That neighborhood kid who is always collecting butterflies and leaves? He just might grow up to be a naturalist, or a scientist who specializes in studying nature.
A biologist whose interest lies primarily in the study of plants or animals can be called a naturalist, although these days it's more likely she'll be called a natural historian, a botanist, or a zoologist. The noun naturalist comes from the Latin word natura, "course of things, natural character, the universe," or literally, "birth." Be careful not to confuse naturalist with naturist, which is another word for a person who prefers to be naked — a nudist.