Plants: Native, Non-native, or Invasive?

Discussions of plant species these days soon turn to native versus non-native, with non-native plants quickly and decisively being labelled invasive. Most reasonable gardeners would probably agree that when choosing plants, choosing native plants is a sound decision. But, what does it mean for a plant to be native? To assert that a plant is native to Europe, or to the U.S., places that plant in a pretty large bucket with many variations of climate and ecosystem type. Picking a point in time and place, such as the moment Henry Hudson entered the Hudson River Estuary[1], seems largely arbitrary, and ignores the influences and changes rendered to the land by European and other settlers since that point in time, up to and including climate change. At least one definition of invasive species, generally and not limited to plants, asserts that a native species cannot be invasive.[2] Again, I would argue, this assumes conditions have not changed. In conversation with Akiko Busch, Erik Kiviat, Executive Director of Hudsonia at Bard College, asserted that either a native or non-native species could be considered an invasive and that “invasive…means an overabundance.”[3] Poison Ivy (Toxicodendron radicans) and Virginia Creeper (Parthenocissus quinquefolia) are both native to the U.S. and both seem to fit that definition of invasive. In her February 29, 2016 article in The New York Times entitled “Invasive Species Aren’t Always Unwanted,” Erica Goode informs readers that the term invasive was not a part of the lexicon until 1958 when Charles Elton wrote a book called The Ecology of Invasions by Animals and Plants in which he employed military-style lingo, post WWII.[4]

Habitat fragmentation and edge influence provide an opportunity for non-native species to gain traction and take hold. Jonathan M. Labaree explains that a healthy ecosystem, of which naturally occurring change is a part, possesses mechanisms to hold in check or repel non-native species but that human-altered ecosystems to which native species have not fully adapted are susceptible to the introduction and success of non-native species.[5] In point of fact, species have been on the move since life appeared and humans have altered ecosystems since they walked the planet. William Cronon explains how very early in the English settling of New England,[6] native grasses and other plants which had not evolved to serve as feed for grazing animals were, over successive years, being diminished and replaced by “‘English grasses’ such as bluegrass and white clover” “which had adapted themselves to the harsh requirements of pastoralism.”[7] Those grasses were “Initially carried to the New World as shipboard fodder and in the dung of the animals that ate them.”[8] He adds that “By the 1640s, a regular market in grass seed existed…and within one or two generations the plants had become so common that they were regarded as native.”[9] According to the “News of the Wild” section of a recent issue of National Wildlife magazine, computer modelling was used to compare the ranges of 13,575 native and non-native species. The results of the study showed that “Nonnative [sic] species are more widespread than native.”[10] The piece concluded with a quote from Bethany Bradley the lead scientist on the study: “The future may already be here.”[11] In his 2013 article “Gardening for Climate Change,” James Barilla argues that greater emphasis in plant choice needs to be given to a diversity of species capable of surviving changes to the climate rather than whether they are native or non-native.[12]

Climate change is already a driver of species migration and is sure to play a greater role. In their 2013 article, “Wildlife Habitat Connectivity in the Changing Climate of New York’s Hudson Valley,” Howard and Schlesinger used computer modelling to project climate change based alteration to the ecosystem onto selected habitat patches in the Hudson Valley, habitat recognized as fragmented. The study focused on 26 at-risk[13] species (including birds, mammals, odanates, amphibians, and reptiles and turtles)[14] in diverse habitat. “The study area was composed of 60% forest; 17% agriculture, grassland, and open habitats; 9% urban development; 8% wetland, 3% water, and 2% shrubland.”[15] The model suggested that climate change would push these species to move northward seeking suitable habitat. The researchers’ objective was to determine whether suitable habitat would be available and whether the species would likely be able to find a path or corridor to the new habitat.[16] There were a lot of confounding factors and limits to the computer projection, for example, the modelling did not allow the researchers a close enough view to recognize real barriers to movement such as small streams or man-made barriers like culverts, walls, or fences. Differences between species such as exhibiting edge aversion or low motility would impact the reality of their migration. In addition, it is reasonable to project an increase in fragmentation and creation of physical barriers during the projected time frame of change.[17]

Computer modelling has also been used to project changing habitat for plants, for example, the whitebark pine. Whitebark pine seeds are a staple food of the grizzly bear and are endangered in their native Rocky Mountains and Sierra Nevada due to a combination of fungus and pine beetles.[18] Computer models show that a portion of British Columbia some 500 miles north will eventually become suitable habitat for the whitebark pine due to climate change.[19] Computer modelling does not have to be limited to the researchers’ labs however. In the case of the grizzly and whitebark pine example, real action, in an attempt to get ahead of the predicted northward movement of habitat, has already been taken. In 2007, a forestry graduate student hiked, carrying 18,000 whitebark pine seeds, into the British Columbian mountains where she planted them.[20] Data reported as of 2012 indicates that 20% of the seeds germinated and as a result, researchers have expressed, at least as far as the transplanting of the whitebark pine, “cautious optimism.”[21] Whether climate change ultimately drives the grizzly population to the same mountains in British Columbia will take more time to observe. (continued)

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