Mask Buffers and Density

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

Mask Buffers and Density

Postby ctlamb » Sat Jul 23, 2016 4:40 pm

Hello,

Quick Q regarding buffers around traps and resulting density. We know our animals range beyond the extent of our traps (red crosses) thus we integrate over an area buffered by ~25 km around the traps, excluding lakes and other features that pose significant barriers (mask shown in grey, lakes outlines in black). My understanding of the buffer is that it simply must be large enough to encompass all regions of non-zero detection probability for the animals detected in the sample. Although I believe we have accurately done that, I am curious about what happens when we have good evidence to suggest that no animals have home range centres within the buffer zone of the mask, despite individuals occasionally roaming into this area, whose home ranges are almost certainly within the trap grid. For example, the area outlined in blue is occupied by animals from our sample, but rapidly becomes unsuitable habitat beyond the western edge of our traps and thus is very unlikely to encompass home range centres, rather this area is only used by individuals that have home range centres on the trap grid who seem to wander west at times.

Image





Therefore, when a poisson point distribution is fit to the data (HR centres as pink dots), is it fit as in A, or B in the figure below? As again, we know animals occupy the blue zone, so we do not expect non-zero detection, but do not expect home range centres in this area, thus A would be incorrect for our purposes and we would hope to modify our models to accommodate.
Image

Cheers,
CL
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Re: Mask Buffers and Density

Postby murray.efford » Sat Jul 23, 2016 8:57 pm

Hi Clayton
The mask defines the area potentially occupied by home-range centres. It does not define the area in which animals roam. So the answer to your question is 'A'. Probably that means you need to construct a more restricted mask to represent your population.
Murray
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Re: Mask Buffers and Density

Postby ctlamb » Sat Jul 23, 2016 9:01 pm

Thanks for clearing this up, Murray. Much appreciated. :D

Clayton
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