One of the most popular justifications for eradicating non-native plants is the claim that they will out-compete native plants, ultimately causing their extinction. Innumerable studies have found no evidence to support that claim, but the belief persists amongst those who demand the eradication of non-native plants.
Islands have been considered particularly vulnerable to extinctions because they contain many endemic species (found only on that island) that have evolved in physical isolation from their ancestors from other places and become unique species. And there were many animal extinctions–particularly of flightless birds–with the arrival of humans who were both their predators and brought predators with them.
However, despite the conventional wisdom that the introduction of new species of plants to islands would result in extinction of their predecessors, there is no evidence that this is indeed the case with introduced plants. In 2008, Dov Sax and Steven Gaines published a study of species diversity on islands. This is what they found:
“Predation by exotic species has caused the extinction of many native animal species on islands, whereas competition from exotic plants has caused few native plant extinctions…By analyzing historical records, we show that the number of naturalized plant species has increased linearly over time on many individual islands. Further, the mean ratio of naturalized to native plant species across islands has changed steadily for nearly two centuries. These patterns suggest that many more species will become naturalized on islands in the future.” (1)
In other words, the introduction of new plants to islands has not resulted in extinctions of the plants that preceded them. Therefore, the result of plant introductions has been greater plant diversity on islands.
But what about the continents?
Recently a new study was published that asked the same question on a global scale: Has the introduction of new plants and animals resulted in the extinction of their predecessors? The answer is a resounding NO! (2)
The study was conducted on a huge scale by an international team of scientists:
- “6.1 million species occurrence records from 100 individual time scales”
- “35,613 species were represented…including mammals, birds, fish, invertebrates, and plants”
- “The geographical distribution of study location is global, and includes marine, freshwater, and terrestrial biomes, extending from the polar regions to the tropics in both hemispheres.”
- “The collective time interval represented by these data is from 1874 to the present, although most data series are concentrated in the past 40 years.”
Like most scientists who expect to find evidence of decline, this team of researchers was surprised to find little evidence of loss. Here are some of their key findings:
- “Surprisingly, we did not detect a consistent negative trend in species richness or in any of the other metrics of α diversity.”
- “There is no evidence of consistent loss of biodiversity among terrestrial plants.”
- “Time series for terrestrial plants exhibit, on average, a positive slope for species richness.”
- “Collectively, these analyses reveal local variation in temporal α diversity but no evidence for a consistent or even an average negative trend.” (Alpha diversity is species richness at the local level.)
- “An analysis of slopes by climate regions reveals that temperate time series have a significantly positive trend…”
In other words, new plants result in more plants, particularly where we live, in the temperate zone. There is no empirical evidence that new plants have resulted in the loss of the plants that were there before they arrived.
So what’s the beef?
You might think that this huge new study would put the controversy to rest. You would be wrong. For every answer we find, there is a new question from nativists. The response of native plant advocates to the good news that the plants they prefer will not disappear if new plants are allowed to live in their company is that the plant world is being “homogenized.” They say that if new plants are permitted to remain, all landscapes will become the same, resulting in the loss of unique landscapes that existed in the past.
They are, of course, mistaken. Their dire prediction will not come to pass because the biotic and abiotic conditions of every landscape are unique. The climates are different. The soils are different. The atmosphere is different. The plants and animals that are there when they arrive are different. If the new plant survives in its new home, it will be capable of adapting to these local conditions and over time it will change, ultimately becoming a unique species. When the first family of monkeys made the voyage from Africa to South America, they were the same species as those they left behind. Now they are unique species as a result of genetic drift and genetic divergence.
The process of adaptation and evolution is often more rapid than we expect. Sometimes such changes have occurred within the lifetimes of scientists who were able to witness these changes. More often, the changes occur more slowly and are only visible in museum collections or fossil records.
Consider the consequences
It is physically impossible to prevent the arrival of new species. Even when they are not intentionally introduced they find a way to piggy back on the daily activities of humans. They arrive on our airplanes and cargo ships. We aren’t going to stop importing or exporting our products all over the world. Nor are we going to quit traveling. We must accept the consequences of the way we live and quit blaming plants and animals for their passive participation in our movements.
Aside from the question of whether or not it is physically possible to stop the arrival of new plants and animals, let’s acknowledge that at least in the case of plants no great harm has come from their introduction. Since we now enjoy more plants than were here when they arrived, just what is it that we’re griping about? We seem to be griping about change. Change will occur whether we like it or not. We can’t prevent change, so we must quit fighting against something that we are powerless to prevent. That is the definition of wisdom.
Finally, we must consider the consequences of trying to eradicate non-native plants that are firmly entrenched in our landscapes. Huge amounts of herbicide are being used in the futile attempt to eradicate them. Fires that pollute the air and endanger our homes are set for the same purpose. Trees that are performing valuable ecological functions are being destroyed. The animals that use these plants and trees for food and cover are being deprived of their homes and their food. We are doing more harm than good.
- Dov Sax and Steven Gaines, “Species invasions and extinctions: The future of native biodiversity on islands,” Proceedings of the National Academy of Sciences, August 12, 2008
- Maria Dornelas, et. al., “Assemblage times series reveal biodiversity change but not systematic loss,” Science, April 18, 2014