Global biodiversity loss from tropical deforestation.

Tropical forests are incredibly biodiverse; they support at least two-thirds of the world’s biodiversity (1) despite covering less than 10% of Earth’s land surface (2). Unfortunately, prospects for tropical forests and the biodiversity therein are becoming increasingly bleak owing to unabated deforestation and forest alteration (3) that stem from human activities such as logging, hunting, agricultural expansion, and human settlement (4, 5) (Fig. 1). Previous studies have summarized how local or subregional biodiversity values differ across primary forests and other landuse types (6, 7), but it is unclear how siteand landscape-level impacts scale up globally. In PNAS, Alroy (8) addresses this challenge by compiling and analyzing a pantropical dataset of 875 local species assemblages sampled in primary forest and 10 other land-use types previously converted from forest. The task of estimating global biodiversity losses that accrue with tropical deforestation might seem straightforward at first. For example, one might approach this problem by collating global distributions of different taxa, removing increasing extents of forest, and then counting the extinctions that accumulate when entire species ranges become deforested. This approach suffers from a number of pitfalls. First, it assumes populations originally occurring in forests can never survive in altered habitats. Indeed, it is well documented that altered forests and other land uses often support some species previously found in continuous primary forests (9, 10) and that biotic responses vary across taxa and disturbance types (6, 7). Further, such an analysis would be necessarily limited to the few vertebrate groups with reasonably complete and reliable global distribution and habitat information (i.e., amphibians, birds,mammals), precluding groups such as arthropods that make up the overwhelming majority of Earth’s terrestrial macroorganismal diversity (11). To examine the impact of a particular disturbance on species communities, ecologists often identify a landscape consisting of undisturbed and disturbed areas and sample ecological communities at sites or transects nested within these areas (6, 9, 10). It is also common for ecologists to characterize only intact forest communities or compare biotic responses across different altered habitats. Because human activities have been so pervasive in and around tropical forests, there is now a large collection of studies sampling local species communities in different land uses across the tropical forest biome. These local field studies are an excellent starting point for estimating global biodiversity loss because they provide information on species occurring in intact forests as well as those species that persist after deforestation or Fig. 1. Human activities that threaten tropical forests and the biodiversity therein. (A) Logging in Peninsular Malaysia. (B) Oil palm monoculture in Kalimantan, Indonesia. (C ) Cattle grazing on pastures converted from forests in Mato Grosso, Brazil. (D) Smallholder cropping (of cassava) in Loreto, Peru. (E ) Harvesting of wood for charcoal production in Benin. (F ) Hunting threatens many forest species, including the Malayan tiger caught here on a camera trap in Peninsular Malaysia. Photographs courtesy of Rimba (A and F ), X.G. (B), Jacob Socolar (C and D), and Orou Gaoue (E ).