This study was published several months ago, but I thought it might be of interest: http://www.plantsciences.ucdavis.edu/plantsciences/features/spring2011/yosemite_toads.htm.
I have a connection to this issue through my days working with the California Cattlemen's Association. In the mid 1990s, the Sierra National Forest was sued over its grazing program by several environmental groups. During the course of the litigation, we organized a field trip to the Sierra's grazing allotments that included the Forest Service, the environmental groups and the ranching families that took cows to the mountains. As you might imagine, the discussions were fairly tense at times.
During one of our last stops, we listened to a researcher (who was from Fresno Community College, as I recall) who had been doing surveys of Yosemite toad populations. He had documented declines in their populations, and he was certain that the decline was due to the presence of cattle in their habitat.
While the gentleman was talking, the kids who were on the trip (ranch kids and the children of the environmental activists) were off playing in the meadow. As he wrapped up his talk, they approached us carrying handfuls of small amphibians - Yosemite toads. To the researcher's credit, he admitted that the kids had found more toads than he'd counted in that meadow that year. "Maybe my sampling technique needs improvement," he laughed.
As the report I've linked to above suggests, there is more going on here than a relationship between grazing and wildlife. To me, that's the cool thing about science - we're forced to challenge our own assumptions at times. Yes, poorly managed grazing can have environmental consequences. And yes, well-managed grazing might be beneficial to the environment. On the other hand, sometimes we need to peel away the scientific approach and see the world through the eyes of a kid!