share: digg facebook twitter (AP) — A grizzly bear killed a man who was hiking with his wife in Yellowstone National Park's backcountry after the couple apparently surprised the female bear and its cubs on Wednesday, park officials said. Details were sketchy but park officials said the bear attacked to defend against a perceived threat. Park officials worked Wednesday to clear the area of people. All trails and backcountry campsites in the area were closed and a warning sign was posted on the trailhead, Yellowstone spokesman Al Nash said. Grizzlies in the Yellowstone region have caused growing problems for people, everything from raiding bird feeders at rural homesteads to the very worst. The big bears require constant vigilance for tourists and park employees alike, said Caleb Platt, a service station manager at Canyon Village who said he has had a handful fairly close encounters with grizzlies while hiking in the park.