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Journal Article

Pollen Dispersal by Hummingbirds and Butterflies: A Comparative Study of Two Lowland Tropical Plants

C. J. Webb and K. S. Bawa
Evolution
Vol. 37, No. 6 (Nov., 1983), pp. 1258-1270
DOI: 10.2307/2408846
Stable URL: http://www.jstor.org/stable/2408846
Page Count: 13

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Topics: Pollen, Plants, Flowers, Dyes, Hummingbirds, Pollinators, Pollen flow, Flower stigma, Foraging, Female flowers
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Pollen Dispersal by Hummingbirds and Butterflies: A Comparative Study of Two Lowland Tropical Plants
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Abstract

In two self-compatible species, Malvaviscus arboreus and Cnidoscolus urens, pollen flow was studied using fluorescent dyes. Experiments were carried out during the dry season in Guanacaste Province, Costa Rica. Our aim was to compare the two species: M. arboreus is a hermaphroditic, hummingbird-pollinated, woody perennial, whereas C. urens is a monoecious, butterfly-pollinated, annual herb and in the population we studied grew in relatively dense stands. Pollen was carried up to 225.5 m in M. arboreus and to 31.2 m in C. urens; the leptokurtotic dispersal curve was similar for the two species with dispersal to greater than 100 m and 8 m being rare for M. arboreus and C urens respectively. Pollen dispersal patterns varied for the same plants from day to day with large plants of M. arboreus having pollen dispersed to as many as 24 other plants over three days. For C. urens pollen dispersal was less extensive with a maximum of dispersal to eight plants from a single plant over three days. From the actual dispersal distances the potential number of plants an individual may donate pollen to over a flowering season was calculated and varied from 8 to 140 for M. arboreus, and from 0 to 61 plants for C. urens. Similarly, individual plants may receive pollen from many different sources. In general, pollen dispersal was much less extensive in the population of C urens than in M arboreus. Butterflies forage over shorter distances than hummingbirds, and their foraging behavior is influenced by their relatively low energy requirements and the need to avoid predators. The difference between the two plant species in population density probably also influenced pollinator behavior and thereby pollen flow. Most pollen removal and deposition occurs in the first few hours of daylight for both species. In M arboreus pollen removal and deposition was followed accurately and it was found that only 3% of pollen presented by a flower reached stigmas of the same or other flowers. In M. arboreus, but not in C. urens, plant size affected pollen dispersal. Pollen flow experiments showed that large individuals with many flowers had pollen carried to more plants than did small individuals with fewer flowers, although there was no difference in the average distance to which pollen was dispersed. The number of grains removed from anthers was also greater for large than for small plants of M. arboreus Thus large and small plants may function to different degress via pollen and ovule pathways to fitness and differences in the diversity of plants from which they receive pollen may affect the genetic variability of their offspring. Some 70% of the pollen grains deposited on stigmas of female flowers in C. urens are from other plants. In contrast hermaphroditism and more systematic working of flowers by pollinators probably results in a greater level of selfing in M. arboreus.

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