You definitely don't need a PhD, or any other qualification for that matter, to work in conservation. But, for some, it can be a great way in.
A while ago I wrote an article about some of the good things zoos do for conservation. Now I hadn't realized how vehemently some people hated zoos (and sadly, how afraid many great zoos are about sticking up for their work). Suddenly, it became one of my most read articles.