High sigma values

questions concerning anlysis/theory using program DENSITY and R package secr. Focus on spatially-explicit analysis.

High sigma values

Postby FAmorim » Mon Oct 15, 2018 1:05 pm


I’m trying to calculate density of bats roosting in bridges (roads). I captured and individually marked bats in five different bridges and in multiple places (traps) within each bridge. Because this was a four-year study with multiple capturing occasions, I’m using a multisession approach in secr. My goal is to know bat density and population size for those bridges. Because we are dealing with roosts, bats can only be in the bridges and, according to my understanding, the mask should be restricted to the bridges. This results in a very narrow strip around the traps.

The models seem to be fitting ok, and I was able to improve AIC by adding some variables (bk, t, session). Although the coefficients show narrow confidence intervals and density does seem realistic, sigma values are extremely high (e.g. 4.52e+12). These values are truly unexpected (and unrealistic), since these bats show some degree of roost fidelity and don’t move a lot between traps (maximum RSPV = 168.34m; maximum distance moved = 1469.66m). I’m wondering if roost fidelity and the low number of movements and recaptures could somehow lead to this high sigma values. I’ve captured a total of 2 686 individuals, of which 294 have been recaptured, but only 112 moved (61 of which less than 10 meters).

Thank you for your help.
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Re: High sigma values

Postby murray.efford » Mon Oct 15, 2018 3:09 pm

I really can't see how the SECR state and observation models would be useful for roosting bats. We would use very large sigma to simulate spatially indiscriminate (not distance-related) detection, and maybe that is what your estimate is telling you. Perhaps think some more about what you want to measure.
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