[R-sig-eco] adehabitat: GNESFA - first and last axes

[Maria Zwart]

Dear list,

I am looking at habitat selection in chaffinches with point data (type I study).
I am trying to do the GNESFA (package adehabitatHS). All goes well up to where the function asks to choose the number of first and last axes. Although there is some information about this in the supplied manual supplied by the package (see below), my knowledge of the function is not enough to be able to understand which axes to choose.

"The first axes correspond to the directions where the utilization distribution as a whole is the furthest from the centroid of the availability distribution (these directions often correspond to directions where the marginality is strong; i.e. the criterion y1 defined previously is maximized). The last axes correspond to the directions where the width of the utilization distribution is the smallest relative to the width of the availability distribution (these directions often correspond to directions where the specialization is strong, i.e. the criterion y1 defined previously is minimized)."

Attached are the eigenvalues plots that I get. My interpertation is that for the first axes I have to choose 3, because after that there is a huge drop. And for the last axes I choose 16, because there is an increase there. The results indicate that they select coniferous woodland and mixed woodland, which makes sense since chaffinches are usually found in those habitats. Any advice is greatly appreciated!

Code:
cp <- count.points(locs, towlaw_spdf) #locs are the coordinates of the locations where the chaffinches are. towlaw_spdf is a SpatialPixelsDataFrame, a map with the different habitat variables.
tab <- slot(towlaw_spdf, "data")
pr <- slot(cp, "data")[, 1]
histniche(tab, pr)
pc <- dudi.pca(tab, scannf = FALSE)
gn <- gnesfa(pc, Focus = pr)

Many thanks,

Mieke
Maria Zwart
PhD student
Room 4.65
School of Biology
Ridley 2
Newcastle University
Newcastle upon Tyne
NE1 7RU
United Kingdom