THE COMPARATIVE LIFE HISTORIES OF TWO ANNUAL LIMNANTHES SPECIES IN A TEMPORALLY

Life histories are commonly described by characters closely tied to age-specific survival and reproduction. In annual plants, these characters may include the proportion of biomass allocated to seed and the number of seeds produced yearly by the plant. Since their numeric values would at first sight seem to correlate closely with Darwinian fitness, why do they vary genetically among populations and species, often being intermediate or nonmaximal? One route for explanation postulates fitness trade-offs between characters such that an intermediate optimum is favored in one or both characters, as in the trade-offs between, for example, reproductive allocation versus competitive ability under r-K-selection (Gadgil and Solbrig 1972), immediate reproduction versus both future survivorship and future reproduction (Charnov and Schaffer 1973; Law 1979), and seed size versus seed number (Harper et al. 1970). When environmentally induced fluctuations of growth, survival, or reproductive rates are added to these models, the fluctuations are often found to affect character optima (Murphy 1968; Schaffer 1974; Giesel 1976; Stearns 1976) or sometimes even be necessary for the existence of the character, as in seed dormancy (Cohen 1966). In those models where environmental stochasticity affects character optima, an implicit assumption is that the affected characters, in turn, influence or mediate the environmental variance. For example, adaptations to plant stress or stress strategies (Grime 1977, 1979) reduce the impact of growthlimiting resource shortages. With these considerations in mind, we will describe a comparative study of the life history characters in two related annual plant species, Limnanthes alba and L.