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Exotic Earthworms or Overabundant White-Tailed Deer: Who is driving this ecosystem?

Jansyn Thaw, Vassar College ’13

Field Station and Ecological Preserve Manager: Keri VanCamp

Listen to Keri VanCamp on Vassar Radio: The Future of the Forest (mp3 - 10.3 MB)

To better understand the forces that are driving ecosystem change, we examined the relationship between invasive earthworms and overabundant white-tailed deer on the 527-acre Vassar Farm and Ecological Preserve (VFEP). Both earthworms and deer have independently been shown to negatively impact herbaceous species diversity, abundance, and tree seedling survival. Some ecologists believe that deer may have a synergistic relationship with earthworms, suggesting that if earthworms and deer both populate a forest the results can be devastating.

In 2008, we began to examine the effects of deer overpopulation at three sites on the Preserve by comparing fenced and adjacent unfenced areas. After four years, a noticeable difference in vegetation can be observed between the two areas. Inside the fence, the species richness, number of woody saplings, and number of shrub seedlings have increased significantly, showing that deer are noticeably affecting forest regeneration.

Though the effects of overabundant white-tailed deer have been well documented on the VFEP, until now the status of the earthworm population has been undocumented. To assess the abundance of earthworms on Vassar’s Preserve, thirty coverboards were placed adjacent to established vegetation subplots both inside and outside of the deer-exclosures. After two weeks, earthworms and slugs under the coverboards were collected, counted, weighed, and measured.

We found that deer-excluded areas had significantly more earthworms, an average of 42 earthworms per square meter, while the open areas averaged only 12 worms per square meter. We suspect that the greater number of saplings in the fenced areas has resulted in more leaf litter and therefore more available resources for the earthworms. Our vegetation data shows increased abundance and diversity in the exclosed areas suggesting that white-tailed deer are having a more profound effect on forest vegetation than earthworms; stunted forest regeneration in the unfenced areas is more likely a result of deer browse than earthworm invasion.