I can think of a couple of wildlife photographers who make a very good income, Thomas Mangelson and Art Wolfe. Mangelson has made an industry out of his "brand name" photography with several shops selling his prints and Wolfe, also a "name brand," does books and photo tours that are highly popular. Both established themselves years ago, pre-digital, when stock photography paid well and the files of stock agencies were not filled with hundreds or thousands of images of every animal on the planet. Stock images twenty years ago could be licensed and re-licensed for fees in magazines and books of $100 to $200 upward. Now, with fewer magazines and images mostly used on the internet, those same images bring in only a few dollars each, if anything at all. With digital cameras in hand, hundreds of amateur wildlife photographers shoot amazing photos and are willing to pay thousands to get to the best locations for wildlife shooting. That may be the wildlife photography market that could still be tapped into, assuming you can establish a name as a knowledgable wildlife photography guide/tour operator.