Bobcats wouldn't be very much of a threat to adult alpaca. They are just too far over their weight class. A bob' might take a young alpaca, but only if no adults were nearby. An alpaca stomped bobcat wouldn't be a threat to a newborn mouse.
Bears aren't really that efficient as predators. Chasing down and killing large animals isn't their thing. If I saw a bear feeding on an alpaca, my first thought is that it was injured or killed by misadventure or another predator rather than taken by the bear.
I'd be more inclined to worry about coyotes, if I were that alpaca herder. I don't know if there are coyotes in that area, but that is the way to bet. Coyotes are amazingly comfortable around humans. They have been seen in some very urban settings.
Bears and mountain lions are one thing, but a bobcat? An alpaca could and would kick the living sh!t out of any bobcat.