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Effects of ants on the reproductive success of wild parsnip

Jing Yang and Dana Dudle

Biology Department, DePauw University

Click here to download:
Jing Yang 's report (microsoft word; 0.5 MB)


Jing Yang and Betsy Feighner collecting
data on the plants

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. In our observations, ants were the most common visitors on the plants in the nature park while past empirical studies indicated that ants are "nectar thieves" of other flowering plants. We examined whether ants on wild parsnips are contributing to the reproductive success of the plants.

We conducted two different sets of pollinator exclusion experiments with either 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.

Our results suggested that flying insects were the main pollinator of the plants. Umbel height was the only factor in our study that contributed to seed production. Even though there were numerous ants on both the open-pollinated and the flying insect-excluded umbels, the ants did not increase or decrease the amount of fruit or seed production. In addition, there were no significant differences between open-pollinated and crawling-insect-exclusion umbels. A seasonal pattern of fruit and seed set was observed. For the later plants, even though they produced the same amount of fruits, they did not make as many seeds as the earlier plants.

Wild parsnip, Pastinaca sativa, grows
in abandoned fields in the Nature
Park. A wire mesh bag has been
placed around the flowers to exclude
flying insect pollinators.

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.