Should population ecology be also part of this this site?
Yes. Population ecology is pretty easy to argue as a "subset" of Biology generally, and is honestly probably the best home for it. Highly sub-divided smaller sites, beyond having trouble getting through the Area 51 process in the first place, are going to suffer from low traffic.
Odds are "Population ecology" as a field is best represented from a mix of questions in Biology, CrossValidated, Math, and Computational Science, depending on the nature of the question.