With the use of the only encyclopedic account of endangered species available (4-6), we compiled a database of the 877 American threatened and endangered species listed by the U.S. Fish and Wildlife Service up until 1995 and the causes of their endangerment that have been operational since passage of the Endangered Species Act. We identified 18 causes of endangerment (Table 1).

Table 1. Causes of endangerment for species classified as threatened or endangered by the U.S. Fish and Wildlife Service. Cause Number of species endangered by cause and rank of frequency* Number of species endangered and rank of frequency Interactions with nonnative species 305  - 1 115  - 8 Urbanization 275  - 2 247  - 1 Agriculture 224  - 3 205  - 2 Outdoor recreation and tourism development 186  - 4 148  - 4 Domestic livestock and ranching activities 182  - 5 136  - 6 Reservoirs and other running water diversions 161  - 6 160  - 3 Modified fire regimes and silviculture 144  - 7 83  - 10 Pollution of water, air, or soil 144  - 8 143  - 5 Mineral, gas, oil, and geothermal extraction or exploration 140  - 9 134  - 7 Industrial, institutional, and military activities 131  - 10 81  - 12 Harvest, intentional and incidental 120  - 11 101  - 9 Logging 109  - 12 79  - 13 Road presence, construction, and maintenance 94  - 13 83  - 11 Loss of genetic variability, inbreeding depression, or hybridization 92  - 14 33  - 16 Aquifer depletion, wetland draining or filling 77  - 15 73  - 15 Native species interactions, plant succession 77  - 16 74  - 14 Disease 19  - 17 7  - 18 Vandalism (destruction without harvest) 12  - 18 11  - 17 * Including Hawaiian and Puerto Rican species. Not including Hawaiian and Puerto Rican species.

Most endangered species are endangered by several causes, and it is rarely possible to determine the relative importance of each cause. By the time a species is endangered, however, any loss of individuals is critical, so that the "relativity" of importance loses relevance for any given species. We suggest, therefore, that the importance of a cause to overall species endangerment generally corresponds to the frequency with which it is found to endanger species.

Dobson et al. (1, p. 552) found that "the overall density of endangered species is correlated with one anthropogenic and one climatic variable (correlation coefficient r² = 0.80, P < 0.01): the value of agricultural output and either average temperature or rainfall." Agriculture is a major cause of endangerment, but it is less important than nonnative species and urbanization (7). Furthermore, there is a host of economic activities that greatly exceeds agriculture in importance, in a cumulative sense (Table 1).

The emphasis of Dobson et al. (1) on the concentration of endangered species in "hot spots" tends to discount the fact that species are endangered in all 50 states (8). (Agriculture alone endangers species in 35 states and Puerto Rico.) Many people might welcome the new study without a concomitant care for the species diversity. It offers policymakers living in 47 states an opportunity to skirt the issue by pointing to Hawaii, California, and Florida and claiming that sanctuaries in those states are sufficient. If population size and per capita consumption are not addressed in the policy arena, then accelerated extinctions will clearly proliferate, and human economy will be severely and forcefully adjusted to fit within the limits of its natural capital stocks. Other efforts (for example, assessments of species distribution) may delay economic adjustment from an administrative time perspective, but can only prolong extinction for a blink of evolutionary time.

Brian Czech
Paul R. Krausman
College of Agriculture,
University of Arizona,
Tuscon, AZ 85721-0043, USA

REFERENCES AND NOTES

1. A. P. Dobson, J. P. Rodriguez, W. M. Roberts, D. S. Wilcove, Science 275, 550 (1997) [Abstract/Full Text].
2. U.S. Bureau of the Census, Statistical Abstract of the United States: 1991 (U.S. Government Printing Office, Washington, DC, 1991).
3. J. H. Zar, Biostatistical Analysis (Prentice-Hall, Upper Saddle River, NJ, 1996).
4. D. W. Lowe, J. R. Matthews, C. J. Moseley, The Official World Wildlife Fund Guide to Endangered Species of North America, vols. 1 and 2 (Walton Beacham, Washington, DC, 1990).
5. C. J. Moseley, The Official World Wildlife Fund Guide to Endangered Species of North America, vol. 3 (Walton Beacham, Washington, DC, 1992).
6. W. Beacham, The Official World Wildlife Fund Guide to Endangered Species of North America, vol. 4 (Walton Beacham, Washington, DC, 1994).
7. B. Edmondson, American Demographics 13 (no. 11), 8 (1991). When a minimum of 1000 people/(1.6 km)² reside in a contiguous area with at least 50,000 people, the area is classified by the U.S. Bureau of the Census as urban. Urbanization endangers species by replacing habitat directly and by depleting resources needed to support urban economies.
8. S. E. Auslander, Arizona Daily Star (10 February 1997), p. A12.

Response: It is not unexpected that the results of our stepwise regression analysis (1) do not parallel perfectly Czech and Krausman's ranking of known causes of endangerment, because the focus and the scale of analysis, as well as the categories used in the two methods differ. Nevertheless, for the continental United States, we identified the value of agricultural output as the top anthropogenic predictor of endangered biodiversity, and agriculture ranks just behind urbanization in the table presented by Czech and Krausman. Moreover, an analysis of threats to endangered species using data from the Federal Register (as opposed to the sources cited by Czech and Krausman) reveals that agriculture affects more endangered species than urban development (1). Their statement that other activities have a greater cumulative effect than a single variable like agriculture misses the point of stepwise linear regression, which ranks the predictive power of dependent variables relative to one another, individually (2).

As do previous commentaries on our paper (3), Czech and Krausman imply that we favor a conservation strategy based solely on endangered species hot spots. We recognize that identifying national hot spots is but one component of many strategies that are required to successfully conserve biodiversity. We maintain, however, that this component is an essential one, given the urgency of the problem. Much as we agree that the increasingly consumptive human population of the United States is the root cause of our environmental crisis, it would not be wise to wait for policy-makers to agree on and implement the fundamental changes to the economy and society required to guarantee the long-term survival of endangered species. With funding for endangered species protection increasing at a much slower rate than the number of endangered species (Fig. 1), it would be irresponsible for scientists to stand aloof from the search for pragmatic, real-world strategies that can be applied in the short term, such as the identification of hot spot areas where focused conservation efforts might prevent the impending loss of hundreds, if not thousands, of species.

A. Dobson
J. P. Rodriguez
W. M. Roberts
Department of Ecology and
Department of Evolutionary Biology,
Princeton University, Princeton, NJ 08544-1003, USA
D. S. Wilcove
Environmental Defense Fund,
1875 Connecticut Avenue, NW, Washington, DC 20009, USA

REFERENCES

1. D. S. Wilcove, in preparation.
2. B. S. Csuti et al., Biol. Conserv. 80, 83 (1997).
3. C. P. Dunn et al., Science 276, 513 (1997); D. Ehrenfeld et al., ibid., p. 515; A. Dobson et al., ibid., p. 516. 23 May 1997; accepted 7 July 1997

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