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IV SIMPOSIO INTERNACIONAL DE PASTIZALES San Luis Potosí, S.L.P. México 22 al 24 de agosto de 2007 Conferencias Magistrales DESERTIFICATION IN GRASSLANDS OF THE ARID AND SEMI-ARID ZONES OF THE GLOBE: BUILDING A SCIENCE FOR DRYLAND DEVELOPMENT James F. Reynolds Professor of Environmental Studies and Biology Nicholas School of the Environment & Earth Sciences; and Department of Biology, PO Box 90328, Duke University, Durham, NC, 27708 USA The world’s grasslands cover approximately 40% of the terrestrial land surface of the earth. Almost half (47%) of these are found in arid and semi-arid zones where land degradation (or desertification) is widespread. Over large areas the native vegetation has been reduced or eliminated, soils are eroding at accelerated rates, and the capacity of the land to support livestock and wild herbivores has been reduced. These areas already have limited potential for crop production because of low and erratic rainfall, low soil fertility, and limited opportunities for irrigation. Land degradation is a serious problem since the development and sustainability of rural people in these regions is dependent largely on their effective use and management of renewable natural resources (“ecosystem goods and services”), including water, soils, plants and wildlife. As a result, in certain areas people are flocking to congested cities because they can no longer obtain a livelihood in their rural home areas. The list of current and future drivers of land degradation consists of both biophysical and human drivers, including climate change (e.g., increased drought, shifting of seasonal precipitation, increased variability of precipitation), land clearing and deforestation, poor land management practices (for example, management attempts in some areas to alleviate stock losses by providing water points and supplementary fodder has exacerbated the deterioration of the vegetation and widened the extent of the area degraded), encroachment by rapidly-expanding urban areas, human alteration of the fire frequency (fire is essential to mesic grasslands worldwide), and afforestation. These drivers lead to accelerated erosion by wind and water, the loss of soil nutrients, increases in the acidity, salination, and alkalinization of soils, and the loss of both soil structure and organic matter. Land degradation has made arid and semi-arid zones highly vulnerable to both environmental and political changes. For example, “grassland afforestation” in the southern regions of South America is leading to a rapid conversion of native grasslands to scrublands and forests, a practice that is often directly supported by governmental policies; in addition, it is highly probable that in the future potential carbon sequestration markets will exacerbate this problem. There is little doubt that land-use changes in arid and semiarid grasslands are altering—and have the potential to further alter—the global budgets carbon, water and trace gases. Many of the dramatic changes observed in grasslands of arid and semi-arid zones in the last century are due to the high growth rate of human populations and, in some instances, climate change. In this millennium, however, these grasslands face a myriad of problems that present tough challenges to the research, management and policy communities. On 2
IV SIMPOSIO INTERNACIONAL DE PASTIZALES San Luis Potosí, S.L.P. México 22 al 24 de agosto de 2007 Conferencias Magistrales the other hand, recent advances in dryland development – in concert with the integrative approaches of global change and sustainability science – suggest that concerns about land degradation, poverty, safeguarding biodiversity, and protecting the culture of native peoples can be confronted with a certain amount of renewed optimism. In this talk, I will briefly review recent lessons about the functioning of arid and semi-arid ecosystems and the livelihood systems of their human residents, and introduce a new synthetic framework, the Drylands Development Paradigm (DDP). The DDP is supported by a growing and welldocumented set of tools for policy and management action and is intended to help navigate the inherent complexity of desertification and dryland development, identifying and synthesizing those factors important to research, management, and policy communities of relevance to the arid and semi-arid zone grasslands of the globe. 3
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