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Quantifying Threats to Imperiled Species in the United States Author(S): David S

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Quantifying Threats to Imperiled Species in the United States Author(S): David S Quantifying Threats to Imperiled Species in the United States Author(s): David S. Wilcove, David Rothstein, Jason Dubow, Ali Phillips, Elizabeth Losos Reviewed work(s): Source: BioScience, Vol. 48, No. 8 (Aug., 1998), pp. 607-615 Published by: University of California Press on behalf of the American Institute of Biological Sciences Stable URL: http://www.jstor.org/stable/1313420 . Accessed: 22/03/2012 16:18 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. University of California Press and American Institute of Biological Sciences are collaborating with JSTOR to digitize, preserve and extend access to BioScience. http://www.jstor.org QuantifyingThreats to Imperiled Species in the United States Assessing the relative importance of habitat destruction, alien species, pollution, overexploitation,and disease David S. Wilcove, David Rothstein,Jason Dubow, Ali Phillips,and ElizabethLosos iologists are nearlyunanimous fine-scale analysis of the types of in their belief that humanity is habitat destruction affecting US in the process of extirpating a Habitat loss is the plants and animals protected under significant portion of the earth's spe- threat the federal Endangered Species Act cies. The ways in which we are doing single greatest (ESA). We also speculate on how so reflect the magnitude and scale of to biodiversity, followed these threats have changed over time human enterprise. Everything from and are likely to change in the future. highway construction to cattle ranch- by the spread of We conclude with a brief discussion ing to leaky bait buckets has been of the implications of our findings implicated in the demise or endan- alien species for the long-term protection of im- germent of particular species. Ac- periled species in the United States. cording to Wilson (1992), most of these activities fall into four major passed by them-is responsible for An overview of the threats categories, which he terms "the mind- endangering species. In general, sci- less horsemen of the environmental entists agree that habitat destruction To obtain an overview of the threats apocalypse": overexploitation, habi- is currently the primary lethal agent to biodiversity in the United States, tat destruction, the introduction of (Ehrlich 1988, Wilson 1992), fol- we tabulated the number of species non-native (alien) species, and the lowed by the spread of alien species threatened by five categories of spread of diseases carried by alien (Wilson 1992). However, apart from threats: habitat destruction, the species. To these categories may be several notable exceptions-includ- spread of alien species, overharvest, added a fifth, pollution, although it ing studies of North American fishes pollution (including siltation), and can also be considered a form of by Williams et al. (1989), endan- disease (caused by either alien or habitat destruction. gered plants and animals in the native pathogens). We restricted this Surprisingly, there have been rela- United States by Flather et al. (1994, coarse-scale analysis to imperiled tively few analyses of the extent to 1998), aquatic organisms by Richter plants and animals occurring within which each of these factors-much et al. (1997), and imperiled birds by the 50 states and falling into any of less the more specific deeds encom- Collar et al. (1994)-few quantita- four categories: all full species of tive studies of threats to have David S. Wilcove is a senior at species mammals, birds, reptiles, amphib- ecologist been conducted. More such studies and fish with status ranks of the EnvironmentalDefense Fund, Wash- ians, DC 20009. David Rothstein re- are needed to provide conservation- "possibly extinct," "critically imper- ington, as ceivedhis J.D. in 1997 fromNortheastern ists, land stewards, and decision iled," or "imperiled," determined UniversityLaw School, Boston, MA 02115 makers with a better understanding by The Nature Conservancy (TNC) and now works for the federal govern- of the relationships between specific in association with the Network of ment. Jason Dubow is an environmental human activities and the loss of Natural Heritage Programs and Con- analyst at the Eastern Research Group, biodiversity. servation Data Centers (Master Arlington, VA 22201; he formerly worked In this article, we quantify the 1991); all full species of freshwater for The Nature Conservancy.Ali Phillips extent to which various human ac- mussels, butterflies and skippers, ti- is a natural resources at The specialist tivities are imperiling plant and ani- ger beetles, and dragonflies and dam- WildernessSociety, Denver, CO 80221. mal in the United States. Our selflies with status ranks of ElizabethLosos is directorof the Center species possibly for Forest Sciences at the analysis has two parts: a coarse-scale extinct, critically imperiled, or im- Tropical examination of the numbers and as determined all Smithsonian Tropical Research Institute, periled, by TNC; of US the full of vascular with Washington, DC 20560. ? 