Optimising monitoring efforts for secretive snakes: a comparison of occupancy and N-mixture models for assessment of population status

Abstract

A fifth of reptiles are Data Deficient; many due to unknown population status. Monitoring snake populations can be demanding due to crypsis and low population densities, with insufficient recaptures for abundance estimation via Capture-Mark-Recapture. Alternatively, binomial N-mixture models enable abundance estimation from count data without individual identification, but have rarely been successfully applied to snake populations. We evaluated the suitability of occupancy and N-mixture methods for monitoring an insular population of grass snakes (Natrix helvetica) and considered covariates influencing detection, occupancy and abundance within remaining habitat. Snakes were elusive, with detectability increasing with survey effort (mean: 0.33 ± 0.06 s.e.m.). The probability of a transect being occupied was moderate (mean per kilometre: 0.44 ± 0.19 s.e.m.) and increased with transect length. Abundance estimates indicate a small threatened population associated to our transects (mean: 39, 95% CI: 20–169). Power analysis indicated that the survey effort required to detect occupancy declines would be prohibitive. Occupancy models fitted well, whereas N-mixture models showed poor fit, provided little extra information over occupancy models and were at greater risk of closure violation. Therefore we suggest occupancy models are more appropriate for monitoring snakes and other elusive species, but that population trends may go undetected.

Using UAV‐mounted thermal cameras to detect the presence of nesting nightjar in upland clear‐fell: A case study in South Wales, UK

“…Nest locations of this sub‐Saharan migrant are difficult to identify due to their cryptic camouflage (Troscianko et al., 2016), crepuscular behaviour and low nesting densities across large areas (Cross et al., 2005; Holyoak, 2001). As such, crypsis can mean lower confidence in population estimates and uncertainty in the regional presence or absence of breeding individuals (Couturier et al., 2013; Ward et al., 2017).…”

Recent Invaders in Small Mediterranean Islands: Wild Boars Impact Snakes in Port-Cros National Park

“…More generally, snakes are secretive organisms notoriously difficult to study in the field. Demographic changes are not easily accessible to evaluation [55,56]. Strong decreases may escape observation and their possible causes might remain unnoticed.…”

Make the Adder Count: population trends from a citizen science survey of UK adders

“…This variation in detectability between individuals and for the same individual over time prevents the use of more complex statistical models which use repeat visits to disentangle detectability and abundance assuming constant detectability and closed populations (e.g. N-mixture models; Royle, 2004;Ward et al, 2017;Barker et al, 2018). The peak count therefore remains the most robust index of population size available under these conditions with the available data.…”

“…We have assumed that changes in the number of adders recorded at a given site reflect genuine changes in population size and are not influenced by systematic changes in their detectability over time. Our analysis accounts for variation in survey effort, which strongly influences snake detectability (Ward et al, 2017), but other factors may also influence detectability. Detectability may increase over time if surveyors become more familiar with a site.…”