als each that are expected to suffer nearly a 50% extinction rate under the nonoptimistic deforestation scenario and an ≈37% loss rate even under the optimistic scenario. Most of these species have small range sizes and are highly vulnerable to local habitat loss. In ensembles of 100 stochastic simulations, we found mean total extinction rates of 20% and 33% of tree species in the Brazilian Amazon under the optimistic and nonoptimistic scenarios, respectively.
To obtain even a very rough estimate of the total number of species in the [Amazonian] forest community, a hypothesis must be made concerning the relationship between the common and rare species.
Pires et al. (1953)
The watershed of the Amazon River and its tributaries is enormous, covering ≈7,179,100 km2, and the longest dimension of the basin is ≈6,815 km. The Amazon Basin contains ≈40% of the world’s remaining tropical forest, much of it still botanically intact, or largely so, particularly in western Amazonia. However, serious concern has been raised about the possibility of large-scale extinctions of tree species in the next several decades, due to the expansion of a network of roads, especially in the Brazilian Amazon (Whitmore and Sauer, 1992; Laurance et al., 2001, 2002; Anonymous, 2006). These roads open undisturbed areas to extractive uses of the Amazon forest in previously inaccessible areas far from rivers and to subsequent clearing of forest for ranching, agricultural crop production, and tree plantations of commercially important species, mostly exotics. Anthropogenic habitat destruction is perhaps the single greatest cause of modern species extinctions (Ehrlich and Ehrlich, 1981; Wilson and Peter, 1988; Wilson, 1989). Climate change may become an even bigger cause of extinction over the next century (Thomas et al., 2004; Williams et al., 2007).
This chapter examines the questions of how many tree species there are in the Amazon and how many of them will go extinct from habitat loss during the next several decades. More than 50 years ago, long before the survival of the Amazon forest became a headline issue, Theodosius Dobzhansky and two experts on the Amazonian tree flora, Pires and Black, made a pioneering attempt to answer the “how many tree species” question from samples of virgin forest in eastern Amazonia in the state of Pará. In their first paper, which reported counts of tree species in several 1-ha plots, they encountered a large fraction of tree species only once (as a single individual) (Black et al., 1950). In their second study (Pires et al., 1953), they increased their plot size to 3.5 ha in the hope that a larger