According to the USDA, “An invasive plant has the ability to thrive and spread aggressively outside its native range. A naturally aggressive plant may be especially invasive when introduced to a new habitat.”
It’s important to be aware of these plants because of the negative effects they can have on the environment, economy, or even human health or well-being. However, it is also important, especially in permaculture, not to conflate the idea of invasive plants with introduced plants or plants that thrive. Our efforts should always be backed by rigorous research and should not be hindered by emotional reactions, prejudices, or the inability to think about new solutions to problems.
Why are Species Classified as Invasive?
There are, of course, many issues that can arise when introducing species (plant or otherwise) to a new habitat. A primary issue that can occur is a loss of biodiversity (particularly the loss of native plants and animals) in the region. This can result because the invasive species is directly harming local populations (as a predator), because it is altering conditions within the ecosystem (like the chemistry of the soil, the introduction of disease, or by causing increasing wildfire intensity), or because it is altering food webs which impact the survival of local wildlife, by outcompeting for nutrition or by replacing a nutritive food source.
Reactions to Invasive Species
Understandably, many people have strong reactions to invasive species, and for good reason. Dutch elm disease, for example, is caused by an invasive fungus, Ophiostoma ulmi. It is responsible for the death of more than fifty percent of all of the elm trees in the northern region of the United States. The disease (contrary to what its name might suggest) originated in Asia—where elms had long since developed a resistance to the fungus. However, European and North American elms have no such resistance. Its introduction in Europe and North America has been called the most significant event in the history of urban forestry. Millions, perhaps hundreds of millions, of elms in Europe, North America, and New Zealand, have fallen to Dutch elm disease. Stories like this one horrify those who are sensibly concerned about the environment.
Controlling an invasive species once it has established itself in a new environment where it is uniquely suited to survival can range from difficult to impossible. These ecological horror stories have naturally had an impact on those who are thoughtful about preserving biodiversity, as well they should. Truly invasive, harmful plants should be controlled and should not be introduced to those areas in which they might negatively impact local ecosystems.
However, it is important that when we engage in permaculture, we carefully consider the pros and cons of introducing plants to an environment based on facts and scientifically conducted research. Unfortunately, the fear of invasive species has caused some people to have a reaction to plants that flourish—in general—that is more emotional than logical, and anything that thrives is classed as invasive by those who think this way. Likewise, any plant that is considered non-native can be considered invasive by those who broaden the definition out of fear rather than knowledge.
Introduced Species, Thriving Species, and Native Invaders
By most definitions of the term, all invasive species are also introduced species—species that have been introduced to a range outside of their native range. But, not all introduced species are invasive. Nor are all successfully introduced species considered invasive, even if they thrive. Finally, there has been an increasingly strong call for an additional classification—native invasive species.
An unambiguous introduced species success story is the honeybee. Honeybees are not native to North America or South America; they were brought by European settlers. Honeybees are now a lynchpin of American agriculture and many wild ecosystems as a pollinator, and an industry in and of themselves. When first introduced, these insects thrived in the Americas, but did not have a negative impact on regional ecosystems. In fact, conservation of this helpful insect is now an important permaculture goal in many regions. Honeybees, while non-native, are in no way considered to be invasive. Most people actually consider them to be native.
Native invaders are species that, often because of a change to the ecosystem wrought by human beings, become aggressive within their niche, increasing their range and population in ways that defy easy control and cause environmental harm. The creation of dams in the Columbia river basin gave the native Northern Pikeminnow an advantage over native salmon, leading to failing salmon populations. Geese can cause nutrient imbalances in lakes when overpopulated; whitetail deer likewise can overgraze on young trees. Both of these native species exploded in population due to humans eliminating the native predators.
Ecosystem Shifts, Permaculture, and “Invasion”
Ecosystems change over time, through both natural shifts and human interference. Neither is inherently good or bad. What is invasive when introduced to one area may be neutral or beneficial in another. We must take care when introducing species to a new area, but we must also refrain from assigning qualities to species based on emotional reflexes rather than sensible consideration. An introduced species can be an important piece of the permaculture puzzle; a native species may threaten permaculture. Study, research, innovation, and thoughtfulness must prevail.