The Focus on Originality
You can only copyright something that is an original work of authorship. In understanding this, two issues are simple: 

• A "work" is a single expression of an idea (and not an idea itself).
• "Authorship" means you thought up the work.

So, the crux is "originality", requiring that the artist added something from his or her own mind (which, then cannot be copied).

Originality in Nature-Based Works
Most of the time, depictions of nature can’t be copyrighted. That is because the law (i.e. society) says that nature is not artistically your own. Only the part of the artistic work that sets nature apart from the way you found it is copyrightable (for instance, your arrangement, positioning, shading, unique coloring, etc.).

Some examples:

• A candle company places cinnamon sticks on the front of a box of cinnamon-scented gifts (Yankee Candle v. Bridgewater Candle, 259 F.3d 25 (1st Cir. 2001)). The company can’t gain protection over putting cinnamon sticks on a gift box because cinnamon sticks are a natural thing. But, the company can get protection over the placement of the cinnamon sticks in a specific pattern and the shading, light source and placement on the box because those were arrangement-based and, thus, original elements.
• A photographer takes a shot of a ballerina’s legs in fifth position (Edwards v. Ruffner, 623 F.Supp. 511 (D.N.Y. 1985)). The photographer can’t get protection over a photograph of a dancer’s legs in fifth position (even if another photographer admits to seeing the photo and using it as inspiration for a similar photo) because even the idea of dancers legs in fifth position is not an original element. But, the photographer may get protection over the angle of the shot, amount of the legs showing, clothing style and background, particularly if there is some unique element to them (like intentionally ripped leg warmers or oddly colored ballet slippers).
   

Putting This into Practice
Drawing from nature is a wonderful start many artists have used to initiate the beginnings of great work. While protections are available, they will likely extend only over those additions. You cannot "corner the market" in making true-to-nature depictions of an idea. So, the trick for the artist, in life and in law, is to add something new.