Evolutionary Ecology of Seed-Bank Annuals in Temporally Varying Environments

Abstract

The production of long-lived seeds by annual plants introduces a unique form of age structure. In a temporarily varying environment the dormant seed may experience many years with different weather, whereas the germinating individual experiences only the weather conditions of a single growing season. Natural selection operates on both the between-year dormancy and on non-seed-bank traits that affect the degree of specialization to conditions pertaining in different year types. We have used an integrated model that permits these two aspects of the life history to evolve simultaneously. This leads to predictions that are not attainable by considering the evolution of each in isolation. Changes in the survival probability of the between-year seed bank select for reinforcing changes in between-year dormancy and specialization. Changes in the probability of occurrence of different year types select for damping changes in between-year dormancy and specialization. Across a gradient in environmental quality, we predict that most change will occur in between-year dormancy, with little change in specialization. If between-year dormancy is fixed, however, a greater change in specialization should occur. The predictions of the model are discussed in terms of environmental gradients, seed-bank versus non-seed-bank annuals, and a variety of plant traits modeled elsewhere that may be involved in specialization to different types of years.