Recently there was some correspondence on the ANPC Facebook group in which a paleoethnobotanist was quoted as having complained about the use of the term invasive in reference to plants. I presume she did not like the word because invasiveness is usually associated with negative human agency and her point was that humans have interacted with, and moved plants around, for thousands (tens of thousands?) of years. This begs the question: if plant distribution was being influenced by human intervention in the distant past, what are the meanings of the terms native, non-native and invasive in today’s context?
Plant movement naturally
First of all, let’s take a look back, way back. A quick check of Google reveals that flowering plants evolved about 130 million years ago, conifers even earlier, and humans about 315,000 years ago. So, plants existed and evolved without any interference from humans for millions of years while obligingly creating the conditions for animal life by creating organic matter for food from sunlight and inorganic substances.
Plants would have moved around for millions of years without human agency, by wind, water, plate tectonics, and transport by animals. Nobody was around then to ponder nativeness.
Re-colonization after glaciation
Looking more recently and locally, fast forward to about 12,000 years ago and the end of the last glaciation. The retreat of the glaciers left the land barren of vegetation except for a few ice-free areas and mountain nunataks. In the northern hemisphere glaciation extended as far south as the northern US and over a lesser area in the southern hemisphere. Humans were around then, but in such small numbers and with such few resources in such a cold and barren landscape that they would have had little impact on the mobility of plants. The latter would slowly recolonize the land from the aforementioned ice-free areas, including those to the south (or north), under their own aegis, largely by wind- and water-borne seeds. (It is no coincidence that there are so many spruce and aspen trees in Canada; in Europe, birch and pine were major re-colonizers.).
Movement of plants by humans
Fast forward again to the last 2-1,000 years when humans were moving around the globe significantly, including embarking on country-wide invasion and colonization. Plants were traded and introduced as food, medicinal, agricultural and horticultural goods and later extensively relocated. Crops grown in one tropical or temperate country could be grown in another with similar conditions. However, during the earlier part of the last millennium, even though people had animals and wheeled transport, and boats, their movement and that of the plants was limited, not least by time (although plants could be carried long distances as seed and remain viable).
Plant movement in modern times
Now time-travelling to the last couple of centuries, we find the reason why the word invasive is so current in biological circles. Human numbers (and therefore our earth-altering activities), mobility and technology have increased tremendously. We now have fast ships and airplanes. I read a comment that in the old days of sailing ships plants would often die and insects accidentally taken on board would also expire on long voyages because after completing their life cycle to adulthood they would not find a suitable environment in which to live. This is less often the case in our age of fast travel and refrigeration. In short, we now have many more people using sophisticated technology to shuttle around the globe, thus deliberately or accidentally taking plants (and other organisms) with them.
Naturalization and invasiveness
Of the number of plants that will find themselves accidentally in a foreign land, the majority will die. Some will settle in and reproduce themselves without human assistance, a process known as naturalization. Not all of these will develop the characteristics that earn them the title of invasive. They may simply be recognized as non-native. A further few will have or will develop the ability to reproduce in large numbers that threaten to overwhelm native ecosystems or (as prolific weeds) cause economic losses. The potential to cause such harm merits the label invasive. Invasive species, both plants and animals, have usually been imported to their new lands by the actions of humans, or have been able to multiply because of human modification of the environment. (They often take advantage of the disturbed habitats created by humans to become weeds.) Invasiveness is therefore a feature of modern life, an unfortunate accompaniment of human technological success and domination of the planet. After habitat loss, it is considered the second major cause of species’ declines and extinction. What makes these plants invaders, developing rapid and aggressive spread when often they have been well-behaved members of their natural communities in their homelands, provides a whole new field of biological enquiry.
Another term used by biologists is adventive. This applies to plants that do not establish or naturalize, persist only for a short time, but re-appear because they are constantly being re-introduced. The term indigenous with a small “I” is also used by biologists to refer to native flora and fauna.
“Native” defined
But let us return to the basic term native. What do we understand by it? Simply put, the concept refers to a plant or animal that is living in the land or region in which it or its ancestors originated and evolved. But given that we can’t know the origin and movement of plants in geologic times, this would seem impossible to determine. Obviously plants that existed in an area before glaciation and then re-colonized it afterwards would be considered native. But what about others from farther afield? For Canada we have a practical workable definition: native plants are those that lived in this country prior to European exploration and settlement, when human mobility became a force majeure.
