Invasion Ecology: Unraveling Invasive Species and Their Impacts
A brief history of invasion ecology & where the science stands today
This piece was originally published in June 2024’s Mergoat Magazine’s Volume 2, Number 1 collection, which you can buy here!
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The autumn olive, Elaeagnus umbellata, is considered a non-native invasive plant species. In late spring, the trees vibrate with pollinators—up to 47 different documented types —happily jumping from flower to flower. The late fall is an explosion of red that many birds gobble up.1 The fruit turns to energy and the seeds, which will be nourished by the nodules on the roots of the tree, are dropped across the landscape and work their way into soil. These nodules fixate nitrogen into the soil, the same chemical found in commercial fertilizers to improve soils for plant growth. Autumn Olive is one of roughly seventy plants in the Elaeagnus genus, only one of which is native to North America.
Our native Elaeagnus, the silverberry, (Elaeagnus commutata), has continued to exist across much of the continent, but largely the midwest where it has historically been managed with fire.2 Smaller, but still full of sugary nectar, the native silverberry has little documentation of pollinator support but seems to support generalist species, much like autumn olive.3 Silverberry shares many traits with the autumn olive, including its massive clusters of red berries gobbled up by birds (though the specific pollinator relationships have not been researched). How can these two trees, so closely related and sharing such similar traits, demand such different management solutions simply because of their point of origin?
This is actually a trick question. I say “native”, but it’s not native to the east coast—and that’s important. Fire regimes and grazing have historically managed the silverberry in a way that would likely manage the autumn olive but are difficult to apply in a modern suburban setting. However, silverberry doesn’t have a track record crossing over the Appalachians, while autumn olive is here to stay.
Given the right conditions, E. commutata could very well become invasive in other landscapes, including the east coast. The black locust for example has fallen into this role–native to a small inland range of the mid-Atlantic, and invasive to the northeast where it continues to spread. Native is a tricky term that we typically assign based on human landmarks instead of ecological conditions, after all. The key management regime that has kept the silverberry in check is more difficult to apply to autumn olive. Still, anyone reading this knows that can’t be the only thing that has kept silverberry from taking over the edges of highways the way autumn olive has across the east coast.4 So what’s going on, exactly?
The short answer? We’re still figuring it all out. There are likely many conditions including hardiness zones, salinity, soil moisture, and human use, all working together in a complex system, and a finite number of researchers investigating.
Looking at these species moving across the land unchecked–well meaning defenders of invasive species offer the perspective that “the earth is healing itself”. This is a thin veil that misses the complexity of why and how species spread, and the enormous role of humans in that story.
A Brief History of Invasion Ecology
Before we can discuss the problems with this perspective, it’s worth fully equipping ourselves with the development of evolutionary ecology concerning invasive species— a fancy way of saying “What the hell is actually happening to our landscape?” While Anthropocene habitats have driven some species beyond their “natural” populations, there are marked increases in exotic species starting from the 1950s onwards, a reflection of the increasing globalization associated with the era of cheap fuel.5 It was at this time that research began to discuss in earnest the idea of “invasions” of non-native species, fundamentally altering the landscape around us.Evidence of this can be found in Charles S. Elton’s 1958 book “The Ecology of Invasions by Animals and Plants”, which recommended that invasion ecology be treated as an independent scientific discipline.
Eight years later, Herbert Baker & G. Ledyard Stebbins released “The Genetics of Colonizing Species”, a publication that altered the development of evolutionary biology regarding invasives. The science of invasives was still poorly understood for decades, and during the second half of the twentieth century, the ideas of “ecological fitting” and the application of the Maximum Power Principle were introduced and applied to rationalize and normalize the impacts of invasive species on the landscape (more on this later).6
As the 1990s and early twenty-first century rolled through, evidence for a more contemporary evolution started to accumulate, most often from studies that focused on invasive species.7 Studying invasive species offered a new glimpse into evolutionary change and the capacity for invasive species as drivers of evolutionary change— for better or worse.8 For example, when Nevada’s population of Edith’s Checkerspot Butterfly evolved a host preference for invasive ribwort plantain, it appeared to be a win for the defense of invasive species. However, as land use changes caused the grasslands to recover and the ribwort plantain could no longer flourish, the Nevadan population could no longer feed on native species and they went extinct.9
Interest exploded as invasives became a more significant piece of conservation work and as the costs of invasive management skyrocketed. The early twenty-first century saw several books focused on the role invasives play in habitat destruction and it was here, particularly in George Cox’s 2004 book Alien Species and Evolution, where the ideas of rapid evolution in invasive species were discussed in depth.
