Habitat Loss and Extinction in the Hotspots of Biodiversity

Nearly half the world’s vascular plant species and one-third of terrestrial vertebrates are endemic to 25 “hotspots” of biodiversity, each of which has at least 1500 endemic plant species. None of these hotspots have more than one-third of their pristine habitat remaining. Historically, they covered 12% of the land’s surface, but today their intact habitat covers only 1.4% of the land. As a result of this habitat loss, we expect many of the hotspot endemics to have either become extinct or—because much of the habitat loss is recent— to be threatened with extinction. We used World Conservation Union [IUCN] Red Lists to test this expectation. Overall, between one-half and two-thirds of all threatened plants and 57% of all threatened terrestrial vertebrates are hotspot endemics. For birds and mammals, in general, predictions of extinction in the hotspots based on habitat loss match numbers of species independently judged extinct or threatened. In two classes of hotspots the match is not as close. On oceanic islands, habitat loss underestimates extinction because introduced species have driven extinctions beyond those caused by habitat loss on these islands. In large hotspots, conversely, habitat loss overestimates extinction, suggesting scale dependence (this effect is also apparent for plants). For reptiles, amphibians, and plants, many fewer hotspot endemics are considered threatened or extinct than we would expect based on habitat loss. This mismatch is small in temperate hotspots, however, suggesting that many threatened endemic species in the poorly known tropical hotspots have yet to be included on the IUCN Red Lists. We then asked in which hotspots the consequences of further habitat loss (either absolute or given current rates of deforestation) would be most serious. Our results suggest that the Eastern Arc and Coastal Forests of Tanzania-Kenya, Philippines, and Polynesia-Micronesia can least afford to lose more habitat and that, if current deforestation rates continue, the Caribbean, Tropical Andes, Philippines, Mesoamerica, Sundaland, Indo-Burma, Madagascar, and Chocó–Darién–Western Ecuador will lose the most species in the near future. Without urgent conservation intervention, we face mass extinctions in the hotspots. Pérdida de Hábitat y Extinciones en Áreas Críticas para la Biodiversidad Resumen: Casi la mitad del total de plantas vasculares del mundo y un tercio de los vertebrados terrestres son endémicos en 25 “áreas críticas” para la biodiversidad, cada una de las cuales tiene por lo menos 1500 especies de plantas endémicas. En ninguno de estos sitios permanece más de un tercio de su hábitat prístino. Históricamente, cubrían 12% de la superficie terrestre, pero en la actualidad su hábitat intacto cubre solo 1.4% del terreno. Como resultado de esta pérdida de hábitat esperamos que muchas de las especies endémicas a es†† email t.brooks@conservation.org Paper submitted December 12, 2000; revised manuscript accepted September 26, 2001. 910 Extinction in Biodiversity Hotspots Brooks et al. Conservation Biology Volume 16, No. 4, August 2002 Introduction Four general principles combine to present one of the greatest challenges for conservation. First, most species have small range sizes relative to the mean range size, increasing their probability of extinction by chance alone (Gaston 1994). Second, species with small ranges also tend to be scarce within those ranges (Brown 1984), so their probability of extinction is increased on two counts. Third, the consistent mechanisms underlying the evolution of small range (Fjeldså & Lovett 1997) mean that most such species co-occur. Stattersfield et al. (1998) demonstrated this convincingly for birds. Finally, most of these areas of co-occurrence of species with small ranges—call them centers of endemism—are disproportionately threatened by human activity (Cincotta et al. 2000). Myers et al. (2000) quantified this. Twenty-five biogeographically distinctive “hotspots” each have 0.5% or more of the world’s flora completely restricted to their boundaries and have already lost 70% or more of their original geographic extent. The hotspots are therefore both irreplaceable and vulnerable (Margules & Pressey 2000). In combination, these hotspots hold the entire ranges of 44% of the world’s