on Mana Island, New Zealand showing (A) the release site, habitat types, and sites of intensive monitoring; (B) locations of Duvaucel's Geckos encountered and/or radiotracked and known area of population occupancy (constructed as a minimum convex hull) in relation to the release location. ACO = Onduline artificial cover object, CFR = closed-cell foam retreat.
Snout–vent lengths (SVLs) of Duvaucel's geckos from the Mana Island (MI) founder population at time of translocation (MI 1998; Miskelly, 2002), the Mana Island population 11–14 yr after release (MI 2009, MI 2010, MI 2011, MI 2012), and from the source population on North Brother Island—both historical (NB 1957, NB 1958, NB 1959) and more recent (NB 1999, NB 2000, NB 2001) (Nelson and Keall, unpubl. data). Numbers above the bars denote the number of individuals measured.
von Bertalanffy curves predicting SVL for geckos aged between 0–25 yr caught from Mana Island 2009–2013 and from North Brother Island 1956–1959 (Barwick, 1982). Each year consists of 225 growth days per year to account for the 20 wk of winter over which Duvaucel's Geckos do not grow. The curve for North Brother Island was constructed from the equation given in Fabens (1965). The vertical red line indicates Barwick's (1982) age of maturity in North Brother Island males and the vertical black line is the size of the smallest gecko captured on Mana Island showing male secondary sexual characteristics. Dashed lines are 95% confidence intervals for the Mana Island curve. Estimated parameters for the Mana Island growth curve were: a = 122.5 (95% confidence interval (CI) = 112.5–132.5), k = 1.005 × 10−3, (95% CI = 6.87 × 10−4–1.32 × 10−3), and β = 0.592 (95% CI = 0.556–0.623). Adjusted R2 of model = 86.90%.
Estimated number of (A) juvenile and (B) subadult and adult geckos using the standardized study site over the six monitoring periods between November 2010 (month 1) and February 2013 (month 28). The solid line represents the trend line derived from linear regression of the estimates and the dashed lines represent the confidence intervals of each estimate as produced by the POPAN model.
Comparison of simulation modeling outcomes against estimated abundance of Duvaucel's Geckos on Mana Island at year 15 post translocation (245 individuals, CI: 179–336). Modeling parameters unique to each model are provided at the top of each graph (B = probability of each adult female breeding, Sj = annual survival rate of juvenile geckos, years 0–5, Sa = annual survival rate of subadult and adult geckos, years 5+). Lines represent the simulated mean population size each year and the associated dotted lines are ± 1 SE.
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