I don't have an exact figure, but I suspect that only a few percent of the area of California is still functional as wildlife habitat for our native species. Obviously, the presence of humans is incompatible with the preservation of all of our biodiversity. The ancestors of New Zealand's Maori people based a culture on hunting moas, but soon made short work of them, both by direct removal and by burning of habitat to clear areas for agriculture. We believe that we own every square inch of the Earth, and have the right to do what we want to it. This is precisely why we are losing, worldwide, dozens of species per day.
Parks are, potentially , one of the most important areas for the preservation of wildlife habitat.
We rejoice whenever we acquire more land and convert it to a park. But if we examine this conversion from the point of view of wildlife, it may not be so rosy. Land that is in private hands may be heavily used, but often it isn't. Without wildlife, a park is nothing but a pile of rocks, which can't hold our attention for more than a few minutes. By far the most interesting part of any park is its wildlife, followed by prehistoric wildlife, prehistoric humans, early humans, native cultures and peoples, and early remnants of our own culture.