. "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
with a stem diameter of >1 cm DBH, with slope and intercept adjusted for species abundance n (Fig. 6.4e and f ).
ESTIMATING TREE SPECIES EXTINCTIONRISK IN THE BRAZILIAN AMAZON
The area covered by tropical forest in the Brazilian Amazon is still very large, but, partly because it is so large, Brazil is also suffering the highest absolute rate of deforestation of any tropical country in the world. Between 1990 and 1994, the mean annual deforestation rate in the Brazilian Amazon was 1.37 million ha·yr−1, which increased 61% to 2.20 million ha·yr−1 a decade later in 2000–2004 (Laurance et al., 2004). This rate of forest loss is equivalent to clearing an area the combined size of the states of Connecticut and Delaware every year. This clearing represents ≈0.43% of the total surface area on the Amazon, not correcting for nonforest area in rivers, lakes, and already deforested portions of Amazonia. When such corrections are applied, conservative estimates of the current rate of deforestation in the Brazilian Amazon are ≈0.7%·yr−1.
What is the actual risk of extinction of Amazonian tree species posed by this deforestation in the near term, i.e., over the next several decades? We can now attempt to answer this question, at least to a first approximation, by confronting our calculations of relative species abundance and range sizes with maps of projected loss of forest cover in the Amazon. Detailed maps produced by Laurance et al. (2001) consist of two graphical scenarios of the future of the Brazilian Amazon. One scenario they considered “optimistic” (Fig. 6.6a) and the other “nonoptimistic” (Fig. 6.6b). They evaluated current and pending road-building projects, agricultural development and urbanization, logging, and mining, and then they classified land use into four categories: “heavy-impact areas,” “moderate-impact areas,” “light-impact areas,” and “pristine areas.” There is a marked increase in the percentage of area in those four categories of impact, in going from the optimistic to nonoptimistic scenarios. The percentages of area in the four land-use categories under the optimistic scenario were 36.7%, 16.1%, 23.1%, and 24.1%, respectively. Under the nonoptimistic scenario, however, these percentages become: 49.4%, 25.4%, 21.0%, and 4.2%, respectively. For our own analyses, we digitized the maps of Laurance et al. at a spatial resolution (pixel size) of 10 × 10-km cells and classified each of these cells into one of the four land-use categories. We limit the analysis to the Brazilian portion of the Amazon because we do not have comparable maps for parts of the Amazon Basin that lie outside Brazil.
Calculating extinction risk for tree species in the Amazon is perhaps the most problematic and the most speculative part of the analysis, but it is a conservation issue of such paramount importance that we feel we