plants and 35% of terrestrial vertebrates in just 1.4% of the land area. Nature has put many of her eggs in a few baskets, and we are in danger of dropping even these. Where are these hotspots? We plotted the distribution and relative densities of endemics across the 25 hotspots as presented by Myers et al. (2000) (Fig. 1). We cannot simply map numbers of endemic species, because the hotspots (and countries) are different sizes and larger areas tend to hold more endemics. Furthermore, this relationship is not linear; it is probably a power function (Harte & Kinzig 1997). Therefore, we factored out area by plotting numbers of endemics against original hotspot area, fitting a power function and taking residuals about this line (Balmford & Long 1995). These residuals represent the relative density of endemics, which we plotted onto the map. The residuals are qualitatively similar if based on remaining hotspot area. Most (15) of the hotspots hold predominantly tropical rainforest; five hold Mediterranean-type vegetation, three temperate forest, one tropical dry forest (Brazil’s Cerrado), and one semidesert (Succulent Karoo). Given these enormous concentrations of small-ranged species in places where most natural habitat has already been cleared, we expect that many of the hotspot endemics will have become extinct (Pimm et al. 1995). Habitat loss accurately predicts species loss in regions where the habitat loss occurred a long time ago (Dial 1994; Pimm & Askins 1995). However, there is a time lag between habitat loss and species loss (Brooks et al. 1999 a ). For well-known taxa, one can detect this time lag in the form of population declines toward extinction. Such information is compiled in the World Conservation Union (IUCN) Red Data books (Baillie & Groombridge 1996; Walter & Gillett 1998). The very act of listing a taxon as threatened should stimulate conservation measures to preempt its decline to extinction (Collar 1996). Nevertheless, for both birds and mammals, the proportion of deforestation in both insular Southeast Asia (Magsalay et al. 1995; Brooks et al. 1997, 1999 b ) and Brazil’s Atlantic forest (Brooks & Balmford 1996; Brooks et al. 1999 c ; Grelle et al. 1999) consistently pretos sitios estén extintas o – porque la pérdida de hábitat es reciente – se encuentren amenazadas de extinción. Utilizamos Listas Rojas de UICN para comprobar esta predicción. En general, entre la mitad y dos tercios de las plantas amenazadas y el 57% de los vertebrados terrestres amenazados son endémicos de áreas críticas para la biodiversidad. Para aves y mamíferos en general, las predicciones de extinción en las áreas críticas para la biodiversidad, basadas en la pérdida de hábitat, coinciden con el número de especies consideradas extintas o amenazadas independientemente. En dos clases de áreas críticas para la biodiversidad la coincidencia no es muy grande. En islas oceánicas, la pérdida de hábitat subestima la extinción porque las especies introducidas han causado más extinciones que las producidas por la reducción del hábitat. Por lo contrario, la pérdida de hábitat sobrestima la extinción en áreas críticas para la biodiversidad extensas, lo que sugiere una dependencia de escala (este efecto también es aparente para plantas). Para reptiles, anfibios y plantas mucho menos especies endémicas son consideradas amenazadas o extintas por pérdida de hábitat. Sin embargo, esta discordancia es pequeña en áreas críticas para la biodiversidad en zonas templadas templadas, lo que sugiere que muchas especies endémicas amenazadas en las poco conocidas áreas críticas para la biodiversidad en zonas tropicales aun están por incluirse en las Listas Rojas. Posteriormente nos preguntamos en que áreas críticas para la biodiversidad serían más serias las consecuencias de una mayor pérdida de hábitat (absoluta o con las tasas actuales de deforestación). Nuestros resultados sugieren que el Arco Oriental y los Bosques Costeros de Tanzania/Kenia, Filipinas, Polinesia/Micronesia no pueden soportar mayores pérdidas y que, si continúan las tasas de deforestación actuales, el Caribe, Andes Tropicales, Filipinas, Mesoamérica, Sundaland, Indo-Burma, Madagascar y Chocó/Darién/Ecuador Occidental perderán más especies en el futuro. Sin acciones urgentes de conservación, habrá extinciones masivas en las áreas críticas para la biodiversidad.