Ch. 2— Importance of Biological Diversity • 53 



pest and disease resistance (18,27,79). Similarly, 

 the average milk yield of cows in the United 

 States has more than doubled during the past 

 30 years; about one-fourth of this increase is 

 due to genetic improvement (89). 



Developing countries have also achieved in- 

 creased production of major crops. The Green 

 Revolution that has transformed heavily popu- 

 lated Asian countries is founded on use of par- 

 ticular genes. High-yielding varieties of rice, 

 for example, rely on a gene from a traditional 

 variety for the "dwarf" stature that enables the 

 plant to channel nutrients from fertilizers into 

 grain production without getting top-heavy and 

 falling over before harvest time. Although the 

 dwarfing trait is effective in many locations, 

 the high-yielding varieties need other genetic 

 characteristics from many different varieties. 

 The rice variety IR36, used in many countries 

 to sustain yield gains, was derived by cross- 

 breeding 13 parents from 6 countries (19,87). 



Progress in tomato improvement in the United 

 States has followed the use of exotic germplasm 

 (traditional cultivars, wild forms of the domes- 

 ticated species, and exclusively wild species). 

 Fruit quality (color, sugar content, solids con- 

 tent); adaptations for mechanized harvesting; 

 and resistance to 15 serious diseases have been 

 transferred to the tomato from its wild relatives. 

 One researcher noted: 



Resistance to some of these diseases is man- 

 datory for economic production of the crop in 

 California, and it is doubtful whether the State's 

 tomato industry would exist without these and 

 other desired traits derived from exotics (77). 



Rice and tomato illustrate the importance of 

 maintaining as much of the genetic variation 

 remaining within the domesticates and their 



wild relatives as possible, because both crops 

 have benefited from genes occurring in a sin- 

 gle population and nowhere else. Asian rice cul- 

 tivars get their resistance to grassy stunt virus, 

 a disease that in one year destroyed 116,000 

 hectares (287,000 acres), from one collection 

 of Oryza nivara (53). The gene for a jointless 

 fruit-stalk (a trait that assists mechanized har- 

 vesting and is worth millions of dollars per year) 

 in tomato is found in a single population of a 

 wild relative (Lycopersicon cheesmanii) unique 

 to the Galapagos Islands (78). 



A variety of genetic resources is being used 

 in the breeding of livestock, particularly cattle 

 and sheep. Grossbreeding Brahman cattle with 

 Hereford, Angus, Charolais, and Shorthorn 

 breeds has had a major impact on commercial 

 beef production in North America (30). A num- 

 ber of African cattle breeds are notable sources 

 of disease and pest resistance (West African 

 Shorthorn to trypanosomiasis, N'dama and Ba- 

 ole to dermatitis. Zebu to ticks) (34). The Finn- 

 ish landrace of sheep was almost lost before 

 its high level of reproductive efficiency was dis- 

 covered. It has now been incorporated into 

 commercial mating lines in the United King- 

 dom and North America (30). 



Yield and quality improvements can continue 

 to be made and defended against pests and path- 

 ogens, provided plant and animal breeding con- 

 tinues to be supported and the genetic diversity 

 that breeders draw on is maintained. Indeed, 

 there is no option but to go on improving crops 

 and livestock if world agriculture is to respond 

 successfully to economic and environmental 

 changes and to the new strains of pests and dis- 

 eases that evolve to overcome existing re- 

 sistance. 



tSITY 



Biological diversity benefits everyone, is val- 

 ued by many (in a variety of ways), but is owned 

 by no one. Thus, its evaluation is fraught with 

 complexity. There are two broad classes of 

 value: economic and intrinsic. 



Economic Value 



Economic evaluation potentially covers all 

 functional benefits described in this chapter, 

 ranging from tangible benefits from harvested 



