Technical Comments

Distribution and Causation of Species Endangerment in the United States

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Science  22 Aug 1997:
Vol. 277, Issue 5329, pp. 1116-1117
DOI: 10.1126/science.277.5329.1116

A. Dobson et al. (1) provide a description of the geographic distribution of endangered species in the United States. They also examine the associations between the density of endangered species and the intensity of human economic activities, with the use of the annual statistical survey of the United States (2). Their effort (1) was too abbreviated for prudent policy implications. The statistical survey of the United States does not provide data on all economic activity, and it says nothing of endangerment causation. Extrapolating correlation to causation is fraught with assumption (3).

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 (Table1).

Table 1

Causes of endangerment for species classified as threatened or endangered by the U.S. Fish and Wildlife Service.

View this table:

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.


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. 11), 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.

Figure 1

Funds allocated for the U.S. Endangered Species Act (ESA) and number of species listed as endangered. Funds in millions of constant 1976 dollars per fiscal year. Number of species listed as endangered per calendar year are current through 31 October 1996. Source: U.S. Fish and Wildlife Service.

Editor's note: Clemett and Zare's mention of nonexistent correspondence resulted from a confusing presentation of materials sent to them by Science. It should have been omitted from their response before publication, and the subsequent correction was also incorrect. Science regrets the two errors.


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