Density Estimation in Terrestrial Chelonian Populations Using Spatial Capture–Recapture and Search–Encounter Surveys
Having an accurate estimate of population size and density is imperative to the conservation of chelonian species and a central objective of many monitoring programs. Capture–recapture and related methods are widely used to obtain information about population size of chelonians. However, classical capture–recapture methods have strict spatial sampling requirements and do not account for lack of geographic closure caused by movement of individuals in and out of the surveyed landscape. Newly developed spatial capture–recapture (SCR) models address these limitations by specification of explicit models for spatial sampling as well as the spatial distribution of individuals in the population. Spatial capture–recapture models have not yet been applied to the study of chelonian populations. Here we demonstrate their application to a population of box turtles in Maryland that has been studied for 75 yr. Results support dramatic declines in population size of box turtles since the 1940s.ABSTRACT
The two study areas, Stickel (white) and Snowden (red), on the Patuxent Wildlife Research Center (PWRC) in Laurel, Maryland. The green line is the Patuxent River.
Season total search effort. Numerical values are the total frequency of observer passes through the center of each grid cell during the 60 d of survey activity (1–4 surveyors each survey day). For example, a value of 40 would mean that a given grid cell was searched 40 times during the season. The buffered track lines, outlined in black, show the search path of one surveyor on Day 5 in the Stickel plot (northwest region) and Day 58 in the Snowden plot (southeast region). The black dots represent capture locations of box turtles over the season.
Daily frequency of box turtle encounters over the 60-d search effort.
The distribution of distances between observer and location of turtle upon capture.
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