Evaluating Survey Methods for Monitoring a Rare Vertebrate, the Sonoran Desert Tortoise

Abstract

Effective conservation requires strategies to monitor populations efficiently, which can be especially difficult for rare or elusive species where field surveys require high effort and considerable cost. Populations of many reptiles, including Sonoran desert tortoises (Gopherus agassizii), are challenging to monitor effectively because they are cryptic, they occur at low densities, and their activity is limited both seasonally and daily. We compared efficiency and statistical power of 2 survey methods appropriate for tortoises and other rare vertebrates, linetransect distance sampling and site occupancy. In 2005 and 2006 combined, we surveyed 120 1-km transects to estimate density and 40 3-ha plots 5 times each to estimate occupancy of Sonoran desert tortoises in 2 mountain ranges in southern Arizona, USA. For both mountain ranges combined, we estimated density to be 0.30 adult tortoises/ha (95% CI 5 0.17-0.43) and occupancy to be 0.72 (95% CI 5 0.56-0.89). For the sampling designs we evaluated, monitoring efforts based on occupancy were 8-36% more efficient than those based on density, when contrasting only survey effort, and 17-30% more efficient when contrasting total effort (surveying, hiking to and from survey locations, and radiotracking). Occupancy had greater statistical power to detect annual declines in the proportion of area occupied than did distance sampling to detect annual declines in density. For example, we estimated that power to detect a 5% annual decline with 10 years of annual sampling was 0.92 (95% CI 5 0.75-0.98) for occupancy and 0.43 (95% CI 5 0.35-0.52) for distance sampling. Although all sampling methods have limitations, occupancy estimation offers a promising alternative for monitoring populations of rare vertebrates, including tortoises in the Sonoran Desert.

Estimating abundance of unmarked animal populations: accounting for imperfect detection and other sources of zero inflation

“…confidence intervals) of estimates varied among methods; estimates from N ‐mixture models had higher precision compared to other methods for lizards (Doré et al . ), but not for tortoises (Zylstra, Steidl & Swann ). Couturier et al .…”

Improving herpetological surveys in eastern North America using the environmental DNA method

“…Our results provide an empirical demonstration of the effectiveness of the eDNA method to efficiently characterize the presence of amphibians and reptiles in natural environments and show that eDNA may provide quantitative data comparable to measurements obtained by classical visual surveys. The extension or contraction of distribution ranges is one indicator of a species trend (Erb et al 2015;Mackenzie et al 2002;Zylstra et al 2010) and is a common indicator in recovery plan objectives (e.g., Environment Canada 2014). Environmental DNA can be used to define species range data from different habitats and merge sampling efforts when resources are limited.…”

Occupancy in continuous habitat

“…These methods use repeated surveys at a sample of sites to estimate the true probability that a site is occupied by a species, the parameter known as 'occupancy'. Occupancy is now widely seen as an alternative to population density (or abundance in a broad sense) in large-scale monitoring programs , Marsh and Trenham 2008, Morrison et al 2008, Mattsson and Marshall 2009, Weir et al 2009, O'Brien et al 2010, Zylstra et al 2010. While not exactly a population parameter, occupancy represents population status in a way that is potentially useful for management.…”