Analyzing sample/site covariates for 1 site only

questions concerning analysis/theory using program PRESENCE

Analyzing sample/site covariates for 1 site only

Postby jlinehan » Thu Dec 20, 2007 4:53 pm

Is it possible to use Presence to identify sample/site covariate effects for one site only?
Our study design is as follows:

We collected presence/absence data for Hyla squirella at one pond during 4 consecutive seasons (Summer, Winter, Spring, Fall). Within each season we sampled the same pond 18 times (a total of 72 surveys) using ground and tree pvc pipes. We collected sample covariates such as temp, rain, etc.

We would like to evaluate the sample covariates (ex. weather) and site covariates (trap type).

Thanks,
Jen
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Postby darryl » Thu Dec 20, 2007 5:11 pm

The effect of the covariates on what? What is it you're trying to estimate? What is 'occupancy' in your situation?
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Postby jlinehan » Thu Dec 20, 2007 5:40 pm

We are trying to evaluate changes in occupancy over the course of the 4 seasons....investigating different trapping methodology and weather variables. We sampled one wetland where we had 30 ground and 30 tree pipes. Could we consider each trap a different site to obtain the POA?
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Postby darryl » Thu Dec 20, 2007 6:06 pm

How are you defining 'occupancy'? What fraction of the wetland has the species of interest, and how that fraction changes through the seasons? Or, is the species in the wetland (yes/no) and does that change?
Darryl
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Postby jlinehan » Fri Dec 21, 2007 9:51 am

We would like to define occupancy as species presence (yes/no) in the wetland and does that change.
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Postby darryl » Fri Dec 21, 2007 5:02 pm

If you're interested in whether the wetland is occupied (yes/no) then the wetland is your sampling unit and you only have a sample size of 1. In this case the only thing you can do with covariates is look at the probability of detection and how it varies. Each trap would just be another survey.

If you're interested in what fraction of the wetland is occupied then you could possibly use the traps as your sampling units. This will perhaps be a more interesting question (because you now have a sample size of 60), but it doesn't sound like it's what you're after.

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