Understanding the factors that influence diversity is a fundamental objective of ecology. In Oneida Lake, New York, two DAPHNIA species (D. GALEATA mendotae and D. PULICARIA) have persisted for decades or centuries despite the fact that their water-column densities are negatively correlated. One species typically is abundant while the other remains in low numbers. This transient dominance lasts for months or years and has been hypothesized to result from a combination of predation and competition. It raises two questions: (1) how is each species able to persist over the long term; and (2) if there is competition, why are both species present in the system?

Both species produce diapausing eggs that remain viable in the lake sediments for over a century, creating populations with overlapping generations. Theoretical models suggest that overlapping generations, in combination with a temporally fluctuating environment, may allow not only population persistence but also the stable coexistence of competitors. Despite extensive theoretical development, this "storage effect" hypothesis has received little empirical attention. In this dissertation I not only explore the prerequisites of the model but also present the first explicit mathematical analysis of the contribution of the storage effect to the dynamics of competing natural populations.

In studying the community dynamics of two species of the herbivorous zooplankton, DAPHNIA, I have not only demonstrated competition between these two species, but have also shown that both species have a long-lived stage. Moreover, recruitment to this long-lived stage is negatively correlated between species, so that both daphnids have years in which they are favored relative to their competitor. When the long term population growth rates are analyzed both with and without the effects of a variable environment, it is shown that DAPHNIA GALEATA MENDOTAE clearly cannot persist without the storage effect. In contrast, D. PULICARIA persists through consistently high per capita recruitment to the egg bank.