How species become invasive is beyond the scope of this discussion— the short of it is that species have the opportunity to reallocate resources that would go to defense in their native habitat towards performance when placed into new environments outside of their specialist natural enemies. This phenomenon is also known as the '‘evolution of increased competitive ability” (EICA).10 This is why, in the example above, while we don’t have clear reasons why silverberry doesn’t exhibit similar traits to autumn olive in terms of invasiveness, we can hypothesize that it is likely due to factors within its native ecological context that limit its ability to focus on performance. This would also help explain why many species seem to show few invasive tendencies until generations later when they explode across the landscape (the way hardy kiwi is today, for example).11
But what makes certain species so good at being invasive, while others never seem to get there? One common thread with many invasives is that they tend to fill generalist roles in the ecosystem. This is also why plant-pollinator and seed-disperser networks are often easily exploited by invasive species.12 Like the example of Edith’s Checkerspot Butterfly, many species are influenced by eco-evolutionary experience (EEE)— that is, the exposure of both native and exotic species to biotic interactions over historical evolutionary timescales. EEE is one of the underlying theories on how and why certain species are more or less suited for ecosystem takeover. For example, native plants in the diverse fynbos biome of South Africa rely on native ants planting the seeds in the ground, out of reach of rodents. The non-native Argentine ant often displaces native ants. Unlike the native ants, Argentine ants are slower to discover native seeds and don’t bury them like native ants do.13 This has led to a fifty-fold reduction in seedling emergence in many native species and has dramatically changed the plant community composition. 14
We see this play out in Appalachia as well with Japanese knotweed. The first documented escape of the plant dates back to 1886 in the United Kingdom.15 Speaking to the surprisingly invasive capacity of one single plant, almost all of the Japanese knotweed population around the world today are related to the plant Von Siebold originally brought to Europe.16 The takeaway is that a number of factors influence the potential for nonnatives to quickly turn to invasives and that taking caution in presenting new species to novel environments comes with a host of potential ramifications.
The arrival of invasive non-natives not only impacts ecosystems in the ways described so far, nor does it simply change the dynamics of the plant and animal communities on the landscape; these novel relationships, as shown in invasion ecology, can also drive evolutionary changes over short timescales for both non-natives (as explained above) and natives themselves.17 This has challenged much of the conventional understanding of evolution as necessarily having a long timeline. This is due, in part, not only to the novel environment, but also because introduced populations— non-natives or invasives— often have some reduction in genetic diversity leading to genetic bottlenecks and founder events, which can cause poor performance of introduced populations at first. This also helps to explain why some exotic species don’t show invasive traits for long periods before exploding.18
In these new environments, natives have many of the same tools as invasives to adapt to their new habitat, except they often lack the benefit of EICA, and thus still need to focus energy on defense against local predation. They can either tolerate the new adverse conditions through plasticity (the presentation of their genetic potential shifts to meet the new, unique environmental conditions), adapt through evolution (and selection), disperse to new environments, or become extinct.19 For invasives, which as we discussed are more often generalists, part of their success is that they can quickly form new associations with other generalist native mutualists or non-natives from their native habitats.20
The challenges for natives aren’t just the non-native invasives themselves, but other novel problems such as toxic phytochemicals produced by the invasive plants, which fundamentally alter the soil, causing native plants to struggle, even once the non-native invasives have been removed.21 Outside of the functional aspects of competition, another major disruption for native species comes from the impacts of hybridization, which, for example, has been detrimental to native mulberry and butternut populations in the United States.22 A recent review reported that 27 out of 69 studies identified hybridization as a specific, immediate threat to native species to the point of extinction.23
To this point, it’s been a pretty damning narrative of invasives— not the plants themselves, but rather the conditions within which they find themselves— and yet despite this, narratives in defense of these invasives have grafted themselves into the world of alternative agriculture in particular. But why?
To answer that question, we have to understand where the narrative came from. We can point to the early realization of the dangers of non-natives— from the arrival of the fungus responsible for chestnut blight, which has left the American chestnut extinct, to the quick realization that kudzu was not a miracle plant, as it claimed hillside after hillside as its territory — but the debate about whether or not the plants were themselves the issue has evolved based on new science.
In Defense of Invasives
Much of the science defending invasives today came at the hand of an Austrian-American mathematician whose initial research was focused on predicting the lag time for the pathogen incubation for malaria in an attempt to better predict outbreaks and to help figure out what the critical vaccination threshold was to stop an epidemic.24 This research was the beginning of his work to provide a mathematical framework for evolution, and it became the basis for his argument that the fundamental foundation of evolution was not random chance but rather that organisms survive based on their ability to capture and use energy more efficiently than their competitors. This idea would later become refined by Howard Odum, who would coin the term ‘Maximum Power Principle’ to describe it.
Odum’s argument was simple: organisms that are best optimized for a specific set of variables are best selected for an environment, and there’s always a balance between efficiency and speed in direct response to the unique environment that the organism lives in. Odum was so confident in this as a rule of how organic matter is organized that he proposed it as the fourth law of thermodynamics.
