Ch. 3— Status of Biological Diversity • 67 



been reduced some 70 to 90 percent during the 

 last two centuries (46,54). In the Sacramento 

 Valley of California, for instance, the estimated 

 loss of riparian vegetation areas is 98.5 percent; 

 for Arizona, the estimate is 95 percent (45). 



The diversity of agricultural ecosystems, or 

 agroecosystems, is also being reduced. System 

 diversity is high in regions where agricultural 

 land is divided into relatively small holdings 

 and each farm uses a variety of crop and live- 

 stock species. As indicated in the preceding 

 chapter, such landscapes support natural ene- 

 mies of crop pests and are likely to contain 

 species and varieties that can resist disease 

 outbreaks and survive abnormal weather. How- 

 ever, on the fertile land of temperate-zone farm- 

 ing regions, where modern machinery and agri- 

 cultural chemicals are used with crop varieties 

 and where livestock are bred to maximize pro- 

 duction, farmers can achieve substantial econ- 

 omies of scale on large holdings that special- 

 ize in relatively few crops or breeds. These less 

 diverse agroecosystems are more productive 

 and more profitable than the older systems (36). 

 As yet, relatively little scientific effort is being 

 made to determine how biologically diverse 

 farming could be made more profitable. Thus, 

 the continuing loss of agroecosystem diversity 

 in the United States and throughout the world 

 seems to be a function of both economic devel- 

 opment and research priorities (10). 



Time-series measurements of agroecosystem 

 diversity are lacking, as is an understanding 

 of the advantages and disadvantages of diver- 

 sity. There is also a delay between the loss of 

 diversity and consequent increased or de- 

 creased profits. Therefore, it seems likely that 

 agricultural system uniformity may continue 

 to increase beyond its economic optimum. 

 Then, a period of restoring some diversity may 

 occur. This process may be underway in some 

 areas of the United States, where multiple crop- 

 ping, crop rotations, and restoration of shelter- 

 belts are becoming more popular practices (51). 



Attempts to increase farm profits by meth- 

 ods that reduce diversity may fail where severe 

 droughts or soil erosion are common and where 

 hot temperatures and high rainfall have resulted 

 in soils with little capacity to hold nutrients. 



Where such development failures occur, res- 

 toration of more diverse farming systems can 

 be difficult, because topsoil, water resources, 

 germplasm, or knowledge of traditional farm- 

 ing methods have been lost (11,25). 



Most countries do not have detailed informa- 

 tion on changes in ecosystem diversity. The 

 greatest concern on a global scale is for reduc- 

 tion of natural areas in the tropical regions, 

 where ecosystems are least able to recover from 

 degradation. Data on deforestation from many 

 tropical countries indicate that the closed- 

 canopy tropical forests are being reduced by 

 about 11 million hectares each year. (The 

 deforestation rates are discussed in some de- 

 tail in ref. 54.) 



Few data are available for the developing 

 countries on degradation of biological diver- 

 sity and other resources within the areas that 

 remain classified as forest. Nor are data avail- 

 able on changes in area or quality of grasslands, 

 wetlands, open-canopy forests, riparian and 

 coastal zones, or aquatic ecosystems. Never- 

 theless, compelling anecdotal evidence indi- 

 cates widespread degradation of all types of 

 ecosystems in developing countries. In Sri 

 Lanka, for example, removal of coral reefs for 

 production of lime has had several conse- 

 quences: 



• the disappearance of lagoons important as 

 nursery areas for fish, 



• the collapse of a fishery, 



• reduction of mangrove areas, 



• erosion of cultivated coconut land, and 



• salination of wells and soil within half a 

 mile of the shore (41). 



Documents from development assistance 

 agencies, such as the U.S. Agency for Interna- 

 tional Development (AID), the World Bank, the 

 United Nations Development Programme, and 

 the U.N. Food and Agriculture Organization 

 abound with observations of resource degra- 

 dation in developing countries. Evidence of 

 ecosystem degradation is found in the environ- 

 mental profile series that AID has been pre- 

 paring since 1979. Usually the evidence is a 

 description of problems caused by resource 

 degradation rather than a report from careful 



