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

Populations of the polychaete Capitella 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 approximately 6 to 8 mo and population peaks ranged from 120,000 to 150.000 indiv. 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. We conclude that 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. We attempt 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, ectothemy 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: