Sexual selection or wind dispersal ? Alternative explanations for the evolution of gigantism in Dahlia species ( Asteraceae )

Several species of herbaceous plants located at Cuerici Biological Reserve, Costa Rica, grow to an unusually large size relative to confamilial species. Lief et al. (1996) provided evidence that this gigantism was due to sexual selection (mediated by pollination) for large, high inflorescences (clusters of flowers). An alternative hypothesis maybe that gigantism is a product of selection for increased seed dispersal distance. We tested these hypotheses with studies of Dahlia sp. , an unusually large species of the Asteraceae. Pollinator visitation rates, but not seed set, were greater in high inflorescences compared with low inflorescences. However, winged seeds released from 4 m traveled 5 m farther, on average, than seeds released from 2 m (4 m and 2 m are representative heights of tall and short flowers). This difference in dispersal distance seems large enough to be biologically meaningful because seeds dispersed from 4 m would commonly escape the proximity of adult plants while seeds dispersed from 2 m would not. Therefore, enhanced seed dispersal is one advantage of increased height in Dahlia sp. The seed dispersal hypothesis, however, does not easily explain the prevalence of gigantic herbs in the high altitude tropics. Additional selection pressures introduced by an unusually high nutrient availability that permits rapid, vegetative growth in these ecosystems may also have contributed to the evolution of gigantic herbs. Observation periods lasted 30 min. and took place between 09:00 12:00 on 29 January 2000. Chi–square analyses were used to test for effects of flower height on the number of individual insects that visited and the total number of visits to flowers (some individual insects visited more than one flower in the patch). The mean number of visitations per pollinator for high and low flowers was compared using a Student’s t-test. To investigate the relationship between flower height and seed production of Dahlia sp. , we sampled a single, closed, inflorescence from 10 plants in each of 2 size classes (high ≥ 4 m; low ≤ 3 m) and recorded the total number of seeds and wings per inflorescence. Preliminary studies also showed that not all flowers set seed, therefore we estimated the proportion of pollinated female flowers in each inflorescence by dividing the number of seeds by the number of wings in each fruit. A Student’s t-test was used to compare the proportion seed set of high vs. low fruits. To test the seed dispersal hypothesis, we first measured the heights of 20 fruiting Dahlia sp. to quantify the natural height range for the species (2.0 – 6.5 m). The frequency distribution of heights showed two modes, one near 2 m and another near 4 m, so we chose these as heights for seed dispersal measurements. We released 50 seeds from each height, and recorded their horizontal dispersal distance. Wind speeds were < 5 mph during all trials. The 4 m release point, accessed by climbing a tree, was directly above the 2 m release point. The dispersal area was relatively free from obstructing vegetation and each seed was untouched from release to contact with the ground. Seed dispersal distances were log-transformed and the averages were compared using a Student’s t–test. We also compared the frequency distributions of dispersal distances from each height.