ON THE NATURE OF POPULATION EXTREMES

Much ecology considers only the typical size of a population, yet extr eme values may be of particular importance. Unusually low numbers may doom a population to extinction and unusually high numbers may pose an economic threat. Extreme values may also determine the evolutionary t raits that predominate. Obviously, even for a fixed variance in annual numbers, the observed maximum and minimum population size will increa se the more years that we count the population. Interestingly, over th e time scales of available data (<100 years), most animal populations have an observed variance in annual numbers that increases the more co nsecutive years we use in its calculation. Consequently, populations w ill meet extreme values more quickly than if the variance were constan t. We quantify the increases in variance for diatoms, insects, and ver tebrates, first correcting the data for overall differences in varianc e. Short- and long-lived species are not consistently different. Speci es that cycle in density have relatively small increases relative to t hose that do not cycle. Species in marine ecosystems have larger incre ases than those in terrestrial and freshwater systems. All these resul ts suggest that the system in which a species is embedded - rather the species' own characteristics - plays the crucial role in determining the nature of population extremes.