1998 Ameri- types species imperiled by species plants can Instituteof BiologicalSciences. major categories of threats, and a status ranks of possibly extinct or August 1998 607 Table 1. Taxonomic breakdown of species used in the coarse-scale analysis. Included possible to assign a particular hu- are species classified as imperiled by The Nature Conservancy and all species, subspe- man activity to one of the major threat cies, and populations that, as of January 1996, are listed as endangered or threatened categories; we excluded these activi- under the Act or have been Endangered Species formally proposed for listing. ties from our coarse-scale analysis. We were able to obtain informa- Number of of Percentage tion on threats for 1880 of the Number of imperiled species imperiled species (75%) imperiled species with threats data with threats data 2490 imperiled species, subspecies, and that met our crite- Vertebrates 541 494 91 populations Mammals 88 85 97 ria for inclusion in this study (Table Birds 101 98 97 1). (For 52 of the species, we could Reptiles 40 38 95 not identify any anthropogenic Amphibians 69 60 87 threats.) We used the resulting data- Fishes 243 213 88 base to determine the relative signifi- Invertebrates 471 331 70 cance of the major threats categories Dragonflies and 33 18 54 and to investigate differences be- damselflies tween species groups in their vulner- Freshwatermussels 150 102 68 ability to particular threats. We com- Crayfish 110 67 61 the distribution of threats Tiger beetles 8 6 75 pared Butterfliesand 46 33 72 among plants and animals, among skippers vertebrate and invertebrate animals, Other invertebrates 124 104 84 and within vertebrate classes. We also the distribution of Plants 1478 1055 71 compared threats among terrestrial and aquatic Total 2490 1880 75 species, Hawaiian and mainland vas- cular plants, and Hawaiian and main- land birds. For all comparisons, sta- critically imperiled, as determined nated as threatened or endangered tistical significance was assessed by TNC; and all species, subspecies, under the ESA), a survey of biolo- using the chi-squared contingency or vertebrate populations listed by gists conducted by Richter et al. test (two-tailed). the US Fish and Wildlife Service (1997) for aquatic species, the Natu- We emphasize at the outset that (USFWS) or the National Marine ral Heritage Central Databases man- there are some important limitations Fisheries Service as threatened or aged by TNC, and interviews with to the data we used. The attribution endangered or officially proposed for specialists in particular species of a specific threat to a species is listing under the ESA as of 1 January groups and geographical regions. We usually based on the judgment of an 1996. (The ESA permits the listing of included only known threats and ex- expert source, such as a USFWS em- species and subspecies of plants and cluded potential or hypothetical ones. ployee who prepares a listing notice animals as well as "distinct popula- We did not attempt to distinguish or a state Fish and Game employee tion segments" of vertebrates.) A between ongoing and historical who monitors endangered species in total of 2490 imperiled species, sub- threats, partly because such infor- a given region. Their evaluation of species, and populations fit these mation is usually lacking and partly the threats facing that species may criteria. because the distinction itself is prob- not be based on experimental evi- Information on the threats to each lematic in the case of habitat de- dence or even on quantitative data. of these species, subspecies, and struction. Nor did we try to distin- Indeed, such data often do not exist. populations was obtained from a guish between major and minor With respect to species listed under number of sources, including the threats to each species because such the ESA, Easter-Pilcher (1996) has Federal Register (i.e., the listing no- information was not consistently shown that many listing notices lack tices published for all species desig- available. In a few cases, it was im- important biological information, Table 2. Percentages
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