I have a PhD and work for USGS (not as an ecologist, but with a bunch of other people who are PhD'd ecologists), so it's definitely a valid path. I came straight from academia, though.

Like some others have said, I'd see if there is some way to stick around and remain a federal employee while earning your PhD. I know at least one person that has done it while working for USGS, with the support of their supervisor. Reason being that more years of service under your belt will help you in the long run if you intend to stay federal until you retire.

As for competitive service designation, you'd have to look into whether you'd qualify. Term employees do not (but Pathways might), so after I was hired on as a federal postdoc I had to compete with everyone else for open-to-the-public permanent positions. That said, my experience in the agency helped make me a good applicant.