Distribution and Causes of Global Forest Fragmentation

Because human land uses tend to expand over time, forests that share a high proportion of their borders with anthropogenic uses are at higher risk of further degradation than forests that share a high proportion of their borders with non-forest, natural land cover (e.g., wetland). Using 1-km advanced very high resolution radiometer (AVHRR) satellite-based land cover, we present a method to separate forest fragmentation into natural and anthropogenic components, and report results for all inhabited continents summarized by World Wildlife Fund biomes. Globally, over half of the temperate broadleaf and mixed forest biome and nearly one quarter of the tropical rainforest biome have been fragmented or removed by humans, as opposed to only 4% of the boreal forest. Overall, Europe had the most human-caused fragmentation and South America the least. This method may allow for improved risk assessments and better targeting for protection and remediation by identifying areas with high amounts of human-caused fragmentation. INTRODUCTION There has not been a systematic analysis of human vs. natural sources of forest fragmentation at the global scale. Frontier forests, defined as “large, ecologically intact, and relatively undisturbed natural forest,” have been mapped globally and a qualitative threat rating has been assigned to each patch (Bryant et al. 1997). Pahari and Murai (1999) demonstrated the high correlation between human population density and cumulative forest loss for regions. Matthews et al. (2000) recognized fragmentation as an issue separate from forest loss, and discussed deforestation and fragmentation caused by humans on a global scale. Riitters et al. (2000) quantified total forest fragmentation across multiple evaluation scales but did not identify human-caused fragmentation. Jones et al. (1999) assessed forest patch vulnerability based on edge shared with anthropogenic and natural land uses but only examined three tropical areas. A systematic global assessment is needed because anthropogenic land uses tend to expand or change over time and, as a result, areas that now experience humaninduced fragmentation are more likely to be areas of changing forest patterns in the future. This is especially true in tropical regions experiencing direct forest loss, but also applies to shifts in land uses in regions where total forest area is more or less constant, such as in North America. Global deforestation has been documented extensively, with an emphasis on the loss of tropical rainforests in Central Africa and Amazonia and the impacts on global climate and carbon budgets (Fearnside 1996, Laurance 2000, Justice et al. 2001, Semazzi and Yi 2001, Zhang et al. 2001). Another consequence is change in forest dynamics from fragmentation. Alteration of forest spatial patterns affects wildlife habitat quality and biodiversity in both tropical forests (Gascon and Lovejoy 1998, Carvalho and Vasconcelos 1999, Scariot 1999, Laurance et al. 2000) and extra-tropical forests (Jules et al. 1999, Hargis et al. 1999, Kurki et al. 2000, Virgos 2001). Forest area statistics are available from several sources. The United Nations Food and Agriculture Organization (FAO) produces the Forest Resources Assessment every 10 years, which estimates global forest area and change over time (Holmgren 2001), and the World Resources Institute (WRI) has published the Pilot Analysis of Global Ecosystems, which includes a forest ecosystems component (Matthews et al. 2000). Estimates for some areas are based on incomplete or inconsistent data, and methodological differences make comparisons difficult (Matthews 2001). Nevertheless, in the preceding decade we have witnessed a continuing reduction in global forest area with apparently substantial reductions occurring mainly in tropical areas. U.S. Environmental Protection Agency, National Exposure Research Laboratory; U.S. Forest Service Forests may be fragmented by a number of activities or events, such as road construction, logging, conversion to agriculture, or wildfire, but ultimately, the fragmenting cause is either anthropogenic or natural in origin. In this paper, we present a method to Conservation Ecology 7(2): 7. http://www.consecol.org/vol7/iss2/art7 calculate the amount of human and naturally caused forest fragmentation on a global scale using 1-km land cover data. The method quantifies fragmentation based on edges between forest and neighboring pixels, and identifies the cause as either anthropogenic or natural. We report the amount of anthropogenic and natural forest fragmentation for six continents by World Wildlife Fund (WWF) biomes (Olson et al. 2001). Because of interest in tropical forest ecosystems, we also report the results by WWF ecoregions for the tropical and subtropical moist broadleaf forest biome (TrMB hereafter) in South America. Table 1. Original IGBP and re-classification used in the fragmentation analysis. IGBP Classification Reclassified to 1 Evergreen Needleleaf Forest Forest 2 Evergreen Broadleaf Forest Forest 3 Deciduous Needleleaf Forest Forest 4 Deciduous Broadleaf Forest Forest 5 Mixed Forest Forest 6 Closed Shrublands Other Natural 7 Open Shrublands Other Natural 8 Woody Savannas Forest 9 Savannas Other Natural 10 Grasslands Other Natural 11 Permanent Wetlands Other Natural 12 Croplands Anthropogenic 13 Urban and Built-Up Anthropogenic 14 Cropland/Natural Vegetation Mosaic Anthropogenic 15 Snow and Ice Ignored 16 Barren or Sparsely Vegetated Other Natural 17 Water Bodies Ignored