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

Abstract

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: (1) eliminate the cyclic population growth or (2) distort and obscure the pattern sufficiently so it cannot be recognized by simply observing population patterns over time.

Secondary production and reproduction of Capitella capitata type I (Annelida. Polychaeta) during a population cycle

“…Our results show that, on a daily basis, the nitrogen requirements of the Capitella capitata population never greatly exceeded the nitrogen input; this is similar to what has been reported by Chesney & Tenore (1985a). Because bacteria and meiofauna are always present in the tray (Alongi & Hanson 1985, Alongi & Tenore 1985 and compete for food with C. capitata.…”

“…8) (Marsh et al unpubl.). These data confirm that the oscillation cycle is driven by changes in food availability (Chesney & Tenore 1985a). …”

“…Weight-specif1.c reproductive outputs (reproductive outputs) were computed after Gremare et al (in press b). Because the distribution of C. capitata was not uniform in the tray (edge effects due to water circulation), we used the sampling strategy described by Chesney & Tenore (1985a).…”

“…Respiration was estimated from secondary production values using the equation of Hurnphreys (1979). A metabolic turnover rate of 8 % of the nitrogen biomass was estimated from the minimum daily-nitrogen requirements reported by Chesney & Tenore (1985a).…”

“…To determine whether the oscillation cycle is due to an overshoot of the carrying capacity requires: (1) an estimation of the carrying capacity of the population, and (2) a measure of changes in growth and reproduction rate as a function of time (Chesney & Tenore 1985a). Previous studies on the oscillation cycles of Capitella capitata have mainly considered changes in secondary production and have not measured changes in reproduction rate (Chesney & Tenore 1985a, b).…”