Wednesday, March 30, 2011

Detecting the Presence of Snakes and Their Conservation


The Western Cottonmouth, 
Agkistrodon piscivorous lecostoma. 
Defense behavior.

Worldwide, more than 82% of snake and more than 84% of lizard species have not been evaluated by the International Union for Conservation of Nature (IUCN) or are classified as having insufficient data to determine conservation status (IUCN, 2010). This is of concern because squamates, like frogs are disappearing. Andrew Durso and colleagues point out that this discrepancy results from the inability of traditional field and data analysis techniques to circumvent unpredictability in reptile detection. Snakes are generally considered the most difficult reptile group to study because of their cryptic behaviors, minimal or sporadic activity patterns, and frequent use of inaccessible habitats such as, subterranean burrows. the forest canopy or murky waters. Therefore high-resolution data on geographic distribution are lacking for many species and few situations exist where population densities have been accurately measured, or population trends tracked over time with confidence. When population declines are suspected it is virtually impossible to distinguish true rarity from poor or unlucky sampling, without knowledge of detection probability. Presence or absence modeling is recognized as an effective technique for monitoring populations of secretive species on a landscape scale, historically considered a daunting or even impossible task. Site occupancy modeling may be the only feasible means for monitoring the population status of some species, in particular for those species with recapture probabilities too low to use mark-recapture effectively. Durso and colleagues provide the first estimates of detection probability and site occupancy for aquatic snake species, and use snakes as a case study for incorporating detection probability in site occupancy monitoring of rare and cryptic species. They surveyed twenty isolated wetlands for aquatic snakes, using multiple replicated sampling events, calculated species-specific parameter estimates of detection probability (p) and site occupancy (w), using the program PRESENCE, and compared single-season models to assess the ability of site-specific covariates to influence these two parameters.They applied this method for seven aquatic snake species: Banded Water Snake (Nerodia fasciata), Florida Green Water Snake (Nerodia floridana), Glossy Crayfish Snake (Regina rigida), Black Swamp Snake (Seminatrix pygaea), Mud Snake (Farancia abacura), Rainbow Snake (Farancia erytrogramma), and Cottonmouth (Agkistrodon piscivorus). This process produced an understanding of how aspects of behavior and ecology influence patterns of detection probability and site occupancy.  They calculated the amount of unsuccessful effort necessary to declare absence of each species with statistical confidence this varied from 5–63 visits; and 150–1890 trap-nights. The study documented considerable interspecific variation in p and w. One species The Banded Water Snake (Nerodia fasciata) was widespread and highly detectable, while the Cottonmouth  (Agkistrodon piscivorus) had low detectability despite its wide distribution. Five other species were secretive, or restricted to specific habitat types, or both, and  those illustrated that complex and sometimes counterintuitive relationships exist between capture rate and occupancy. They conclude that incorporating p and w is essential to the success of large-scale monitoring programs for elusive species.


Citation
Durso, A. M., J. D. Willson, C. T. Winne. 2011. Needles in haystacks: Estimating detection probability and occupancy of rare and cryptic snakes. Biological Conservation, doi:10.1016/j.biocon.2011.01.020

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