Oviposition by mutualistic seed-parasitic pollinators and its effects on annual fitness of single- and multi-flowered host plants

Abstract

The balance of intimate relationships between plants and seed-eating pollinators can depend on pollinator behaviour in relation to floral characters, such as flower size and flower number. Here, we examined how pollinator oviposition in relation to these traits affected annual fitness (seed output) of single- and multi-flowered Trollius europaeus along altitudinal gradients in subarctic Sweden and the French Alps. Small flies (Chiastocheta spp.) pollinate T. europaeus and their larvae feed on developing seeds. Assuming that late flowers in multi-flowered plants attracted flies to the earliest flower on the same plant, we expected more eggs and higher seed predation in early multiple flowers than in single flowers. Relative seed predation would thereby increase with flower number. Both in Sweden and the Alps, more eggs were placed on large flowers. Early multiple flowers were slightly larger than single flowers, and about twice the size of secondary flowers. As a result, and possibly combined with the effects of secondary flowers, early multiple flowers attracted more ovipositing flies and experienced relatively higher seed predation. However, this did not generally result in higher seed predation of multi-flowered hosts. Multiple flowers had greater seed output than single flowers at all altitudes, also in the high alpine and subarctic sites, where single flowers were more abundant. We hypothesise that the distribution of multiple flowers generally is enforced by environmental factors, rather than by fly-host plant interactions, because only very rarely (in triple-flowered alpine plants) was seed predation disproportionate, and the relationship skewed to the disadvantage of the host. The outcome of the mutualistic interaction was often similar in alpine and subarctic populations, but the underlying factors were different. Subarctic flowers had high abortion and low predation rates, while alpine flowers experienced the reversed situation. The higher fly abundance in the Alps suggests a more intense mutualistic interaction than in Sweden. Despite varying ecological and environmental conditions at these sites, the mutualistic relationship was generally in balance. However, when it was unbalanced, this could be explained by fly behaviour in response to floral traits, and by differences in fly abundance.

Speciation in the Globeflower Fly Chiastocheta spp. (Diptera: Anthomyiidae) in Relation to Host Plant Species, Biogeography, and Morphology

“…Few cases of such extremely specific pollination mutualisms have been documented: the Ficus spp.-Agaonidae wasp (Bronstein, 1992;Anstett et al, 1996), the Yucca spp.-Tegeticula spp. moth (Powell, 1992), and the Trollius europaeus-Chiastocheta fly interactions (Pellmyr, 1989;Jaeger and Després, 1998;Hemborg and Després, 1999;Jaeger et al, 2000Jaeger et al, , 2001. In accordance with the cospeciation hypothesis, in the fig-fig wasp system, one plant species is usually associated with one pollinating insect species (Machado et al, 2001).…”

Geographic and within-population variation in the globeflower–globeflower fly interaction: the costs and benefits of rearing pollinators’ larvae

“…including six populations sampled for 3 consecutive years (1995,1996,1997). Study sites represent a wide range of ecological conditions (Jaeger and Després 1998;Hemborg and Després 1999). This resulted in a total of 38 records, each representing a group of globeXowers sharing their pollinators, hereafter called a "population".…”

Relationship between capitulum size and pre-dispersal seed predation by insect larvae in common Asteraceae

“…These studies suggest that seed eaters may exact considerable fitness penalties on plants, which should favour individuals whose floral traits reduce such predation. Previous studies have suggested a link between floral display and seed predation, for example in Ipomopsis aggregata (Polemoniaceae;Brody 1992;Brody and Mitchell 1997), Lathyrus vernus (Fabaceae;Ehrlén 1996), and Trollius europaeus (Ranunculaceae; Hemborg and Després 1999). Among the Asteraceae, it has been shown that the number of eggs laid by the weevil Rhinocyllus conicus in flowerheads of Cirsium vulgare and Silybum marianum is positively related to the capitulum diameter (Zwölfer and Preiss 1983).…”