Life History Evolution in Seven Milkweeds of the Genus Asclepias

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Abstract
Life history differences among A. exaltata, A. incarnata, A. purpurascens, A. syriaca, A. tuberosa, A. verticillata and A. viridiflora in SE Michigan [USA] are correlated with differences in microhabitat, exposure to herbivores, and competition. Components of each species'' reproductive strategy include number of stems per plant, number of umbels per stem, number of flowers and pods per umbel, number of seeds per pod, seed weight and annual increase in reproductive potential. Components of each species'' selective regime include the herbivore load (measured by the frequency of plants damaged by predators or animals parasites), competition (measured by the proportion of non-flowering plants and by the density of competitors), and environmental uncertainty (measured by annual mortality rates). The study involved observation of 3482 individually labeled plants during portions of 3 growing seasons. The species are long-lived perennials and have a common pollination system and mechanism of dispersal. Within this adaptive zone the species have a variety of mechanisms of seed production adjusted to the pattern of herbivore damage and opportunities for seedling establishment. A. exaltata is a woodland species with a high herbivore load and low annual reproductive output, but with a remarkable ability to withstand herbivore damage from deer and the chrysomelid beetle Labidomera clivicollis. A. incarnata is a wetland species with relatively low annual survival due to erosion of pond banks; seedling densities are often very high and each plant produces many pods, each with a few large seeds. A. purpurascens is rare in the study area; it is similar to A. exaltata but has more seeds per pod. A. syriaca is a ubiquitous weed of open upland habitats and is the only species with vegetative propagation. It has many seeds in each pod and a prolonged flowering season. The cerambycid beetle Tetraopes tetrophthalmus and the weevil Rhyssomatus have a coevolved relationship with A. syriaca. A. tuberosa invades the highly diverse and stable old-field forb community. One established this species has a very high annual survival, virtually no herbivore load, and low annual seed output. A. viridiflora is widely distributed in old-field habitats. It has a heavy herbivore load, long flowering season and large seed crop. A. verticillata occurs in scattered colonies on dry open hillsides. It has a long flowering season, low herbivore load, long prereproductive period and a large crop of small seeds. This study demonstrates that interpretations of the evolution of reproductive patterns should involve considerations of herbivore loads and environmental uncertainty as well as competition and the growth trajectory of the population.