Detecting nativeness
For those of us who operate in the present, with some knowledge of present-day plant distribution, the distinction between native and non-native is relatively clear. Nevertheless, recognizing nativeness can be complex. It should be noted that plants can be native over a wide territorial range, encompassing several countries. For example, lingonberry or rock cranberry (Vaccinium vitis-idaea) is circumboreal, ranging from Greenland across North America to northern Eurasia.
A further complication is that a single species can be present in an area as both native and non-native genotypes. In North America, examples are the grasses reed canarygrass (Phalaris arundinacea) and common reed (Phragmites australis). For the latter, Phragmites australis subspecies americanus (American reed) is the native version of the grass, while Phragmites australis subspecies australis (European reed) is considered introduced and invasive. These genotypes are virtually indistinguishable to look at but behave very differently.
A rough guide to distinguishing natives from non-natives is that the former occupy natural (often long-existing) habitats such as original prairies, old-growth forests and wetlands, whereas the non-natives tend to occupy disturbed habitats such as fields, waysides, gardens and other urban environments. However, invasive plants will also occur in natural plant communities and if they are abundant these communities have been recognized as novel, hybrid or semi-natural ecosystems. These are usually the norm now in heavily settled areas. The Canadian plant database Canadensys VASCAN features maps showing in which parts of Canada a given species occurs, and whether it is native or introduced or extirpated.
Plant history provides a new dimension of knowledge
Maintaining the distinction between these terminologies can often lead to intriguing forays into ethnobotany, ecology and plant history, in other words, getting more bang for your buck by knowing your plants! The sycamore tree (Acer pseudoplatanus) is such a basic component of the British landscape that I was quite old before I discovered that it was actually naturalized in Britain, native only to central, eastern and southern Europe. Likewise, the famous horse-chestnut tree of English village greens is non-native; it was introduced in the late 1500s and, less invasive, it does not so readily penetrate woodlands as the sycamore. Learning this gave me a whole new perspective on the ecology of the British Isles, which, although a group of islands, has the human influence of a strong maritime history, connecting it with continental Europe and farther afield. It thus has an extensive naturalized flora. Unfortunately, although the naturalized species make valid ecosystem contributions to their communities, continual human expansion means that old plant communities, such as ancient woodlands, are being lost, and with it the original native, non-mobile species. This same maritime technology also enables Britain to “export” species.
Conserving native ecosystems
The ethnobotanist’s interest is to explore human use of, and interactions with, plants. The role of organizations and native plant societies such as ENPS in growing native plants and promoting their use in human and natural environments is somewhat different. Our interaction with native plants has the goal of compensating in some small way for the loss of native populations occasioned by human colonization and population growth, urbanization and industrial expansion, particularly in contemporary times.
Putting value on semi-natural ecosystems
The ethnobotanist’s post touched on another point, the increasing assumption, as she saw it, that only natural ecosystems uncontaminated by human influence have value. In fact, we do value semi-natural ecosystems with a mixed composition of natives and non-natives immensely, but strive to preserve their proportion of naturalness against the onslaught of invasiveness!
Smooth brome grass (Bromus inermis) is native to Europe but invasive in western North America where it was introduced as a forage grass early in the twentieth century. It has since become naturalized. Its harm derives from the fact that it invades and overruns ecosystems such as native grasslands. However, because of its use as a forage grass it is not legally considered to be a weed.
Footnote to Invasiveness
Can introducing non-native species to another environment ever be a good thing?
There seems to have been an emergence of the scarlet malachite beetle (Malachius aeneus) in my Edmonton neck of the woods (Rio Terrace). I have had several in my house and outside. According to Wikipedia, it was introduced to North America in 1852 and is now widespread. In Britain, however, where it is native, it has become extremely rare and is the subject of conservation efforts. Apparently, the adults like open meadows full of flowers.
Still on the subject of introductions, I recall hearing a Eurasian skylark, a bird about which I am particularly emotional for some reason, singing in New Zealand, to which it has been introduced, and thinking, “Thank goodness it’s here!” It has declined appreciably in Britain, due to changes in agricultural practices.
I don't yet know of any plant examples, although I’m guessing there are some.
This raises the question, can introduction and subsequent naturalization be a way of preserving species? Presumably the introduced species find more available habitat as well as less competition in the new land. And can they happen without harming the native populations?
