. "6 How Many Tree Species Are There in the Amazon and How Many of Them Will Go Extinct?--STEPHEN P. HUBBELL, FANGLIANG HE, RICHARD CONDIT, LUIS BORDA-DE-ÁGUA, JAMES KELLNER, and HANS TER STEEGE." In the Light of Evolution, Volume II: Biodiversity and Extinction. Washington, DC: The National Academies Press, 2008.
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In the Light of Evolution: Volume II—Biodiversity and Extinction
als, calculated range sizes were smaller than a single pixel, and each of these was assigned to a single pixel of a single land-use category. We then computed the fraction of the species range that was within each land-use category. Range areas were originally calculated as circles whose radii were the mean distances to the nth nearest neighbor, but in our simulations, we allowed the compass orientation and shape to vary from circles to ellipses up to a maximum aspect ratio of 4:1 for species whose ranges exceeded 100 km2 (minimum pixel size), with the long axis at a random angle with respect to north. The relative sizes of the Brazilian Amazon and some of the larger species range sizes are shown in Fig. 6.6c on the same scale as the maps in Fig. 6.6a and b.
With regard to the second problem—the responses of species to the land-use categories—we have run three extinction scenarios, the first of which we believe is most likely. The first scenario obeys a middle-of-the-road conservative rule of the three extinction scenarios—conservative in the magnitude of predicted extinction rates. The rule is that a species goes extinct if, and only if, its range lies entirely in heavy-impact areas. Of the four land-use categories, the heavy-impact areas, are most likely to result in tree extinctions among species restricted to these areas because they have lost virtually all of their primary forest cover, and what forest remains is in very small, isolated, and highly disturbed remnants.
Although speculative, we think is it likely that the other land-use categories will have minimal impact on elevating species extinction above background rates. Despite the visually alarming appearance of the nonoptimistic scenario map (Fig. 6.6b), the descriptions of the land-use categories in online supplementary material to the paper by Laurance et al. (2001) do not describe impacts that are likely to cause many, if any, tree-species extinctions, in our opinion. For example, light-impact areas still retain nearly intact primary forest cover (>95%) but can “experience illegal gold-mining, small-scale farming, hunting, hand-logging, and nontimber resource extraction (e.g., rubber-tapping).” Even moderate-impact areas still have mostly intact primary forest cover (>85%) but “contain localized forest clearings and some roads, and may be affected by logging, mining, hunting, and oil and gas exploration.” The fourth land-use category, pristine, is, by definition, the reference or “natural” state in which extinction occurs at background rates. These areas are described as having “fully intact primary-forest cover and are free from anthropogenic impacts aside from limited hunting, fishing, and swidden farming by traditional indigenous communities.”
One can, however, erect a plausible second extinction scenario in which species could go extinct in moderate-impact areas at a higher than background rate, and even in light-impact areas. This is potentially the least conservative scenario, the one that predicts the most extinctions.