Stress
Ecologists are scientists, which means they're always testing, observing, testing more, then coming up with hypotheses, theories, and scientific models. Often they don't even know what they're looking for until they find it.
Welcome to the uncertainty principle, and to working with it every single day. And not just with your experiments—there are those little things called "grants" you may need to worry about, too. So goes your funding, so goes your paycheck.
That said, aside from worrying about grant money and worrying about the environment, you shouldn't be too stressed as an ecologist.