Evaluation of trap capture in a geographically closed population of brown treesnakes on Guam

Abstract

Summary 1.Open population mark-recapture analysis of unbounded populations accommodates some types of closure violations (e.g. emigration, immigration). In contrast, closed population analysis of such populations readily allows estimation of capture heterogeneity and behavioural response, but requires crucial assumptions about closure (e.g. no permanent emigration) that are suspect and rarely tested empirically. 2. In 2003, we erected a double-sided barrier to prevent movement of snakes in or out of a 5-ha semi-forested study site in northern Guam. This geographically closed population of >100 snakes was monitored using a series of transects for visual searches and a 13 × 13 trapping array, with the aim of marking all snakes within the site. Forty-five marked snakes were also supplemented into the resident population to quantify the efficacy of our sampling methods. We used the program mark to analyse trap captures (101 occasions), referenced to census data from visual surveys, and quantified heterogeneity, behavioural response, and size bias in trappability. Analytical inclusion of untrapped individuals greatly improved precision in the estimation of some covariate effects. 3. A novel discovery was that trap captures for individual snakes consisted of asynchronous bouts of high capture probability lasting about 7 days (ephemeral behavioural effect). There was modest behavioural response (trap happiness) and significant latent (unexplained) heterogeneity, with small influences on capture success of date, gender, residency status (translocated or not), and body condition. 4. Trapping was shown to be an effective tool for eradicating large brown treesnakes Boiga irregularis (>900 mm snout-vent length, SVL). 5. Synthesis and applications. Mark-recapture modelling is commonly used by ecological managers to estimate populations. However, existing models involve making assumptions about either closure violations or response to capture. Physical closure of our population on a landscape scale allowed us to determine the relative importance of covariates influencing capture probability (body size, trappability periods, and latent heterogeneity). This information was used to develop models in which different segments of the population could be assigned different probabilities of capture, and suggests that modelling of open populations should incorporate easily measured, but potentially overlooked, parameters such as body size or condition.

Ecology and Control of an Introduced Population of Southern Watersnakes (Nerodia fasciata) in Southern California

“…These biases are widespread among snake studies (e.g., Tyrrell et al 2009), and are established for watersnakes including N. fasciata (Willson et al 2008. Such caveats about detection probabilities also apply to most of the historical literature with which we can compare our results (although this bias was largely unappreciated in historical literature), reducing our confidence in the strength of comparisons.…”

“…We captured more females (187) than males (119) for a ratio of 1.57:1 that differed from 1:1 (x 2 5 15.1, df 5 1, P , 0.001). We were unable to quantify variation in capture probability by sex (e.g., Tyrrell et al 2009) and also funnel traps have known size biases in capture probabilities (Willson et al 2005(Willson et al , 2008; therefore, this ratio is not necessarily representative of the overall population.…”

Scented Guide Ropes as a Method to Enhance Brown Treesnake (Boiga irregularis) Trap Capture Success on Guam

“…ID, to account for latent individual-based capture heterogeneity, was included as a random variable because it appreciably influenced capture rates in this population (Tyrrell et al, 2009).…”

Burmese Pythons

“…Such traps, using live mouse lures, are only partially effective at capturing snakes 700-900 mm SVL, and not effective for smaller juvenile snakes (Rodda et al 2007b;Tyrrell et al 2009) due to a strong ontogenetic prey preference for small lizards and nonresponsiveness to rodent-based lures (Savidge 1988;Lardner et al 2009a;Siers 2015). Ef cacy of different lure systems and eld operational evaluations are covered below.…”