by Whit Gibbons

October 28, 2002

How many kinds of animals are there? The answer is one of the most evasive in conservation ecology. The question has often been asked, but seldom satisfactorily answered even for particular wildlife refuges, national parks, and coastal islands that have distinct boundaries, not to mention the tropics in general or the entire planet. The estimated number of species has ranged from a few million for the world to more than a tenth of a billion just in the oceans.

Biologists disagree on how many animals exist in the world, or even in a defined geographic region, for many reasons. One complicating issue is whether a particular species or other identifiable group of organisms represents one kind or several different kinds.

The endangered Perdido Key beach mouse is an example of how naming animals can become an issue. Surviving populations of the small, sand-burrowing mouse exist naturally in Gulf State Park, Baldwin County, Alabama, and in part of the Gulf Islands National Seashore, Escambia County, Florida. However, the beach mouse belongs to a more common species known as the oldfield mouse that has a much wider geographic range, covering much of the Southeast. So why is anyone worried about this "species"? Why do we need to be concerned about extinction of a species when in most places it is doing fine?

The matter of defining how many kinds of mice are represented by the species becomes even more convoluted. Consider that, in addition to the mainland oldfield mouse and the Perdido Key beach mouse, the species comprises other distinguishable groupings of beach mice, including the Alabama beach mouse and, in Florida, the Santa Rosa, Choctawhatchee, and St. Andrews beach mice. Consequently, even though none of the several populations of beach mice have evolved sufficiently to become separate species from the oldfield mouse, each beach group is distinctive.

Part of the politics of conservation is making a convincing argument that a species can be endangered in one part of its range but not another. After all, if a species is thriving in one region, how can we say the species is endangered? Wouldn't the species be either all right from an environmental perspective or not all right? Fortunately, the Fish and Wildlife Service (FWS) solved that conservation conundrum several years ago by designating some species as officially endangered or threatened in one part of their geographic range although the same species might be doing well elsewhere.

By being recognized as ecologically distinct from the mainland oldfield mouse and less likely to persist, some of the beach mice were given endangered species status by the FWS. Much of beach mouse behavior remains similar to that of mice on the mainland, including a tendency to be primarily nocturnal and to dig underground burrows for protection. Part of the distinction of beach mice when compared to their mainland counterparts is that their pelage is a lighter color, being almost white in some of those from Santa Rosa Island. Presumably this change in coat color is a product of natural selection, in that mice with dark coats on a sandy beach would be more obvious to night predators such as owls. Hence, genes that favored light-colored mice would have a higher probability of being passed down to the next generation than genes that made a mouse dark.

In a philosophical debate about whether beach mice, as members of an otherwise common species, should be protected, the issue arises of how many kinds of mice are represented. Conservationists might argue that beach mice are different kinds of animals from each other and from mainland oldfield mice. An equally convincing argument might be made that all of them are the same kind of animal because they are the same species, although their habitats and environmental requirements differ. The question has no perfect answer from a conservation perspective. Also, it seems unlikely that even mammalogists will ever fully agree on the designations of how many kinds of "oldfield/beach" mice there are, let alone how many kinds of mice exist in the world.

(Next week: How Do We Count Animals We Cannot See?)

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