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

Ward, Robert J., Griffiths, Richard A., Wilkinson, John W., Cornish, Nina (2017) Optimising monitoring efforts for secretive snakes: a comparison of occupancy and N-mixture models for assessment of population status. Scientific Reports, 7 (18074). ISSN 2045-2322. (doi:10.1038/s41598-017-18343-5)

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.

Item Type:	Article
DOI/Identification number:	10.1038/s41598-017-18343-5
Projects:	[UNSPECIFIED] Status and conservation of the grass snake in Jersey
Uncontrolled keywords:	occupancy, snake, abundance, monitoring, dataset, Natrix helvetica, Jersey, N-mixture, detection
Subjects:	Q Science Q Science > QH Natural history Q Science > QH Natural history > QH301 Biology Q Science > QH Natural history > QH541 Ecology Q Science > QH Natural history > QH75 Conservation (Biology) Q Science > QL Zoology
Divisions:	Faculties > Social Sciences > School of Anthropology and Conservation Faculties > Social Sciences > School of Anthropology and Conservation > Biodiversity Conservation Group Faculties > Social Sciences > School of Anthropology and Conservation > Biodiversity Management Group Faculties > Social Sciences > School of Anthropology and Conservation > DICE (Durrell Institute of Conservation and Ecology)
Depositing User:	R.J. Ward
Date Deposited:	27 Dec 2017 17:24 UTC
Last Modified:	29 May 2019 20:08 UTC
Resource URI:	https://kar.kent.ac.uk/id/eprint/65571 (The current URI for this page, for reference purposes)
Ward, Robert J.:	https://orcid.org/0000-0002-2343-8473
Griffiths, Richard A.:	https://orcid.org/0000-0002-5533-1013