According to the Photographic Society of America (PSA)
Nature photography records all branches of natural history except anthropology and archaeology. This includes all aspects of the physical world, both animate and inanimate, that have not been made or modified by humans.
The most important part of a Nature image is the nature story it tells. High technical standards are expected, and the image must look natural.
Objects created by humans, and evidence of human activity, are allowed in Nature images only when they are a necessary part of the Nature story.