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Reproductive success of wild parsnip

Jing Yang and Dana Dudle
Biology Department, DePauw University
Summer 2006

Introduction
Pastinaca sativa, the wild parsnip, is a biennial plant species in the family Apiaceae and is common in abandoned fields. During the flowering season, the plants are visited by many insects, such as beetles, bees, and flies. We observed ants visiting the plants most frequently in the Nature Park.  Past studies indicated that ants are "nectar thieves" of other flowering plants. We examined whether ants contribute to the reproductive success of wild parsnip.

Methods
We conducted two sets of pollinator exclusion experiments with two or four types of treatments: (1) open pollination, (2) crawling insect exclusion, (3) flying insect exclusion and (4) all insect exclusion. We observed pollinators and measured plant morphology (number of flowers, umbel height, light intensity, inflorescence density, etc.) in early June. Fruit set and seed set were assessed in July.

Results
Flying insects were the main pollinator of the plants. Umbel height was the only factor in our study that contributed to seed production. Although there were numerous ants on open-pollinated and flying insect-excluded umbels, the ants did not increase or decrease the amount of fruit or seed production. There were no significant differences between open-pollinated and crawling-insect-exclusion umbels. We observed a seasonal pattern of fruit and seed production. Later plants produced the same number of fruits but did not make as many seeds as the earlier plants.

Discussion
Flowering timing of the wild parsnips was one of the crucial factors in determining the outcome of this experiment. Because we began our study relatively late in the season, the differences in intensity of competition may have caused the number of fruit and seed set to vary. It is also important to increase the sample size and the number of observations of pollinators for more information and to reduce the statistical errors. Moreover, there are alternative studies that might be helpful to understand the roles of the ants. These studies include investigations of nectar of flowers while observing visitors and the quality of fruits/seeds of the flowers.