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The Listing of a of Species:
M. Lynne Corn 1
December 15, 1992
The 103d Congress will debate the reauthorization of the Endangered Species Act (ESA) 2 which expired on October 1, 1992. The Act has recently generated controversy, even though it passed in 1973 with virtually no opposition. Much of the debate concerns specific actions that would jeopardize particular species or populations. However, the controversy has been fueled by the discrepancies between two sets of legal definitions and the subtle biological realities that they approximate
First, the ESA offers different levels of protection to vertebrates (fish, amphibians, reptiles, birds, and mammals), invertebrates (all other members of the Animal Kingdom), and plants. Some environmentalists have reacted against these differences by pressing for ecosystem protection in cases like the northern spotted owl, and arguing that recoveries could be more successful if all organisms were given equal protection.
Second, the Act defines populations and species without highlighting the subtleties of their distinctions or the difficulty of their determination. As a result, several complicated issues have arisen about the protection of organisms like the marbled murrelet and Florida panther. The distinctions among vertebrates, invertebrates, and plants, and between populations and species will be important issues in the ESA reauthorization debates.
Without biological evidence to justify the distinction among vertebrates, invertebrates, and plants, each category receives different protection under the ESA. Individual vertebrate populations can be listed as well as whole species, and listed vertebrates are protected on both public and private lands. For example, bald eagle populations in Washington, Oregon, Minnesota, Wisconsin, and Michigan are listed as threatened, and populations in the other contiguous 43 states are listed as endangered despite a healthy population in Alaska (1) bald eagles are not listed in Hawaii, an area outside of their natural range).
In contrast, although listed invertebrates are protected on both public and private lands, individual invertebrate populations cannot receive protection. For example, the monarch butterfly occurs in two populations: one migrating to California and the other to Mexico for winter. Although the Mexico-wintering population is declining due to habitat destruction, it cannot receive protection under the ESA as long as the California-wintering population is abundant. The bald eagle, on the other hand, receives protection in all contiguous 48 States despite its numbers in Alaska.
Listed plants receive the least protection, with no protection for populations or on private land without the landowner's consent. Historically, plants on private lands have been considered the property of landowners and therefore not controlled by government. In keeping with tradition, the ESA generally does not prohibit private landowners from damaging or destroying listed plants on their own property. Before its 1988 amendments, the ESA did not even protect listed plants from trespassers on private lands. The Virginia round-leaf birch was brought closer to extinction by trespassers, motivated by fear of government regulation, who purposefully destroyed trees on private lands. With the 1988 amendments, parties breaking other laws at the time they damaged or destroyed a listed plant can be charged using the ESA. However, landowners can still legally destroy listed plant species on their property if they are not breaking other national or state laws in the process.
In contrast to these legal distinctions, biological evidence would justify a reverse ranking in many cases - with plants receiving the most protection and vertebrates the least. Plants, which are less easily dispersed than most animals, supply energy for the rest of the ecosystem by converting the sun's light into a form accessible to animals. Invertebrates often assist in converting this plant energy into a form vertebrates can eat by digesting plant fibers and collecting the energy into concentrated bundles. Illustrating their importance, individual plant species have been found that house over 1,200 animal species, mostly invertebrates. This food chain can be symbolized by a house, with plants serving as the foundation, invertebrates as the walls, and vertebrates as the roof. Though the foundation and walls can stand without the roof, the roof cannot stand without the walls or foundation.
On the other hand, vertebrates can confer their enhanced protection on plants and invertebrates when the distribution of a vertebrate population overlaps significantly with that of a habitat type. Such a vertebrate species is called an indicator species. The northern spotted owl serves as an indicator for old growth forests of the Pacific Northwest. Therefore, protected spotted owl populations confer some of their protection on the other species occurring in old growth forests. This conferred protection may be particularly important because many species, especially invertebrates, have not been discovered or described yet.
