Global Rates of Habitat Loss and Implications for Amphibian Conservation
A large number of factors are known to affect amphibian population viability, but most authors agree that the principal causes of amphibian declines are habitat loss, alteration, and fragmentation. We provide a global assessment of land use dynamics in the context of amphibian distributions. We accomplished this by compiling global maps of amphibian species richness and recent rates of change in land cover, land use, and human population growth. The amphibian map was developed using a combination of published literature and digital databases. We used an ecoregion framework to help interpret species distributions across environmental, rather than political, boundaries. We mapped rates of land cover and use change with statistics from the World Resources Institute, refined with a global digital dataset on land cover derived from satellite data. Temporal maps of human population were developed from the World Resources Institute database and other published sources. Our resultant map of amphibian species richness illustrates that amphibians are distributed in an uneven pattern around the globe, preferring terrestrial and freshwater habitats in ecoregions that are warm and moist. Spatiotemporal patterns of human population show that, prior to the 20th century, population growth and spread was slower, most extensive in the temperate ecoregions, and largely exclusive of major regions of high amphibian richness. Since the beginning of the 20th century, human population growth has been exponential and has occurred largely in the subtropical and tropical ecoregions favored by amphibians. Population growth has been accompanied by broad-scale changes in land cover and land use, typically in support of agriculture. We merged information on land cover, land use, and human population growth to generate a composite map showing the rates at which humans have been changing the world. When compared with the map of amphibian species richness, we found that many of the regions of the earth supporting the richest assemblages of amphibians are currently undergoing the highest rates of landscape modification.Abstract
The 21 ecoregions used in this assessment (note that Greenland and Antarctica have been excluded because they lack amphibians).
Amphibian species richness. The map legend depicts classes of species richness, where each successive class represents twice the number of species as the previous class.
Maps of human population growth rates for three time intervals. There is a general decrease through time in growth rates for countries in temperate and higher latitudes in contrast to a substantial increase in human population growth rates in tropical latitudes.
(A) Percent change in the extent of closed-canopy forest cover since pre-settlement conditions (referenced to 1996). (B) Number of extirpated and threatened tree species as of the 1990s.
(A) Changes in cropland area from 1961–1997. (B) Change in the percent of cropland that was irrigated during 1961–1997. (C) Percent change in average annual fertilizer usage during 1961–1997.
Composite rates of population-, agricultural-, and forest-related change. The resulting patterns show the relative magnitude of the rates at which the landscape is being altered in ways that are likely detrimental to amphibian habitat. To generate the composite map, rates of change in the source maps (1-km resolution) were transformed to values from 1 (very low rate of change) to 5 (very high rate of change). The mean value was calculated across the source maps for each grid cell to yield this composite map.
Contributor Notes
Section editor: S. A. Schaefer.