The Mechanism of Competition for Pollination between Two Forest Herbs

Abstract

The primary mechanism of competition for pollination between the two forest herbs Stellaria pubera and Claytonia virginica in piedmont North Carolina is interspecific pollen movement. The most common visitor, the bee fly Bombylius major, forages indiscriminately among flowers of the two species. In only one of five experiments did the presence of C. virginica reduce the pollinator visit rate per S. pubera flower, and in other experiments addition of C. virginica enhanced visit rate. Thus these plant species exhibit little or no competition through pollinator preference. In some natural populations, however, visits to S. pubera are frequently immediately preceded by a visit to C. virginica, and a flower receives less conspecific pollen and produces fewer seeds following such an interspecific visit than if the visitor has arrived directly from a conspecific flower. Interspecific pollen movement is responsible for most, if not all, of the reductions in seed set of S. pubera due to pollinator sharing.Although insects deposit a substantial amount of S. pubera pollen on stigmas of C. virginica, little C. virginica pollen is found on S. pubera stigmas. Moreover, application of foreign pollen to the stigma does not influence seed production of S. pubera. The effect of interspecific pollen movement is due to loss of conspecific pollen, not stigmatic interference.

Coexistence induced by pollen limitation in flowering-plant species

“…We may observe this type of species coexistence as the result of rare chance incidents that lead to the successful establishment of an alien species from lots of trials, most of which have resulted in failure and left the resident population unchanged. Our model not only supports the idea that sympatric plants have interspeci¢c di¡erences in their timings of £owering as the consequence of competition for pollinators (Waser 1978a(Waser , 1983Pleasants 1980;Campbell 1985;Campbell & Motten 1985;Ashton et al 1988), but also elucidates a feasible speci¢c mechanism for interspeci¢c di¡erentiation of the timing of £owering. Also, it has been reported that many genera of alien plants in the British Isles £ower in either an early or a late season of the year, which avoids overlap with the native plants (Crawley et al 1996).…”

“…It has been suggested from the results of various ¢eld studies that competition for pollinators contributes to the reproductive e¤ciency and £owering phenology of outbreeding plants (Waser 1978a(Waser , 1983Pleasants 1980;Campbell 1985;Campbell & Motten 1985;Ashton et al 1988). Waser (1978a) and Campbell (1985) revealed that interspeci¢c overlap in the timing of £owering in outbreeding plants causes a decrease in seed production, suggesting that interspeci¢c competition for pollinators occurs only between plants that £ower at the same time.…”

Partição de recursos florais de espécies de Sida Linnaeus e Malvastrum coromandelianum (Linnaeus) Garcke (Malvaceae) entre Cephalurgus anomalus Moure & Oliveira (Hymenoptera, Andrenidae, Panurginae) e Melissoptila cnecomala (Moure) (Hymenoptera, Apidae, Eucerini)

“…Em uma comunidade os padrões temporais de floração podem ter conseqüên-cias ecológicas sobre as espécies de plantas e seus visitantes florais (H EITHAUS 1974;LrNSLEY 1978;RA THCKE 1983 ;ARMBRUSTER & HERZIG 1984 ;CAMPBELL & MOTTEN 1985;MURRAY e/ aI. 1987).…”

Indirect interactions between invasive and native plants via pollinators

“…One type of indirect interactions between flowering plants occurs when two plant species compete for a common pollinator, with negative consequences for the reproductive success of one or both species (Waser 1983;Campbell and Motten, 1985). For example, pollinators may neglect certain flowering species because neighbouring plants offer larger amounts of nectar (Chittka and Schürkens 2001), or pollinators may transfer large quantities of heterospecific pollen that interfere with fertilisation by conspecific pollen (Campbell and Motten 1985;Feinsinger 1987). Such situations are possible when alien plant species invade new communities and start interacting indirectly with native species through shared pollinators (e.g., Richardson et al 2000;Stout et al 2002;Ghazoul 2004).…”