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ABSTRACT:

Leaves of Datura wrightii in California (USA) are covered either with glandular trichomes ("sticky" plants) or non-glandular trichomes ("velvety" plants), and sticky plants are resistant to many insect herbivores. Theoretical models suggest that variation in resistance to herbivores may persist if resistance is costly and herbivore damage is variable. If so, then natural selection should favor resistant plants when damage is high and disfavor it when damage is low. However, without long-term equivalence between costs and benefits, then natural selection either should drive the trait to fixation if it has a net benefit or eliminate it if it has a net cost. Previously we showed that the production of glandular trichomes carried a net cost in short-term studies, suggesting that this expensive resistant trait might be eliminated from D. wrightii populations. To test this hypothesis we monitored survival and seed production of sticky and velvety D. wrightii in the presence of herbivores in 11 natural populations over four or five years. In eight populations where both types occurred, the finite rate of increase for velvety plants was 60-274% greater than for sticky plants. Plant survival averaged between two and three years and did not differ significantly between types. Because seed production consistently favored velvety plants, our prediction that the proportion of sticky plants should decline was met within five years in seven of those eight populations. If the variation in trichome morphology in D. wrightii is indeed maintained by variation in costs and benefits of glandular trichomes, then the periodicity of such variation extends over more than five seasons.