Adding to Zlatko's fine definition
When I think of "nature writers" I think of writers who involve themselves in the natural world by celebrating it, exploring it, defending it, observing it, and communicating it.
I'm sure, too, that some writers who are tagged with such a label might reject it as being too narrow a scope for their work. Peter Matthiessem for instance, certainly transcends the "nature writer" moniker by producing well-crafted, and in some cases ("Far Tortuga") experimental, fiction. Yet Matthiessen's body of work incorporates some of the finest "nature writing" of the 20th century. To list a few of his works in this genre (the first three concern themselves with Africa):
The Tree Where Man was Born
Blue Meridian: The Search for the Great White Shark
Wildlife in America
The Shore Birds
The Birds of Heaven: Travels with Cranes (2001)
I would suggest that a writer who is ecologically & environmentally concerned, and who produces works that address those concerns (whether in fiction, non-fiction, poetry, etc.), could fall under the rubric.
Gary Snyder comes to mind. Certainly Edward Abbey, John McPhee, perhaps even Jim Harrison (although his swaggering, consumptive approach to life, despite expressed reverence for the natural world, strays beyond the boundries of this genre as we've set them so far).