[R-sig-eco] repeated measures

[Steve_Friedman at nps.gov]

Hi,
This is a question regarding technique rather than an R specific issue.  I
have been asked to evaluate a 30+ year long term continuous survey of bird
presence/absence data that has an associated ocular estimate of the
vegetation community percent coverage.  The data are organized by
subpopulations (5), and by year ( 1991 - present).  We are interested in
gaining understanding on whether bird presence is 1) related to vegetation
community cover and 2) whether that relationship is variable over the
period of record.  Ultimately, I can associate management to this, but that
data has not be pulled together yet for this survey.

My first thought was to simply look to a clustering (vegetation conditions)
and nmds  (veg condition and bird presence/absence) to examine which sites
were more similar given the vegetation community. This analysis would help
us understand which sites were similar/dissimilar from one another.
Following this I had intended to repeat that analysis using year as a
grouping variable to see if the clusters change. This would then provide
the basis for a trajectory analysis.

Ok, so that might be rather very simple, I'm now considering a repeated
measures approach, but do not have much experience here.  Since every site
is the same in all years, what changes is the bird count and perhaps the
vegetation community coverage estimates.

Can someone let me know if this alternate approach is a better approach and
how to conduct the analysis in R?  If not what methodology do you suggest?

Thanks very much.  If additional information is needed I can provide it,
but I didn't want to fill mail boxes with unnecessary info at this time.

Thanks
Steve

Steve Friedman Ph. D.
Spatial Statistical Analyst
Everglades and Dry Tortugas National Park
950 N Krome Ave (3rd Floor)
Homestead, Florida 33034

Steve_Friedman at nps.gov
Office (305) 224 - 4282
Fax     (305) 224 - 4147