Propagule Size, Number, and Dispersion Pattern in Ambystoma and Asclepias

The empirical study of life history patterns has revealed a complex of interrelated adaptations that cannot easily be explained by reference to the models of demographers. Parental investment can be expended in many ways by manipulating propagule size, propagule number, and the pattern of propagule dispersal in time and space. Each aspect of reproduction entails a balance between the benefits and cost of current reproduction versus future reproduction. Natural selection results in the pattern of reproduction that maximizes the lifetime contribution of an individual to future generations. The compromise between propagule size and propagule number is modeled by the effects of competitive challenges during the early juvenile period, dispersal mechanisms, and mechanisms of predator defense. The position of the balance is modeled by the interaction of the level of investment per clutch and the functional dependence of early juvenile survival on propagule size. The spatial pattern of egg deposition is modeled by the energetic cost of nest construction and the mortality risk to the female and the foraging efficiency of nest predators. The advantage of multiple nests is related to the variance of the number of survivors rather than the mean number of survivors. The timing and size of successive clutches is modeled in relation to the length of the growing season, the extent of parental care, female mortality patterns, and the rate at which eggs can be produced. Successive clutches may be smaller and more widely spaced in time if the rate at which females can produce eggs or female survival decreases as the season progresses. In a group of four sympatric salamanders of the genus Ambystoma the pattern of egg production can be interpreted as a suite of adaptations to the annual uncertainty of the environment in which the larvae live. The salamanders differ in egg size, egg number, egg dispersion, size at metamorphosis, length of juvenile period, size at maturity, parental investment, and adult survival rate. They all reproduce during a few weeks after the ponds have thawed, have aquatic, carnivorous larvae, and are secretive during the terrestrial adult stage. In a group of six milkweeds of the genus Asclepias seed production can be interpreted as a suite of adaptations to the risk of herbivore damage, adult survival, colony size, and the spatial and temporal pattern of opportunities for seedling establishment. The species have different growth forms, habitat distributions, flower number and arrangements, number of follicles per plant, number of seeds per follicle, and individual seed weights. All species have similar mechanisms of seed dispersal, pollinator relationships, and herbivore defense mechanisms. These examples demonstrate some fundamental differences between angiosperms and vertebrates. The sedentary habit of plants often results in extreme phenotypic plasticity. The frequent occurrence of multiple shoot units per plant permits a diversity of breeding systems that may yield genetic diversity among half-sibs produced at the same time as well as a high level of spatial (different fruits) and temporal heterogeneity in seed production. Seeds may be dispersed long distances and may lie dormant for years until conditions are promising for seedling success. Vertebrates are constrained by their bilateral body form and single reproductive system. Clutches are usually discrete and probably are generally composed of full sibs that are born at nearly the same time. A wide range of developmental diversity is possible, from a prolonged larval stage to direct development. Mobility permits a high level of habitat selection, parental control over the microhabitat of the hatchlings, and the evolution of complex behavioral systems of parental care.