Hairston, Jr., N.G.: Temporal dispersal: ecological and evolutionary implications of prolonged egg diapause

Zooplankton egg banks are the accumulation of diapausing embryos buried in the sediments of many aquatic ecosystems. These eggs, which are analogous life history stages to the seeds of many plants, can survive in a ready-to-hatch state for periods ranging from decades to centuries. Their presence in ponds, lakes and near-shore marine environments has substantial implications for the maintenance of biotic diversity, both among and within species, and for the rates and trajectories of ecological and evolutionary change. Their role in maintaining diversity derives from the interaction of two factors: generation overlap created by prolonged diapause and environmental fluctuations that cause different types (species or genotypes) to be favored at different times. Hatching of diapausing eggs influences population and community response to environmental change by introducing to current environments species or genotypes laid at times in the distant past through what can be thought of as temporal dispersal. A completely different aspect of egg banks derives from the fact that the sediments of lakes are often structured in historical sequence (with the most recent sediments on top and progressively older sediments deeper in the lake bottom). When this is the case, diapausing eggs extracted from different sediment ages can provide a means of studying past changes in community or population-genetic structure. These two distinct aspects of egg banks (i.e., their direct impact on ecological and evolutionary processes versus their usefulness in reconstructing historical changes), are potentially in conflict because for old eggs to hatch, the sediments must be at least partially mixed, but this same mixing degrades the accuracy of the historical record. Both aspects are possible, however, when sediment-mixing intensity is spatially heterogeneous.