Home Range & Animal Density

Three methods for determining population density from GPS collar data are presented below. Density is usually the number of adult individuals in a given area (adults/km2). It is common in studies of large carnivores to report density in terms of # of adults per 100km2. Your project may involve additional considerations for density (mean annual counts, sex-specific density, etc.), but no matter how you do it, you need to clearly state what you mean by density and show how you calculated it.

Printable page with illustrations here:
Density Methods_SkyeCooley_2014_GIS4Geomorphology.pdf

1.) Study Area Densitydensitymethod1
– If an animal utilizes any portion of the study area, then it is 100% inside the study area. Essentially, you are asking “Is this a study animal or not?”.

– The study area is assumed here to be a single polygon drawn by the research team (a posteriori knowledge). Study areas boundaries for projects involving large mammals are commonly defined by natural geographic features (i.e., large rivers, high divides, watersheds) or coincide with administrative/political/cultural boundaries (i.e., game management units, county lines, freeways).

– You’re not concerned with overlap of one home range with another, only that each home range overlaps with any portion the study area boundary.

– Tools needed:
Attribute table calculation functions
Excel for compilation

 

2.) Proportioned-count Densitydensitymethod2
– Determine the area overlap of each animal’s 95% kernel home range polygon with the study area boundary polygon. The study area is a polygon drawn by the research team (a posteriori knowledge).

– The fraction of the KHR polygon equals the fraction of the animal counted towards density with respect to the Study Area. For example, if 60% of an animal’s KHR polygon overlaps the study area polygon, 0.6 of one animal is counted towards the density calculated for the study area.

– You’re not concerned with overlap of one home range with another, only the % overlap of each with the study area boundary.

– Tools needed:
Intersect tool (ArcToolbox > Analysis > Overlay > Intersect tool)
Editor toolbar (Customize > Toolbars > Editor)
Attribute table calculation functions
Excel for compilation

 

3.) Composite Study Area Densitydensitymethod3
– Compile all home range collar points into a single shapefile (use the Append tool).

– Calculate a KDE surface for all points, using the appropriate h-factor.

– Slice the KDE at 95% to create the kernel home range polygon(s).

– The area represented by this “composite 95% KHR” polygon becomes the new study area. You abandon the study area boundary used in the Study Area and Proportioned-count Density methods and replace it with the composite polygon.

– Tools needed:
Append tool (ArcToolbox > Data Management > General > Append tool)
Attribute table calculation functions
Excel for compilation

 

Refs
H.S. Cooley et al. (2009) Ecology 90
H.S. Cooley (2011) PhD Dissertation, Washington State University

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