Invasive species in New Jersey - Series 6 Article 1 - Introduction to Invasive Species

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By Dave Parsons    

   Invasive plants are a surprisingly misunderstood phenomenon.  Several thousand new species of organism get introduced to American soil every year and roughly 15% will become “invasives.”[1][2]  This article seeks to define what an invasive plant species is and discuss the impact these species can have.  Later articles in this series will discuss popular plant varieties that remain readily available in the marketplace despite having already had significant impact as invasives.
     Most sources point to Presidential Executive Order (PEO) #13112--signed by then President Bill Clinton on February 3, 1999--as the defining moment and document for what constitutes an invasive organism. Accordingly, PEO #13112 defines an invasive as, “An alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.”[3]  The United States Department of Agriculture’s (USDA) current working definition for an invasive plant is, “A plant that is both non-native and able to establish on many sites, grow quickly, and spread to the point of disrupting plant communities or ecosystems.”[4]   The common threads in these (and most other definitions) of an “invasive” are (1) it is not native to the particular natural system affected and (2) it is causing some type of damage within that system. Defining an “invasive” may seem pretty straight forward; however, an organism’s impact is often insidious and it may take years or even decades to recognize a particular species as a problem.

Giant Hogweed--Heracleum mantegazzianum in full bloom.

Terry English, USDA APHIS PPQ
www.forestryimages.org
     Some invasives such as Giant Hogweed (Heracleum mantegazzianum) pose a direct threat to human health and are quickly identified as problems.[5]  Other invasives do their damage by marginalizing native plants or even causing their ultimate disappearance. One study estimated that nearly 50% of our native plants species categorized as “imperiled” face pressure from invasives.[6]

Japanese Stiltgrass--Microstegium vimineuminfestation.

Chris Evans, River to River CWMA
www.forestryimages.org

     Japanese Stiltgrass (Microstegium vimineum) is an example of an invasive that utilizes a variety of tactics to outcompete native species. It flourishes in a wider variety of lighting and soil conditions than similar native plants and it can populate disturbed areas more rapidly than local flora.  M. vimineum grows in dense stands that crowd out potential rivals.  It has a longer growing season than similar native grasses.  This allows it to sprout earlier in the spring to get a head start against competitors and remain active later into the fall to allow it to spread into spaces where other grasses have gone dormant.  Each individual M. vimineum plant disperse 1000s of seeds each season which in turn can remain viable for years.  To add insult to injury, M. vimineum is also last on the menu for many herbivores who ignore it in favor of dining on native alternatives.[7]  With many advantages over native floras it’s easy to understand why M. vimineum has the potential to disrupt native ecosystems.
     Not surprisingly many non-native plants introduced to American soil intended for use in landscaping or agriculture have the potential to become invasives.  What is surprising is how long a popular species remains in the marketplace after it is identified as an invasive.  An explanation for this persistence is the lag time it takes for some species to reveal themselves as invasives.  A certain ideological inertia occurs when a once popular plant becomes is labeled as invasive.  That favorite shrub or tree which has been a part of the landscape for years cannot possibly be a threat!  A responsible gardener recognizes the real impact of invasives and avoids or eliminates them from use regardless of how long it takes to identify them as such.  A series of articles will follow, that examine in turn the impact of invasive species that remain readily available (if not popular) in the marketplace despite the negative effects they have imposed on native ecosystems  and--in at least one case--the threat it represents to human health.



[1] In this series of articles the informal term “invasive(s)” is used to refer to any invasive species.
[2] Nancy Knauss, Master Gardener Manual, (PennState Extension, College of Agricultural Science, The Pennsylvania Sate University, 2016), 644.
[6] David S. Wilcove et al., “Quantifying Threats to Imperiled Species in the United States,” BioScience, 48, no. 8 (Aug., 1998): 607-615.

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