Benchmarks for Managing Ecosystems: Are Human Activities Natural?

For conservation biologists one of the fundamental goals of managing ecosystems is to maintain or restore their natural structure and ftinction (Noss & Cooperrider 1994). In ecological reserves we strive to attain this goal as completely as possible. Outside reserves we accept some compromise in reaching this goal-for example, by designing timber harvests to emulate natural disturbances. In both cases we have to decide on the benchmarks for designing and evaluating our management-in other words, the natural structures and functions that we seek to maintain, restore, and emulate. The most difficult part of this question often involves deciding what is natural. In the context of conservation, "natural" might mean without human influence. Used in this way it is an antonym of adjectives such as "anthropogenic," "cultural," and "artificial." This usage establishes a clear, simple dichotomy based on the idea that humans are a unique species. Some people, however, are uncomfortable with the idea that humans are not natural because they are not making distinctions based on the context in which a word is used. For example, these people might think, incorrectly, that describing humans as natural in contexts such as "she's a natural athlete" means that human effects on ecosystems must also be natural. Even within the context of conservation some people are concerned that recognizing a dichotomy between nature and humanity will perpetuate abuses of nature. Conversely, some other people are primarily interested in being able to extract commodities from ecosystems without their activities being condemned as unnatural. Ambiguities in the definition of "natural" are particularly evident in discussions of benchmarks for managing ecosystems in the Western Hemisphere because there are two distinct periods of human influence, one beginning roughly 12,000 years ago with colonization by Asian people across Beringia and a second beginning about 500 years ago with European colonization. When asked to choose a temporal benchmark for managing ecosystems, most conservation biologists in this hemisphere choose pre-European colonization. This decision implies that European management of ecosystems has not been natural, but that Native American management was natural. One argument that Native American activities (most notably setting fires) were natural emphasizes that species have had a very long time12,000 years-in which to evolve in response to these manipulations. It is true that 12,000 years is 24 times longer than the period of European influence, but this is not a long period in evolutionary time. If we are concerned with the ability of species to evolve in response to human activities, one million years is a more relevant figure. One million years is a rough estimate of the average longevity of a given species (Jenkins 1992) and it suggests that species that have evolved in the western hemisphere will have experienced human influence during only a small portion of their existence. Another argument is that Native American activities were natural because they lived with nature in a more harmonious fashion than Europeans have. Certainly the limited population density and technology of Native Americans mitigated their impact, but consider two points. First, some Native Americans (e.g., Aztecs, Incans, and Mayas) did live in dense populations that must have had profound impacts on local ecosystems. Dense populations probably existed elsewhere (e.g., the southeastern United States; Delcourt et al. 1986), but we tend to overlook these groups because they left few monumental structures or because they were decimated by diseases introduced by the earliest European explorers and had largely disappeared when later settlers arrived. Second, many biologists believe that hunting by Native Americans played a major role in the extinction of over 30 genera of large mammals, such as antelopes, mammoths, horses, and ground sloths (Martin & Klein, 1984). In contrast, the only large mammal extinctions attributable to European colonization (full species, not subspecies) were the loss of the Steller's sea cow and the Caribbean monk seal. In short, whereas the overall ecological impact of Native Americans was much less than that of Europeans, it was significant in certain times and places. How well do these two arguments apply in the East-