This fourth law is ecological, mathematic, and political— it is predicated on self-organization as the foundation of systems design, production capacity, and ultimately the system’s efficiency. These systems are dynamic and rely on what Odum called a ‘pulsing paradigm’. The subsystems that ‘pulse’ allow energy to transfer and reorganize as they find new ways to become more efficient. The key is that these systems don’t overpulse; overpulses can cause systems to collapse, in the same way a novel predator might drive its prey to extinction. Hold that thought.
In 1972 (or 1978, depending who you ask) the term “permaculture” was first coined by Bill Mollison in Tasmania, Australia— a portmanteau of “permanent” and “agriculture”. David Holmgren later described this period of experimentation as focused on developing “an interdisciplinary earth science”.25 While biodynamic, organic, and agroecological movements had taken shape in various parts of the globe, permaculture found its home on bookshelves with the book “Permaculture One - a Perennial Agriculture for Human Settlements” in 1978. The same oil crisis that had driven Odum’s theory to prominence in the 1970s catapulted permaculture—the seemingly simple concept of local food grown permanently—through the 70s and 80s.
Part of permaculture’s movement is framed in Odum’s work; the Maximum Power Principle is a foundational component of understanding the relationship between native ecosystems and invasive species.26 Today, Geoff Lawton carries this torch forward, and when confronted with the question of whether or not we should be using non-native invasive species, his response is:
I think it is good… and invasive species are good. They indicate to us that there is a problem with the native ecology, but they also demonstrate to us that there’s nothing wrong with novel ecosystems. Novel ecosystems are the future and they always have been and it was always going to happen.27
He even goes so far as to compare driftwood carrying novel species to the 11 billion tons of shipped goods containing various nonnative bacteria, fungi, insects, and more.28
In 2015, Tao Orion wrote the book “Beyond the War on Invasive Species: A Permaculture Approach to Ecosystem Restoration”, in which David Holmgren himself wrote the forward. The book offers this same perspective as Lawton, under the guise of scientific data, focusing on studies pointing to, for example, birds choosing nonnative berries over native berries as proof that invasives offer good ecological services. While Orion offers a bit more nuanced discussion than the title lends itself, Holmgren’s forward militantly defends invasive species, arguing that the book exposes ethical corruption in ecological science.29
What’s worth exploring is how the Maximum Power Principle is misapplied. Instead of being applied to understand self-organization in nature, it’s wielded as a holistic argument in defense of the ‘natural’ process of new species crossing the planet. Ignoring scale, it refocuses on oversimplification– allowing the fittest to survive and ignoring the cascading impacts of these changes. While, for example, Japanese knotweed may be better at quickly producing biomass (which is often touted as the measure of plants’ productivity in these types of arguments), this ignores the impacts it has on energy transfer throughout an ecosystem. Researchers, however, have repeatedly shown that trophic efficiency— that is, how efficiently energy transfers through an ecosystem after that initial biomass production— is significantly diminished in ecosystems with high amounts of invasive species.
For example, one study focused on invasive species in the Sylt-Rømø Bight ecosystem in Denmark showed trophic efficiency declines of up to 65% in Lake Oneida.30 In northern Japan, non-native rainbow trout not only led to reductions in native fish populations but also, due to their consumption of aquatic insects, there was a 65% reduction in the density of riparian-specialist spiders in the forest. This caused significant disruption to the flow of resources between interconnected ecosystems, which can be difficult to measure or anticipate when attempting to catalog the impacts of species invasions.31 Time and time again, the problems are not limited to the species and its existence in the landscape, but rather the downstream impacts of how it relates to the landscape around it.
The Invasion— Another Perspective
Ecosystems don’t exist in a vacuum away from people. Our modern global society, driven by capitalism, is homogenizing life across the globe. We’re trapped in a modern landscape of profit that treats land as assets, to be maximized, destroyed, and traded, without respect or dignity for the coherence of living beings and ecosystems. Water systems have been eliminated to produce hydroelectric power without consulting the role of salmon runs in the nutrient cycles in the landscape. Wolves and mountain lions have been removed from the east, and hunting is heavily restricted, leaving deer so overabundant they starve themselves to death. Dirt is constantly being disturbed for endless construction projects of trivial human significance. Many Indigenous Peoples have been blocked from accessing the land–along with their traditional fire management practices.
These injuries leave the landscape bruised and broken. Native systems which could have inhibited or even welcomed non-native species, now fall victim to out-of-control invasives bulldozing the landscape.
Some claim invasives are only a problem in these disturbed landscapes, arguing that the backwoods are safe, or that the real solution is to return the natural systems to the landscape. While there is truth in the call to restore healthy land management practices - it’s not realistic to assume this is a switch we can flip in the near future.
