Invasive Nature

What is an Invasive Species (and, Are We an Example)?

The idea of invasive species often conjures up negative connotations in our ecological narratives—organisms that don't belong, running rampant, pushing native life to the brink.

Kudzu vines from Asia blanketing landscapes in the southeast of the U.S., zebra mussels from the Black and Caspian Seas taking over the Great Lakes region and blocking intake pipes, cane toads from Central and South America introduced to Australia for pest control quickly becoming uncontrollable themselves, and the inadvertent release of hybrid africanized honeybees in Brazil leading to the spread of these more dangerous bees across the Americas are all among the best known examples of invasive species.

But invasiveness isn’t just a matter of where a species comes from—the term often carries hidden assumptions about human responsibility and control. And perhaps more provocatively, a fun question to ask is if we humans reshape and impact ecosystems wherever we go, does that make us invasive too?

And I’ll even go one step further: can a planetary biosphere itself be seen within the larger ecology of the cosmos to be invasive?

Invasion of the Species

In ecology, a species is generally considered invasive when it meets certain criteria, including that it is non-native to the ecosystem, it’s been introduced (often by humans but potentially in other ways, as I’ll argue below), and causes harm to the local ecosystem.

The UC Riverside Center for Invasive Species Research suggests the following:

Invasive species, alien species, exotic pests, bio-pollution, non-indigenous species, or invasive alien species, are common names that categorize non-native animals, microbes, diseases, or plants that are pests. These pests are not native in areas in which they cause problems and they are considered "invasive" because they invade and establish populations in new areas and the resulting uncontrolled population growth and spread causes economic or environmental problems.

Similarly, a paper in the Journal of Extension titled Invasive Species Terminology: Standardizing for Stakeholder Education attempts to define seven specific ecological terms for standardized use amongst various stakeholders in regard to potential invasive species impacts:

I generally like those definitions since they attempt to show differences between a variety of terms often used when discussing these matters. Though I also would argue that the main difference between native and nonnative is time—and we have no standard view of how much time is necessary before a species goes from being nonnative to native in a given ecosystem (and the concept pushes on some cultural issues if we spread that to human populations).

Those definitions also highlight the importance of humans in introducing species to new ecosystems. For our modern world, that is very much how we often view introduced and invasive species, but certainly throughout the history of life on our planet there have been times when species have been introduced into new ecosystems through major weather events, earthquakes and volcanism, changes in currents in the ocean, flooding, landslides, and more.

But as that table of terms can help to show, not all introduced species necessarily become invasive.

Tomatoes, many kinds of honeybees, and lots of ornamental plants have traveled across continents without disrupting local ecosystems. We humans ourselves have often taken crop plants and plants for producing spices far and wide in our travels and our trade. We’ve also taken sheep, cattle, pigs, horses, cats, dogs, and other animals on extensive journeys to new homes (though we could definitely argue that some of these have caused lots of harm—free ranging house cats, for instance, kill billions of birds and mammals annually).

But some species, once freed from the checks and balances of their native predators, pathogens, and physical environment, can find an ecological niche so open to them that they can flourish unchecked and sometimes even drive trophic cascades and ecosystem collapse.

The zebra mussel, from my previous example, filters vast amounts of plankton from the water column, outcompeting native mussels and dramatically altering freshwater food webs. The cane toad spreads across Australia, toxic to any predator that makes the fatal mistake of eating it, allowing for populations to grow rapidly. Grey squirrels, introduced from North America to parts of Europe, have displaced native red squirrels not only by competing for food but by spreading squirrelpox, a virus harmless to themselves but deadly to their smaller cousins.

In some cases, when the introduced species rapidly alters the local ecosystem, it can trigger a trophic cascade—a domino effect where the addition of this single species drastically alters the local web of interdependencies between organisms. Such a cascade can even lead to a larger ecosystem collapse, whereby many species go extinct and the ecosystem itself is irreparably harmed.

And yet, ecology is never static.

Over long enough timelines, ecosystems adapt. Succession—the natural progression of ecosystems through stages of development—means that even an invasive species may eventually be absorbed into a new local balance. The line between disruption and adaptation becomes blurry. Is the ecosystem of today less natural than that of a century ago, simply because it has changed?

