[R-sig-eco] (no subject)

[calenge at biomserv.univ-lyon1.fr]

Hello Roy,

> Could anyone shed any information on why the area of home ranges as
> estimated in adehabitat can be several times higher when the kernelUD \
> kernel.area functions are used compared to estimates from the mcp
> function.  I would have expected them to be roughly the same, rather
> than up to an order of magnitude different.  The feature is obvious even
> in the example puechabon dataset that comes with the adehabitat package.

Because the two methods differ: the MCP computes the smallest polygon
encompassing a given percentage of the relocations, while the kernel method
tries to estimate the bivariate probability density function to relocate the
animal at a given place (the utilization distribution, or UD). The kernel home
range is estimated from the UD as the smallest area on which the probability to
relocate the animal is equal to a given probability (e.g. 0.95, 0.7, etc.).

Because the two methods do not rely on the same mathematical bases they are not
expected to return the same results. Very often, the kernel home-range estimate
is /smaller/ than the MCP (because the MCP may include large areas not actually
used by the animal, while this is not the case for the kernel home range). When
it is higher, it may be because the smoothing parameter h is misspecified (as it
is the case for the "puechabon" dataset). Alternatively, it can be just a
characteristic of your data.

Indeed, the ad-hoc or so-called "reference" method to estimate the smoothing
parameter, i.e. the most widely used method in the ecological literature (and
the default in adehabitat, but it can be changed with the parameter "h" of
kernelUD), tends to overestimate the home-range size, as this method supposes
that the UD is bivariate normal, which is actually rarely the case. There is a
large literature on this issue. You may begin with:

Worton, B. 1995. Using Monte Carlo simulation to evaluate kernel-based home
range estimators, Journal of Wildlife Management, 59, 794-800
Worton, B. 1989. Kernel methods for estimating the utilization distribution in
home range studies. Ecology, 70, 164-168.

Hope this helps,


Clément.

-- 
Clément CALENGE
Office national de la chasse et de la faune sauvage
Saint Benoist - 78610 Auffargis
tel. (33) 01.30.46.54.14