[R-sig-Geo] spatially balanced sampling to reduce geo-political bias

[Claire Teeling]

Dear all,

I am carrying out some species distribution modelling based on a database of
species occurrence records of a single tree species, encompassing the entire European
continent. The records are primarily historical and heavily biased towards
western, northern Europe. A few of the counts of records by country are shown
below to illustrate.

CHE     12
CZE     1
DEN     6
DEU     1742
DNK     12
ESP     237
FIN     1
FRA     6536
GBR     3294
GEO     39
GRC     47
HUN     2

I am very new to R and I'm trying to find a way to subsample in order to obtain
a more spatially balanced sample of 300 records, from a total of 16794. I have
looked at some packages, e.g. sp, spcosa, spsurvey, spdep, have searched the
manuals and searched for similar examples.
I have also tried to stratify the data but can't find a stratum which reduces
the impact of the bias. I also have a field containing inclusion probabilities
for each record, based on country.
I just can't seem to work out how best to perform sampling to reduce the effect
of geopolitical bias.
Any advice, for an R novice, would be very gratefully received.

Thanks,

Claire