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economists have trouble with decisions where one of the options cannot be reversed. This is an especially important problem for biodiversity. If we decide to have a dam and give up a species, blowing up the dam won’t bring the species back.
Second, we are forced to make present decisions under conditions of uncertainty—another problem for assigning present values. Our ignorance of species is mind-boggling. Suppose you’re walking on a hillside in Mexico. Your eyes fall on a few tufts of nondescript grass. Would you guess that grass is worth $6.82 billion annually? Only if you knew that it was a member of the corn family, that it is a perennial, that…, and so on. Scientists believe that they have identified and named approximately 15% of the species on Earth (Myers, 1979), and we have rudimentary knowledge of the life characteristics of only a few of them. It is an understatement to refer to this level of ignorance as mere “uncertainty.”
A third problem with the divide-and-conquer method derives from ecological knowledge. Species do not exist independently; they have coevolved in ecosystems on which they depend. This means that each individual species depends on some set of other species for its continued existence. A species may depend on just one other species for food, or it may depend on an entire complex of interrelated species. This seems to imply that if we now take actions that cause the extinction of any species, then the loss in future benefits should include losses accruing if any other dependent species succumbs as well. Species on which others depend therefore have contributory value in addition to their direct uses (Norton, in press). To extinguish a species on which two other species depend is to extinguish three species. Thus to get the full value of a species, we would somehow have to determine the values of all the other species that depend on it.
It also appears that some species are keystones in their ecosystems. For example, when the Florida alligator populations dipped dangerously low about 15 years ago, wildlife biologists noticed that many populations of other species also declined. During the dry winters in the Florida Everglades, other species depended on alligator wallows as their source of water (Taylor, 1986). Must we say then, that the value of the alligator includes the value of most of the wildlife in the Everglades?
In principle, these ecological facts add no complication. We need only factor in the ecological information regarding the interdependencies among species in ecosystems. Then, we could tally the direct uses and option values of a species and add to this the uses and option values of all dependent species, and so forth. But, of all the areas of biology and ecology, few are less understood than interspecific dependencies. Ecologists cannot even identify all the interdependencies in the systems they understand best. There is no hope that sufficient information will become available for us to determine the interdependencies in tropical forest ecosystems before the forests are destroyed.
Aside from all these problems, the divide-and-conquer method is not even asking the right question. The value of biological diversity is more than the sum of its parts. Even if we could place a value on the biological diversity represented by all species, we would be only part way to an answer to the question, “What is the value of biodiversity?” To answer that question, we would have to include also the genetic variation within species across populations and the variety of interrelationships in which species exist in different ecosystems.