If we attribute intrinsic value in some degree to other sentient creatures, then the optimal scale of the human niche would be smaller than if only human sentience counted. Investment in natural capital would then have the additional benefit of increasing life-support services to nonhuman species whose enjoyment of life would no longer be counted as zero, though it certainly should not be counted as equal to human life enjoyment. To recognize that a sparrow’s intrinsic value is greater than zero does not negate the fact that a human is worth many sparrows. But not even theology, much less economics, can say how many sparrows are worth a human.
—Herman Daly, Beyond Growth
Schools & Ecosystems 