108 • Technologies To Maintain Biological Diversity 



ber harvesting and then linking the islands by 

 corridors of old-growth vegetation. This design 

 would presumably provide mobility for species 

 like the cougar and bobcat— far-ranging carni- 

 vores that would have populations too small 

 for survival and continued evolution if confined 

 to a single habitat island. Proposals like this 

 must be considered planning hypotheses, sug- 

 gested by general theory; and as such must be 

 subjected to close, case-by-case scrutiny before 

 implementation. 



Genetics 



Genetic considerations are another dominant 

 concern in the literature on population viabil- 

 ity and conservation. Attempts are being made 

 to determine the smallest number of interbreed- 

 ing individuals that will enable a species to 

 survive indefinitely— adapting to changing envi- 

 ronmental conditions without suffering the neg- 

 ative effects of a small population size (popu- 

 lation instability, erosion of genetic variability, 

 inbreeding). Because each individual carries 

 only part of the genetic variation characteris- 

 tic of its species, the size of a population — and 

 thus, the amount of genetic variation — may de- 

 termine how much and how fast a population 

 can evolve. 



Application of genetics to the issue of popu- 

 lation size and viability has led to theoretical 

 estimates of minimum populations for success- 

 ful conservation of birds and mammals. One 

 such estimate, known as the 50/500 rule, is that 

 effective population size (in genetics sense) of 

 50 breeding adults is the minimum needed to 

 sustain captive breeding programs over dec- 

 ades or a century (e.g., as in zoos), but a popu- 

 lation 10 times as large is needed to sustain a 

 species in its natural habitat as it evolves over 

 millennia to survive changing environmental 

 stresses (25,45). 



The 50/500 rule is an approximation based 

 on studies of only a few species. But the effect 

 of population size depends on several factors 

 that differ for various species, such as sex ra- 

 tio, age structure, mating behavior, and be- 

 haviors such as feeding. Thus the rule, when 

 applied to a particular species, could project 



a need for populations larger than 50/500— 

 perhaps orders of magnitude larger. Empirical 

 or experimental evidence is lacking to deter- 

 mine how resilient a "genetically viable" pop- 

 ulation would be when confronted with other 

 pressures (e.g., demographic, environmental, 

 or catastrophic uncertainty) (72). 



ilati< 



imics 



Scientists have long recognized that, in gen- 

 eral, smaller populations are more susceptible 

 to extinction than larger ones, because death 

 for individual organisms is an event determined 

 largely by change, and populations are collec- 

 tions of individuals. Models of the impact of 

 change on individual births and deaths were 

 developed decades ago (e.g., ref. 21), and these 

 have been applied to estimate the extinction 

 time for particular species under various cir- 

 cumstances. Models also have been developed 

 to evaluate the effect of chance environmental 

 variations and chance population-wide catas- 

 trophes. 



Recently, more sophisticated models of sto- 

 chastic population dynamics have been formu- 

 lated specifically to investigate questions of 

 population viability. These models do not give 

 specific prescriptions for minimum population 

 size to assure survival, but they are leading to 

 a better understanding of the role of chance in 

 populations. They indicate that to avoid extinc- 

 tion resulting from the impact of chance on in- 

 dividual births and deaths may require only a 

 few hundred breeding individuals. But larger, 

 perhaps much larger, population sizes are nec- 

 essary if the condition of the species' environ- 

 ment varies, and still larger populations are 

 needed for species that are susceptible to catas- 

 trophes (72). 



The modeling approach is useful but has sig- 

 nificant limitations. First, data for population 

 models encompassing both environmental and 

 genetic factors exist for only a few species. Also, 

 species experience the effects of chance at in- 

 dividual, environmental, population, and ge- 

 netic levels. But models that could simultane- 

 ously simulate all these factors would be too 

 complex for existing analytical capabilities. 