Advocates of equal protection argue that even vertebrates would benefit from better protection of plants and invertebrates because many vertebrates, especially indicator species, suffer primarily from habitat destruction. To support their argument, these advocates list three gaps in the conferred protection that vertebrates can give. First, not all ecosystems have indicator vertebrates. Rocky intertidal ecosystems worldwide, for example, lack significant vertebrate presence. Second, conferred protection does not necessarily protect invertebrates or plant populations from targeted activities. Many cacti, for example, are threatened by plant collectors whose actions only damage the cactus population and not the surrounding desert. Therefore, the cacti cannot receive conferred protection from a listed vertebrate unless they are critical to its survival. Finally, conferred protection may under-represent the actual loss when certain actions are exempted, or permitted despite the ESA. For example, other species both described and undescribed) may be harmed by the 13 Bureau of Land Management timber sales that were exempted from the ESA's protection of the northern spotted owl if these sales proceed.
Others use these gaps to argue for ecosystem protection, or an endangered ecosystem act. They argue that ecosystem protection would protect described and undescribed species in ecosystems with and without vertebrate indicators equally. In addition, they argue that ecosystem protection would be simpler for both developers and the Fish and Wildlife Service (or the National Marine Fisheries Service-NMFS) because they would only have to go through one listing or exempting process to determine the fate of an area, rather than a new process for each species in that area. However, ecosystems are even more difficult to describe and categorize than populations, subspecies, and species, although, as the next section will illustrate, the distinctions among populations, subspecies, and species are far from clear.
The ESA treats populations, subspecies, and species as if they are neatly distinct categories. However, nature appears much blurrier to biologists. Populations, subspecies, and species are not static, but change with the ongoing processes of habitat modification and evolution. Therefore, biologists do not expect to always find distinct boundaries among these categories. Just as economists define terms like "poor," "middle income," and "rich," and then struggle to apply them, biologists find it easier to define a population, subspecies, and species than to assign these categories to actual groups in nature.
A population has been defined as "a group of organisms coexisting at the same time and place and capable for the most part of interbreeding" 3 and as a group of conspecific organisms that occupy a more or less well defined geographic region and exhibit reproductive continuity from generation to generation; it is generally presumed that ecological and reproductive interactions are more frequent among these individuals than between them and the members of other populations of the same species. 4
The Fish and Wildlife Service (FWS) has classified populations in both contexts, e.g., by geographic areas (States) in the case of the bald eagle and by reproductive interactions (interbreeding) in the case of the marbled murrelet. Most biologists would agree that the second definition encompasses the concept of populations more fully than the first. In concurrence, the NMFS has recently decided to list Pacific Salmon populations only if they are evolutionarily significant units, defined as "substantially reproductively isolated" and "an important component in the evolutionary legacy of the species". 5 However, the study of interbreeding requires greater sophistication and effort than the study of geographic distributions.
The difficulty in studying interbreeding highlights why some populations are hard to differentiate. Two groups of organisms can range from never interbreeding to nearly complete interbreeding. The marbled murrelet, a puffin-like bird, was proposed for listing as threatened in Washington, Oregon, and California. Its listing has been complicated by debate about whether the Washington State birds interbreed with more abundant populations in Canada and Alaska. It has been difficult to assess the degree of interbreeding because these birds move long distances and reproduce discretely high up in the canopy of old growth forests. Even when the degree of interbreeding is understood, it might be low but not trivial. In such a case, it could be a judgement call as to whether the birds from Washington, Canada, and Alaska were separate populations, and therefore, whether the Washington State birds could be listed.
Differentiating between subspecies also has led to controversy. The ESA defines a species as "any subspecies of fish or wildlife or plants." 6 In contrast, biologists consider subspecies and species to be separate categories. They divide a species into two or more sister subspecies if each occupies a distinct location in time and space, and varies in some important characteristic from its sisters.
When two distinct subspecies interbreed, their offspring is called a hybrid. Since the biological definition of species (see below) requires that each species is reproductively isolated, natural hybrids are not possible between true species (although they can occur between true species in certain artificial situations). Si nce sister subspecies are members of the same species, they are by definition reproductively compatible and hybrids between them, though unusual, are natural. While artificial hybridization between true species can produce bizarre and unnatural mixes, hybridization between subspecies can be the only hope for some endangered subspecies. Groups of organisms require genetic variability to be able to respond to environmental changes, even over short time-scales. If a group becomes too small and does not receive genetic input from another source, its chances of recovery are significantly decreased. In these cases, hybridization can add genetic variability and increase the chances of recovery.
