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.
Even if it should have a separate site, it is better to start on Biology and spawn the site later -- this way the ecological community on SE will be larger and more active (this is more less the way Astronomy left Physics).
There's a proposal for an Ecology site now. I think there's some argument for dividing out this field, because the communities are pretty distinct in the research world. It may make sense to migrate questions tagged ecology, population biology, etc., to there.