Ch. 3— Status of Biological Diversity • 65 



Box 3-A. — Biological Concepts 



Trends in changing biological diversity cannot be measured directly; so many species exist that 

 costs of inventories would be too high. Rather, trends must be inferred by applying biological con- 

 cepts and by observing changes in habitats. Several biological concepts are relevant: 



• Species-area relationship: Large sites tend to have more species than small sites. [So] When 

 the areas of diverse natural ecosystems are reduced by land development or by degradation, 

 diversity is reduced. From analysis of many sets of empirical data, scientists have derived a 

 mathematical equation that can be used to predict the decrease in number of species that can 

 be expected following a reduction in habitat area (5). 



• Provinciality effect: Diversity of species and populations separated by geographic barriers, usually 

 increases over time. But when species or varieties are carried across these barriers, as with 

 the introduction of an exotic organism, provinciality is abruptly lost. Rapid loss of diversity 

 can follow if native species have no defense against an exotic pathogen or pest, or if the exotic 

 organism competes more aggressively for habitat (44). Examples include the introductions of 

 Dutch elm disease to the United States, of cattle to California, of the paperbark tree to Florida, 

 and of goats to many oceanic islands. 



• Narrow endemism: Some species occur only within very restricted geographic ranges. This 

 group includes many species that have evolved on islands, in mountaintop forests, in isolated 

 lakes or other aquatic zones such as coral reefs, in areas with Mediterranean climates, includ- 

 ing California, Western Australia, the Cape of South Africa, Chile, and the Mediterranean Ba- 

 sin countries. Areas with a high proportion of narrow endemic species contribute to global 

 diversity more than other areas with similar numbers of species but less endemism. Thus, bio-j 

 logical degradation in such areas reduces diversity more than it would elsewhere. i 



• Species richness: Some ecosystems have many more species than others. Generally, species 

 richness is greatest in equatorial regions, and it decreases toward the poles. It is generally greater 

 in warmer or wetter places than in colder or drier places. Thus, the hot, wet tropical forests, 

 which cover only 7 percent of the Earth's land area, may have about half of the Earth's terres- 

 trial plants and animals (30). 



• Species interdependence: Interdependence can take a variety of forms. Symbiosis occurs when 

 one or both of two species benefit from association. Mutualism occurs when neither species 

 can survive without the other under natural conditions. Commensalism refers to associations 

 in which one benefits and the other is unaffected. 



• Natural vulnerability: Vulnerability to extinction varies with several factors. Narrow endemics 

 are perhaps most vulnerable. Rare species may be less susceptible to catastrophe if widely dis- 

 persed, but dispersion may lessen their chances for successful mating. Other species relatively 

 vulnerable to extinction include the following: top-level carnivores, species with poor coloniz- 

 ing ability, those with colonial nesting habits, migratory species, those that depend on unrelia- 

 ble resources, and species with little evolutionary experience with perturbations. ^ 



same landscape after most of the wood- 

 lands are converted to grassland and 

 cropland. 



Species diversity: A rangeland with 100 

 species of annual and perennial grasses 

 and shrubs has more diversity than the 

 same land after grazing has eliminated or 



greatly reduced the frequency of the peren- 

 nial grass species. 



Genetic diversity: Economically useful 

 crops are developed from wild plants by 

 selecting valuable inheritable characteris- 

 tics. Thus, the wild ancestral plants con- 

 tain many genes not found in today's crop 



