On the nature of population extremes

Summary Much ecology considers only the typical size of a population, yet extreme 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 traits that predominate. Obviously, even for a fixed variance in annual numbers, the observed maximum and minimum population size will increase the more years that we count the population. Interestingly, over the time scales of available data (<100 years), most animal populations have an observed variance in annual numbers that increases the more consecutive years we use in its calculation. Consequently, populations will meet extreme values more quickly than if the variance were constant. We quantify the increases in variance for diatoms, insects, and vertebrates, first correcting the data for overall differences in variance. Short- and long-lived species are not consistently different. Species that cycle in density have relatively small increases relative to those that do not cycle. Species in marine ecosystems have larger increases than those in terrestrial and freshwater systems. All these results 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.