Oscillations of laboratory populations of the polychaete Capitella capitata (Type I): their cause and implications for natural populations

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Abstract
Populations of the polychaete C. capitata (Type I) were grown in the laboratory under constant conditions of food input, temperature and salinity. Population biomass and numbers oscillated with a period of .apprx. 6-8 mo. and population peaks ranged from 120,000-150,000 individuals m-2. Estimates of carrying capacity of the population along with analyses of the reproductive output show these populations are overshooting their equilibrium density (K) due to high intrinsic growth rate (r) and time lag associated with reproduction. Populations of opportunistic species, such as C. capitata, colonizing an unexploited environment in nature will also overshoot the carrying capacity of the environment when conditions favor rapid population growth. When combined with normal environmental variability (i.e., temperature, food supply) the tendency to overshoot carrying capacity can result in explosive population growth followed by sharp declines. These growth patterns can result in their eventual replacement by other species. An attempt is made to reconcile the frequent observation of cyclic population growth in laboratory studies with the lack of examples in field studies. A variety of factors in nature affect population growth pattern, such as environmental variability, species attributes (i.e., fecundity, reproductive cycle, territoriality, ectothermy and endothermy) and species interactions (i.e., rate of predation and the relation between prey and predator growth rates). Changes in population growth caused by these factors can eliminate the cyclic population growth or distort and obscure the pattern sufficiently so it cannot be recognized by simply observing population patterns over time.