78 • Technologies To Maintain Biological Diversity 



maize, potato, tomato, and bean. Such assess- 

 ments are needed to understand the dynamics 

 of crop genetic change and its relationship to 

 social and economic change (39). 



The status of diversity conservation for eco- 

 nomically important timber trees lies between 

 that of wild plants gathered for economic use 

 and that of agricultural plants. Some commer- 

 cial tree species are protected by parks and 

 other protected natural areas, and the diver- 

 sity of some is at least partially captured in 

 offsite seed collections. In many extensively 

 managed forests, commercial tree species re- 

 generate naturally after logging, fire, or other 

 disturbances, and local genetic diversity is 

 maintained. However, replanting with stock 

 propagated from selected parents and from 

 tree-breeding programs is a common practice 

 with some trees, such as Douglas fir, and gene 



pools for these species are being gradually 

 altered (19). 



The genetic resources of commercial trees 

 and other economic plants and animals in de- 

 veloping countries are being eroded by conver- 

 sion of forest areas to agriculture or grazing 

 use. An international panel recently identified 

 some 300 tree species or important tree popu- 

 lations (presumably with unique genetic char- 

 acteristics) that are endangered (17). Thus, in 

 the United States and developing countries 

 where U.S. agencies provide assistance, pro- 

 tection of natural gene pools of commercial 

 trees and other nondomesticated economic spe- 

 cies could become an objective in development 

 planning (see ch. 11). At present, economic spe- 

 cies not used in agriculture or horticulture are 

 poorly represented in genetic conservation 

 programs. 



CAUSES OF DIVERSITY LOSS 



Forces that contribute to the worldwide loss 

 of biological diversity are varied and complex, 

 and stem from both direct and indirect pres- 

 sures. Historically, concern has focused on the 

 commercial exploitation of specific threatened 

 or endangered species. But now attention is also 

 being focused on indirect threats more sweep- 

 ing in scope (30). 



Development and Degradation 



Economic development usually entails mak- 

 ing sites more favorable for a manageable num- 

 ber of economic activities. Consequently, the 

 changed landscape has fewer habitats and sup- 

 ports fewer species. Habitats may be affected 

 by offsite development too — by pollution, for 

 example, or changed hydrology. Some devel- 

 opment, such as logging in a mosaic pattern 

 through a large forested area, mimics natural 

 processes and may result in a temporary in- 

 crease in diversity. 



But poorly planned or badly implemented de- 

 velopment, such as agricultural expansion with- 

 out investment in soil conservation, can se- 

 verely disrupt biological productivity, and it can 



start a self-reinforcing cycle of degradation. For 

 example, soil erosion reduces soil fertility, 

 which in turn can reduce growth of plants for 

 cover, leading to more soil erosion and to rapid 

 depletion of diversity as the site becomes suit- 

 able, for fewer and fewer species (51). Other 

 causes of site degradation include grazing pres- 

 sures, unnatural frequency or severity of fires, 

 and excessive populations of herbivores (such 

 as rabbits) where predators are eliminated. Arid 

 and semiarid sites are especially susceptible to 

 degradation from such pressures. Species may 

 be reduced by one-half to two-thirds without 

 outright conversion of the land use (33). 



Modernization of farming systems is also a 

 cause of diversity loss. Such systems often in- 

 clude many species of crops and livestock; 

 genetic diversity is typically high, because cul- 

 tivars and breeds adapted to the vagaries of site- 

 specific conditions are used. To achieve pro- 

 ductivity gains, however, traditional systems 

 are being replaced with modern methods. Cap- 

 ital inputs, such as manufactured fertilizers and 

 feeds, are used to compensate for site-specific 

 differences. Thus, it is possible to replace tradi- 

 tional crops and livestock with fewer varieties 



