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Invasive Plants on Peary’s Eagle Island

By Priscilla Seimer, Harpswell Invasive Plant Initiative

The arrival of invasive plants on Peary’s Eagle Island is depressing, but not surprising. As noted below, they are very good at what they do – colonize and spread. What can be more confusing to people is just what an invasive is, and why controlling them is so important.

An invasive species is defined by the USDA as “non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health. Invasive species can be plants, animals, and other organisms (e.g., microbes). Human actions are the primary means of invasive species introductions.”

The reasons plants are initially introduced to an environment can vary:

  • Japanese barberry, Berberis thunbergii, Arnold Arboretum at Harvard, 1875.
  • Garlic-mustard, Alliaria petiolata, introduced on Long Island as a medical or edible option in the 1860s.
  • Multiflora rose, Rosa multiflora was introduced around 1886, then used in the 1930s as Dust Bowl fence breaks.
  • Canada thistle, Cirsium arvense, came over in the 1600s attached to sheep’s wool.

Invasives are very good at what they do. Generally, they leaf out much earlier than native species, grow and spread aggressively, are hard to control, and often impossible to fully eradicate. Many love disturbed sites, and one common way they spread is with fill used for construction. Fill may be full of invasive seeds without anyone knowing it.

With a plant such as garlic mustard, we spread it easily on our footwear, and animals spread the seeds as they move through areas where its seeds are present. The seeds are tiny, and the plant is prolific. Garlic mustard can survive happily in the understory – rare for an invasive – and can completely cover an area.

While birds spread some seeds, such as those from the Asian bittersweet and Autumn olive, most native fauna don’t like invasives. Nothing likes purple loosestrife, which can take over cattail marshes, removing an important source of food for animals and birds. Only ticks seem to like Japanese barberry. A Connecticut study has shown higher concentrations of ticks with barberries, although why is less clear. Critical to all this is that many insects do not like invasives (there are exceptions) and as invasives crowd out the natives our insect population declines, which impacts an entire system.

Our native plants and animals (including the all-important insects) evolved together. When invasive plants take over, this relationship is disrupted, and an ecosystem may be harmed or even lost altogether. Biodiversity is lost.

Invasive species are difficult to control, but it is critical that we do the best we can. Particularly on islands such as Eagle Island, where for much of the year no one is around, giving invasive plants more time to establish themselves.

The Harpswell Invasive Plant Partnership (HIPP) provides excellent information about invasives in our area through the Harpswell Heritage Land Trust website. https://hhltmaine.org/hipp/

For information on invasive plants in Maine: http://www.maine.gov/dacf/mnap/features/invasive_plants/invsheets.htm

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Thanks to Ted Elliman, New England Wildflower Society for some of the information used in this article.
Additional information from the United States Department of Agriculture website.

Black Swallow-wort (Cynanchum louiseae) on the beach at Eagle Island. One of the worst invaders on the Island (photo by Priscilla Seimer)