For example, today's Florida panther has descended from two subspecies, the remnants of a North American population and a small number of panthers from South America that were brought to Florida and released. The Florida panther population is quite small (under 50) and shows signs of low genetic variability, even with the two types of ancestors. As biologists would predict, the panthers that are most hybridized show the most vigor, while the panthers with little or no South American heritage have impaired ability to reproduce. The Florida panther will be more likely to recover because of the introduction of South American panthers, and other endangered subspecies may require increases in their genetic variability via intentional hybridization with sister subspecies in order to recover. Therefore, small subspecies with low genetic variability face the paradox of loss of protection if they are hybridized, and greater chances of extinction if they are not. Although the ESA definition protects unique subspecies, it is unclear whether hybrids should be granted the same protection; the FWS policy excluding hybrids from protection has been controversial.
Finally, biologists even have difficulty distinguishing between one true species and another. Definitions range from
The definitions have two similarities: that a species is a group of organisms that actually or potentially interbreeds under natural conditions, and that the concept is difficult to define. How can it be so difficult to differentiate between two species?
First, surprisingly few species have been studied in nature. According to Malcolm Hunter, Jr., "It is only for a tiny portion of species that we have more than a vague notion of their abundance and distribution and ecological roles." 9 Given this constraint, it is difficult to assess whether two organisms can potentially or do actually interbreed.
Second, the boundaries between species, like populations, vary in their clarity. Some species, like African elephants, cannot produce viable offspring with any other. Other species, such as many ducks, can produce viable offspring with other species of duck in captivity, but do not do so in nature due to differences in courtship behavior, With these sometimes subtle barriers against interbreeding, it can be challenging to distinguish one species from another.
In sum, the ESA's preference for clear distinctions between populations, subspecies, and species clashes frequently with biology's necessarily blurry categories.
Living organisms are not treated equally in the ESA. Vertebrates are given first class status, invertebrates second class, and plants third class. These rankings, however, may be the reverse of their contributions to many ecosystems. Although indicator vertebrates can serve as umbrellas to both described and undescribed species, they leave several gaps in their protection of co-existing species. While some interests have used these gaps to support ecosystem protection, ecosystems would be even more difficult to define than populations, subspecies, and species.
Populations, subspecies, and species all describe regions on the spectrum of interbreeding, from nearly complete interbreeding (a single population) to never interbreeding (separate species). In contrast to legal definitions that draw clear boundaries between these categories, the actual boundaries are blurry and made even more hazy by the lack of information on most organisms. As a result, many proposed listings will continue to be complicated by debates over population structure and interbreeding. In addition, the use of hybridization as a conservation tool will undoubtedly continue to promote controversy. Therefore, it is crucial that Members of Congress understand the biological evidence both for and against current policy and the subtle distinctions that biologists must measure.
1 Under the supervision of M. Lynne Corn, Joshua P. Nowlis, doctoral candidate at Cornell University, researched and wrote the final draft of the report.
2 Act of December 28, 1973, Pub. L. No.93-205 (as amended), 87 Stat. 884, codified at 16 U.S.C. 1531 et seq.
3 Paul Colinvaux. Ecology. New York. John Wiley & Sons. 1986. p. 134.
4 Dougias J Futuyma. Evolutonary Biology. Sunderland, Mass. Sinauer Associates, Inc. 1979. p.506.
5 56 FR 58612.
6 ESA, Sec. 3 (15).
7 p. 631 in Paul R. Ehrlich and Jonathan Roughgarden, The Science of Ecology. Macmillan Publishing Co. New York. 1987. 710 p.
8 p. 507 in Douglas J. Futuyma, Evolutionary Biology. Sinauer Associates, Inc. Sunderland, Mass. 1979. 565 p.
9 Malcolm L. Hunter, Jr. "Coping with ignorance: the coarse-filter strategy for maintaining biodiversity." In Balancing on the Brink of Extinction. Kathryn A. Kohm, ed. Washington. Island Press. 1991. p.267.
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