Being under the gun to save a species from extinction might be considered stressful to some people. Being the one person who can advise a rancher about how to avoid wildfires in the dry, western forest where his 2,000 acres lie may be somewhat nerve-wracking occasionally.
If you really think about it, it's hard to come up with a profession that doesn't offer some modicum of stress. Just making sure that you get up in the morning to make it to work on time can be a source of stress.
But the thing is that you got into this career because you care deeply. And you know that caring deeply about anything is always going to bring up some strife, some worry and yes, some stress. But sometimes knowing that it might be stressful at times can help ease the severity of that stress.