A comparison of line-transect distance sampling methods for estimating gopher tortoise population densities

Abstract

Line-transect distance sampling (LTDS) is increasingly used to estimate gopher tortoise (Gopherus polyphemus) densities. The process requires detecting tortoise burrows, and then determining occupancy. We compared 3 LTDS approaches that differ in how burrow detection and occupancy data are integrated, and 2 search strategies. Surveys were conducted at Avon Park Air Force Range in South-central Florida, USA, from April through October 2009. These include 1) LTDS using data from occupied burrows only; 2) LTDS with burrows treated as "clusters" of size 1 or 0 depending on occupancy; 3) LTDS to estimate burrow densities, with occupancy modeling to estimate occupancy rates; and 4) 2 search strategies: standard versus expanded searches. The LTDS method produced reliable burrow density estimates, and tortoise densities were estimated most precisely and with least effort using data from occupied burrows only. However, any method could be biased if burrow occupancy is uncertain. Indeed, in the Florida scrub and pine flatwoods habitats, video-camera scoping was hindered by groundwater or obstructions, resulting in undetermined occupancy for 45% of burrows. Improving scope technologies and surveying during the dry season has improved subsequent results for xeric habitats; however, research is needed to improve detectability of tortoises in mesic habitats with flooded burrows. If occupancy is uncertain, we recommend collecting data on all burrows, irrespective of occupancy, so that burrow densities can be estimated even if occupancy cannot. We do not recommend expanding the search area with sigmoid search paths adjacent to transects owing to potential violations of assumptions, nor use of occupancy modeling, because model requirements may lead to unacceptable exclusion of data. Ó 2015 The Wildlife Society.

Techniques for Estimating the Size of Low-Density Gopher Tortoise Populations

“…We recommend that all surveys report the number of burrows where occupancy could not be determined along with the survey results. If field conditions preclude the ability to effectively scope burrows (e.g., as in Castellón et al 2015), the method may not be appropriate.…”

“…Gopher tortoises can occur in naturally low densities in suboptimal habitat (Breininger et al 1994;Castellón et al 2012;Legleu 2012) or may persist at low densities because of past exploitation for food or inadequate habitat management (Hermann et al 2002). Surveys of low-density populations require considerable effort in terms of total transect length, and small sample size can reduce precision (Smith et al 2009;Castellón et al 2015). In these circumstances, there is a need for new approaches for LTDS surveys.…”

“…In standard LTDS surveys, tortoise observations (including any tortoises at the surface) are used to calculate population estimates (Smith et al 2009). Burrow cameras can provide a reasonably accurate assessment of occupancy, although in some instances burrows are occluded by roots or debris, make sharp turns, or are temporarily flooded, such that the occupancy cannot be determined with certainty (Smith et al 2005;Castellón et al 2015). However, with appropriate equipment and trained observers, this source of error is generally small (,5%; e.g., Smith et al 2009;Stober and Smith 2010).…”

Adapting coastal management to climate change: Mitigating our shrinking shorelines

“…We estimated burrow densities using hierarchical distance sampling models in the package Unmarked in R (R version 3.2.0, http://www.R-project.org, accessed 31 Aug 2013) based on our LTDS data (Royle and Dorazio , Fiske and Chandler ). We used this method, as opposed to standard LTDS analyses established by Smith and Stober () based around single point and model estimates, and tested by Castellón et al (), to build higher confidence in our population estimates, while accounting for variation in burrow detection and density in a unified framework. We surveyed dune crest lines for tortoise burrows along the entire length of the constructed dunes and the 800‐m sections of each natural dune.…”

“…We scoped burrows only in winter because this is the period of lowest activity for tortoises at our site, and it minimized the chances of falsely declaring a burrow as unoccupied. The current recommendations for gopher tortoise surveys include scoping all burrows encountered (Smith and Stober , Castellón et al ). If this takes >1 day/site, tortoises may move between surveyed and unsurveyed burrows, increasing error rates in burrow occupancy estimates because of the potential to double count tortoises.…”