Patterns and Consequences of Competition for Pollination between Introduced and Native Species with Different Floral Traits.

Abstract

Competition for pollinators and avoidance of heterospecific pollen transfer have been assumed to be important factors promoting the evolution of specialized (restricted) floral morphologies. To test these assumptions, I performed field and greenhouse studies on prairie plants in Iowa. I also examined whether self-incompatibility reduces detrimental effects of heterospecific pollen. In a field study, neither Sisyrinchium campestre (Iridaceae) with an unrestrictive floral morphology nor Viola pedatifida (Violaceae) with a restrictive floral morphology showed a decrease in fruit or seed set when growing near patches of introduced Euphorbia esula (Euphorbiaceae) indicating that competition for pollination was weak or absent. In a survey of 29 species, plants with restrictive floral morphologies experienced less pollinator overlap with Euphorbia and received significantly less Euphorbia and other heterospecific pollen than those with unrestrictive (open) floral morphologies, as predicted. However, flowers with unrestrictive morphologies had significantly larger stigmas, and the density of heterospecific pollen (per stigmatic area) did not differ significantly between floral morphologies. Thus, correlated effects of stigma size may explain the patterns of heterospecific pollen receipt. Large quantities of Euphorbia pollen added to stigmas prior to conspecific pollen had no effect on fecundity for two self-incompatible species and reduced fecundity for three self-compatible species, as predicted. However, two other self-compatible species were not significantly affected, indicating that this relationship needs further testing. Additional experiments with two species demonstrated that Euphorbia pollen receipt is unlikely to affect fecundity in nature because reducing the quantity of Euphorbia pollen applied or eliminating the time delay between Euphorbia and conspecific pollen application reduced effects of heterospecific pollen receipt. To further explore the effects of relative abundance, pollinator visitation rates, and pollen carryover on competition by interspecific pollen loss, analytical and simulation models were developed which indicate that rare plants receiving few visits are most affected by pollen loss. Furthermore, increased pollen carryover is expected to result in the same average pollen receipt, but with a more uniform pollen distribution. This could increase or decrease competition by pollen loss, depending on the relationship between pollen receipt and pollination success.