Plant Invasive Species

http://www.bernards.org/Environmental%20Commission/
Document/Springtime%20Trees_files/image017.jpg

Many plant invasive species plague the Pacific Northwest and disrupt ecosystem functioning. An example of one of these invasive plant species is English ivy, Hedera helix, which has invaded at least 28 states in the United States (Yang, 2013). English ivy is is a vine that can easily out-compete other native plant species and forms a dense monoculture groundcover (Richards, 2008). It can grow over shrubs and tree trunks, which can inhibit the growth of native vegetation. English ivy can block shrubs off from sunlight and can make plants structurally weak because of the weight of the vine on the plant. Trees tend to survive but can result in decreased vegetation on the tops of the trees. English ivy grows incredibly rapidly and needs very little water or light once established. This vine was first established in the Pacific Northwest as an ornamental plant but has now become a very serious invasive species that needs to be controlled.

One study done in Seattle parks showed that "total cover increased from approximately 75% in non-invaded plots to over 100% in ivy-invaded plots (Dlugosch, 2005)." These sites with increased cover had an additional layer of ivy that decreased sunlight availability for the plants below, thus inhibiting the growth and germination of those plants. Overall, English ivy was found to have a substantially long-term impact on understory cover by increasing vegetative cover and suppressing dominant native shrubs, thus disrupting the species composition and diversity of the forest community. 

Dlugosch, 2005
Percent cover of native and non-native herbs and shrubs in plots that
 have been invaded by English ivy and ones that have not been invaded.

English ivy tends to establish itself well in habitats that have been disturbed, so it often settles down in areas that have been anthropogenically affected. Ivy can rapidly reproduce and germinate and has been shown to produce as much as 87.1 g/m^2 of biomass a year with an almost 100% germination rate (Biggerstaff, 2007). Another study shows that the dense ground cover that English ivy forms inhibits the emergence of other plants and has significant allelopathic effects on seed germination when the ivy is present. However, ivy does not damage the seed bank formation or germination of the local plant community, so there would not be any enduring negative effects if the ivy was removed (Biggerstaff, 2007). 

http://www.washingtonpost.com/rf/image_606w/2010-2019
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There have been many studies focused on the removal and eradication of English ivy. One of the more unique eradication methods studied was whether or not grazing by goats had an effect on levels of English ivy. Interestingly, it was found that English ivy cover dramatically decreased in plots that were heavily browsed once or twice (Ingham, 2010). However, if you don't have a pet goat, you can help with the ivy problem in your area by manually removing it and by deciding not to plant it as an ornamental plant.