Cite as: 515 U. S. 687 (1995)
O'Connor, J., concurring
to hypothetical or speculative, death or injury to identifiable protected animals. Second, even setting aside difficult questions of scienter, the regulation's application is limited by ordinary principles of proximate causation, which introduce notions of foreseeability. These limitations, in my view, call into question Palila v. Hawaii Dept. of Land and Natural Resources, 852 F. 2d 1106 (CA9 1988) (Palila II), and with it, many of the applications derided by the dissent. Because there is no need to strike a regulation on a facial challenge out of concern that it is susceptible of erroneous application, however, and because there are many habitat-related circumstances in which the regulation might validly apply, I join the opinion of the Court.
In my view, the regulation is limited by its terms to actions that actually kill or injure individual animals. Justice Scalia disagrees, arguing that the harm regulation "encompasses injury inflicted, not only upon individual animals, but upon populations of the protected species." Post, at 716. At one level, I could not reasonably quarrel with this observation; death to an individual animal always reduces the size of the population in which it lives, and in that sense, "injures" that population. But by its insight, the dissent means something else. Building upon the regulation's use of the word "breeding," Justice Scalia suggests that the regulation facially bars significant habitat modification that actually kills or injures hypothetical animals (or, perhaps more aptly, causes potential additions to the population not to come into being). Because "[i]mpairment of breeding does not 'injure' living creatures," Justice Scalia reasons, the regulation must contemplate application to "a population of animals which would otherwise have maintained or increased its numbers." Post, at 716, 734.
I disagree. As an initial matter, I do not find it as easy as Justice Scalia does to dismiss the notion that significant impairment of breeding injures living creatures. To raze the last remaining ground on which the piping plover cur-
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