Without a doubt, some invasive species would lose their harmful qualities in a healthy world. But that’s not the world we live in.
Those who critique the negative depiction of invasive species, often return to a few specific arguments. We’ve already discussed one - the so-called efficiency of these species. Some novel species may ‘fit’ more effectively and distribute energy more efficiently on a superficial level in their new homes. But there are untold downstream efficiencies lost as the system collapses - species adapted to specialized niches that vanish as the novel species fill the space and take control of the first link in the chain of energy transfer. As a system collapses, the efficiency from numerous downstream evolutionary functions are erased for the benefit of a slightly more efficient species at the beginning of the chain of energy transfer.
Permaculturalist arguments meanwhile focus on food systems — often defying basic ecology and instead advocating for the wholesale colonization of every nutrient-poor landscape with nitrogen fixers, to turn the planet into an enormous ‘food forest’. Their claims often revolve around fertility and food production being the ultimate land use. This ignores the vibrant endemic landscapes that naturally offer us models of how to live in alignment and celebration of unique ecological conditions. Not to mention the numerous ecosystem services offered by systems that don’t resemble the permaculture dreams like ‘greening the desert’.
The remaining arguments made by invasive apologists are more sociopolitical, suggesting that xenophobia and white supremacy are the root causes of the criticism of invasive species. In their view, the only reason to defend native landscapes is out of ignorance; that recognizing non-natives and particularly invasives flattens Indigenous cultures stuck in static environments, that never utilized new species when they became available. Of course they did– but we are often comparing one species added to an otherwise healthy environment as opposed to hundreds (if not thousands) of new species in degraded ecosystems, struggling to survive.
The often repeated question from this line of thinking is that if we want to remove these non-native invasives, what’s our benchmark– the question is “restore to when?” (e.g., if you are doing habitat restoration, to what era are you seeking to return?), as though the dense webs of ecological systems are a philosophical debate with a ‘gotcha’ moment.32
And this is where the conversation of native plant defense and restorative solutions as a tool of white supremacy fall flat— we aren’t recognizing one plant as being the best, but rather that one plant is the best for those unique conditions of their living ecosystem, in the same way one person is particularly suited for a specific type of role within a community. Attempting to sealion discussions about the devastating effects of invasives under our current ecological conditions by focusing on semantic positions like specific dates or specific anomalies erases the overwhelming evidence posited by a majority of researchers focused in this subject area today, much like we see with climate change denialists.
Conversely, while many people defending native plants point to invasives as a tool of colonization (one that erases food and cultural history by actively removing species and their relations from the landscape) I think a more apt metaphor would be whiteness, rather than colonization. Whiteness doesn’t point to a place, a time, a history, or a context; it is better understood through its relations with power. In this case, invasives are the ones with power, and their ability to fundamentally change the landscape and erase its vast history speaks to the capacity of the broad strokes changes, and homogenization the entire planet faces at the hands of invasion ecology.
In short, the long-standing narrative of ‘the Earth is healing’ erases nuance to address the massive loss of ecosystems, the erasure of plants and life and history, and the diverse human relations with the species that have fed and cared for us. This mantra lifts the arc of progress while trivializing those living and those who will be lost to history if we do not act to protect the places that exist today, the ancestors who have watched the landscape since before the terms native and non-native existed. The argument has never been that invasive species are bad, but rather that their place on the landscape is a reminder of the damage that has been done and the responsibilities we have now for that damage.
On a long enough timeline, all living things attempt to have optimal energy usage for health, replication, and ultimately niche expansion. We know this through the concept of mutualism (as species learn to co-depend on one another) and this is a point Peter Kropotkin wrote about as an evolutionary biologist and anarchist. However, our concern is in the short- and medium-term.
Our responsibility today is to the plants that survive and thrive despite habitat destruction, invasion ecology, and suburbanization, and whose roots stretch back to our ancestors and the lands they called home.
Therefore in response to “restore to when”, we offer a different question - “Restore to whom?”. Ecosystems and the Indigenous Peoples who have expertly stewarded them for millenia are defined by interrelationships defined by both evolution and mutual care. When we restore, we seek to reunify these relationships such that they can persist as whole vibrant living communities.
To honor our home is to honor those whose bodies compose the soil beneath our feet— the plants, animals, and humans within it. What does it mean to restore the landscape in the face of accelerating climate change and impending collapse? How do we settle that debt under the weight of the future? While there are no sure answers, the one thing we can do is keep the history of our native plants alive - and to do that we must keep our native species from disappearing. This means we keep them on the landscape— for one season, for one year, for one human lifetime. Our responsibility today isn’t to have a solution, but rather to be sure no more species are lost for the next generations.
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I discussed this very situation regarding autumn olive in Texas here: https://www.thesurvivalpodcast.com/beyond-food
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