I have added the combined PSA, FIAP and RPS nature photography definition here :-
Nature photography is restricted to the use of the photographic process to depict all branches of natural history, except anthropology and archeology, in such a fashion that a well-informed person will be able to identify the subject material and certify its honest presentation. The story telling value of a photograph must be weighed more than the pictorial quality while maintaining high technical quality. Human elements shall not be present, except where those human elements are integral parts of the nature story such as nature subjects, like barn owls or storks, adapted to an environment modified by humans, or where those human elements are in situations depicting natural forces, like hurricanes or tidal waves. Scientific bands, scientific tags or radio collars on wild animals are permissible. Photographs of human created hybrid plants, cultivated plants, feral animals, domestic animals, or mounted specimens are ineligible, as is any form of manipulation that alters the truth of the photographic statement. No techniques that add, relocate, replace, or remove pictorial elements except by cropping are permitted*. Techniques that enhance the presentation of the photograph without changing the nature story or the pictorial content, or without altering the content of the original scene, are permitted including HDR, focus stacking and dodging/burning. Techniques that remove elements added by the camera, such as dust spots, digital noise, and film scratches, are allowed. Stitched images are not permitted. All allowed adjustments must appear natural. Color images can be converted to greyscale monochrome. Infrared images, either direct captures or derivations, are not allowed. Images used in Nature Photography competitions may be divided in two classes: Nature and Wildlife. Images entered in Nature sections meeting the Nature Photography Definition above can have landscapes, geologic formations, weather phenomena, and extant organisms as the primary subject matter. This includes images taken with the subjects in controlled conditions, such as zoos, game farms, botanical gardens, aquariums and any enclosure where the subjects are totally dependent on man for food. Images entered in Wildlife sections meeting the Nature Photography Definition above are further defined as one or more extant zoological or botanical organisms free and unrestrained in a natural or adopted habitat. Landscapes, geologic formations, photographs of zoo or game farm animals or of any extant zoological or botanical species taken under controlled conditions are not eligible in Wildlife sections. Wildlife is not limited to animals, birds and insects. Marine subjects and botanical subjects (including fungi and algae) taken in the wild are suitable wildlife subjects, as are carcasses of extant species. Wildlife images may be entered in Nature sections of Exhibitions.
* I read this to include ANY cloning of parts of the image to remove grass stalks, jessie's etc.