That's why it pays to diversify, not just your stock, but your skills-set too.
I've got experience in mainframes, Unix, PC development, telecomm, and Web development. It just happens that the IT field as a whole took a hit, and I never got around to getting my electrician's license. There is only so many hours in the week, and spending too much time diversifying means that you don't have time left to maintain your skills in your area of specialty
What happens to you if the biotech field as a whole suddenly goes south, with China and India deciding that it's the "next big thing" and cranking out a couple of million grads?