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Interspecific pollen loss by hummingbirds visiting flower mixtures: effects of floral architecture
Ecology, March, 1996 by Carolina Murcia
For example, the data of Tables 3 and 5 and Fig. 2 suggest that a competitive asymmetry may exist between short- and long-flowered species even if their pollen placement on the pollinators they share is entirely distinct. Corollas and other parts of long flowers are likely to scrape against the pollen loads deposited by shorter flowers, while the pollen loads deposited by the long flowers themselves might well be untouched by any part of short flowers. Among the five intervening species studies here, long-flowered Hansteinia and Dicliptera affected Palicourea pollen transfer the most (Table 3C), and, like other species, removed most of the grains from Palicourea pollen loads carried by birds [ILLUSTRATION FOR FIGURE 2 OMITTED]. However, few Palicourea grains contaminated the exserted sexual parts of Hansteinia in return (Table 5), and Palicourea pollen loads on birds scarcely affected the ability of Hansteinia to place pollen [ILLUSTRATION FOR FIGURE 2 OMITTED]. This reasoning might apply within polymorphic species as well: for example, pollen from long-stamened, short-styled individuals of distylous species may face fewer risks of scraping and may transfer more effectively to long-styled, short-stamened individuals than pollen travelling in the opposite direction (cf. Feinsinger and Busby 1987). In more general terms, the Pollen Scraping Hypothesis could provide new slants on theories about the evolution of flower form (e.g., Grant and Grant 1965, Leppik 1972, Levin 1978, Willemstein 1987, Nilsson 1988, Vogel 1990, Fenster 1991, Herrera 1993) or the evolution of pollen: ovule ratios (Cruden 1977).
The Sexual Architecture Hypothesis as a special case
Where flowers have constricted tabular, trumpet-shaped, or otherwise complex and three-dimensional corollas, predictions derived from the Pollen Scraping Hypothesis should differ substantially from those derived solely from the Sexual Architecture Hypothesis, providing a clear test between the two. However, where the principal floral structures to contact pollinators are, in fact, the anthers and stigmas, the Pollen Scraping Hypothesis in effect becomes the Sexual Architecture Hypothesis. For example, in both the Neotropical genus Dalechampia (Armbruster 1985) and the Australian genus Stylidium (Armbruster et al. 1994), only the anthers and stigma are likely to scrape against the pollinator. In these cases, predictions based strictly on sexual architecture reflect biological reality and are upheld. Similarly, special mechanisms in other plants may minimize the potential for pollen loss to any structure on heterospecific flowers other than the stigma itself; again, the Sexual Architecture Hypothesis is relevant. For example, among some orchids the precise placement and impressive adherence of pollinaria on visitors' bodies (e.g., van der Pijl and Dodson 1966, Ackerman 1981, Dressler 1981) usually minimize "interspecific pollinarium transfer" even when pollinators rub against other floral structures. Nonetheless, for many pollination guilds, such as flowers pollinated by lepidopterans or syrphid flies as well as those pollinated by hummingbirds, we expect the Pollen Scraping Hypothesis to provide the most robust predictions.
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