Endangered Species and Peripheral Populations: Cause for Reflection
Peterson, A. Townsend, Endangered Species Update
Endangered species lists constitute critical foci of conservation attention. Species on such lists are given special attention in prioritizations for conservation (Peterson et al. 2000), with the Endangered Species Act affording immediate protection to areas known to hold populations of endangered species. Hence, decisions regarding "endangered" status of species have profound effects on conservation action (Tear et al. 1995).
Oddly, though, endangered species lists seem to be assembled with little attention to the biology of species involved. The purpose of this commentary is to point out that many "endangered" species in the United States are actually peripheral populations when the entire range of the species is considered (Godown and Peterson 2000). Such populations are often not viable populations to begin with (Holt and Gaines 1992), constituting population "sinks," and indeed are often species of little conservation concern in the main portion of their geographic distributions. Inclusion of these species in status lists dilutes the effectiveness of endangered species legislation and conservation action, and changes geographic foci of endangered species richness (Godown and Peterson 2000).
Endangered species lists
The U.S. endangered species list includes 44 avian taxa (excluding Hawaii). Of these taxa, 19 are endemic or nearly endemic to the United States, such as the California Condor (Gymnogyps californianus), Mississippi Sandhill Crane (Grus canadensis pulla), Attwater's Greater Prairie-chicken (Tympanuchus cupido attwateri), and Florida Scrub-Jay (Aphelocoma coerulescens). These taxa are clearly appropriate for inclusion in the list, given that conservation efforts will either prove successful in the United States, or the taxon will be lost to extinction. Another 19 of the endangered bird species, while not endemic to the United States, have substantial populations in the country that can be an appropriate focus of conservation efforts.
Six avian taxa on the list, however, are represented in the United States only by peripheral populations when the entire range of the species is considered: Masked Bobwhite (Colinus virginianus ridgwayi), Audubon's Crested Caracara (Polyborus plancus audubonii), Northern Aplomado Falcon (Falco femoralis septentrionalis), Thick-billed parrot (Rhynchopsitta pachyrhyncha), Cactus Ferruginous Pygmy-Owl (Glaucidium brasilianum cactorum), and Wood Stork (Mycteria americana). Some of these species are indeed in global peril of extinction (Masked Bobwhite), and perhaps the attention that they might receive in the United States is worth the effort. However, others are species that are quite common south of the U.S.-Mexico border. For example, the Ferruginous Pygmy-Owl is a common resident of tropical forests, the Crested Caracara is a common resident of open and disturbed habitats, and the Wood Stork is a frequent species of open wetlands; all three are found broadly throughout the tropical Americas.
These peripheral populations lie at the limits of species' ranges, where they are often not viable in the long term. Hence, whereas extinction of the U.S. populations of Northern Aplomado Falcon would be regrettable, they always have been marginal, and as such likely have a tenuous hold on long-term survival. Worse still, for species such as the Crested Caracara and the Ferruginous Pygmy-Owl, although U.S. populations are limited in distribution, the species is abundant, and far from threatened with extinction, just a few hundred kilometers south in Mexico.
State endangered species lists are similarly of curious composition. Some states have very clear and straightforward lists: Michigan, for example, includes Peregrine Falcon, Bald Eagle (Haliaeetus leucocephalus), and Piping Plover (Charadrius melodus), all of which are represented by breeding populations in the state, as well as Kirtland's Warbler (Dendroica kirtlandii), for all intents and purposes endemic to the state as a breeding species; 18 other states have similarly straightforward endangered species lists. …