I got out of the Corps 43 years ago. I started down several career paths before deciding to go into biology. I ended up working 30 years as a fishery biologist, and must say that it was, all and all, a great career. I think the same is true for the guys working in wildlife, parks and to some extent game wardens.
The down sides: 1) You need a Master of Science degree to get a job, so you will need to go to college for at least 6 years. A Bachelors degree might get you a technician job, but many agencies will look past technicians in order to hire a person with a MS. Get the MS, or at least realize that you will eventually have to. 2) If you get an opportunity to work for a government agency for the summer, take it, even if it pays little or nothing. Experience counts. 3) There is lots of competition for jobs. However, since natural resource agencies have lost staff during the recession, when the economy comes back, there should be quite a few jobs. 4) The money isn't as good as many jobs, especially at the start, but it's sure rewarding working in the natural resource field.
Semper Fi, Marines
Casey 1997-2011 RIP