Is there a website that shows population sizes?
You can access the Imperial College global population dynamics database.  They will have time series data at specific locations.  http://www3.imperial.ac.uk/cpb/databases/gpdd
There is a sister database as well that might be useful. http://lits.bio.ic.ac.uk:8080/litsproject/

These contain several hundred time series, and you can see a paper that used them here:  http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2011.01702.x/abstract