Here’s where the question of human responsibility enters.

Most of what we classify as invasive species have been introduced—deliberately or inadvertently—through human travel, trade, agriculture, and globalization. Shipping containers carry stowaway insects and their bilge waters if not filtered or dumped appropriately can bring new species to alien waters, ornamental plants can escape gardens and join local ecosystems, pet species are sometimes released into the wild, and an ever changing climate opens new frontiers for once-constrained organisms. In this way, the idea of “invasion” as we see it usually is focused mostly on human behaviors and how the introduced or invasive species impacts us in the near and long term.

Some people may even see us humans as an invasive species. As we’ve spread out around the globe, introducing ourselves to new environments, we’ve certainly caused harm (sometimes irreparable harm) to other species and entire ecosystems. We’ve introduced new nonnative species to environments where they sometimes have become invasive. The misanthropes among us might think of humanity in general as causing great harm to the world we share with so much other life. Agent Smith in The Matrix even refers to us as a disease:

“I'd like to share a revelation that I've had during my time here. It came to me when I tried to classify your species. I realized that you're not actually mammals. Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment, but you humans do not. You move to an area, and you multiply, and multiply, until every natural resource is consumed. The only way you can survive is to spread to another area. There is another organism on this planet that follows the same pattern. Do you know what it is? A virus. Human beings are a disease, a cancer of this planet, you are a plague, and we are the cure.”

― Agent Smith (portrayed by Hugo Weaving)

It might be easy to see the bad in our actions. We see so much pain and loss and destruction in human history. But there is also so much good. We have love and passion and ambition and care. While we certainly have done harm to other beings and to ecosystems—sometimes inadvertently and other times intentionally—I think the sheer fact that we can think about these things now and have open conversations about our impacts on the planet and how we might be better tomorrow are enough to give us some hope.

But there’s another idea that’s been rattling around in my head that I want to explore (and the reason I started writing this):

Can the biospheres of worlds themselves be thought of in similar terms? Will some types of life generally be more likely to cause drastic harm, alteration, or extinction of other forms of life in the cosmos? Can a biosphere become an invasive species?

Biospheres as Cosmic Actors

Thinking of what constitutes an invasive species and whether we humans on a larger scale should be considered invasive (not just as a species ourselves, but also as the vectors of introduction of non-native species that can become invasive), then maybe we can go a bit further.

If we step back far enough—beyond the boundaries of nations, ecosystems, and species—and consider the biosphere of a planet as a single, emergent entity, then one way to view life in the universe isn’t to think on the scale of individual cells or multicellular forms that we think of as individuals but rather to consider ecological processes acting on entire worlds.

Our own world is one where life happened and then slowly and dynamically came to be intertwined with the planet itself. The atmosphere, our rocks and minerals, our oceans are all impacted by life and vice versa. And while our biosphere has mostly been constrained to this one rock in space for eons, we humans have now become the vector of change for allowing the biosphere to slowly escape out into space, traveling to other worlds on our spacecraft, and even out into deep space. And we already know that we have cause for concern of meeting alien lifeforms out there—they could be harmful to us just as much as we could be harmful to them.

But what if not just creatures as we see them can impact other life? We think a lot about the potential threats from introducing Earth life to some extant biosphere on Mars or Europa and also have reason to be concerned at the potential for bringing some alien virus or other creatures back to Earth. But what if in a larger sense, worlds themselves may become invasive relative to how they behave in the cosmos. Maybe we could treat worlds themselves in such a case as individuals/populations in a larger cosmic ecology.

What would be the nature of interactions between such biospheric or planetary entities? In terrestrial ecology, populations interact through competition, predation, mutualism, and commensalism. Across interstellar distances, direct resource competition or predation seems a bit far-fetched. However, could there be a cosmic analog to species migration and various ecological dynamics? Panspermia—the concept of transfer of life across space—could be seen as a natural, albeit slow and likely undirected, form of colonization or reproduction for microbial life. One world, rich in resilient extremophiles, might inadvertently seed a sterile but receptive neighbor over eons.

The concept of an invasive world then takes on a new dimension. Is a naturally occurring panspermic event invasive, or simply part of a larger, natural galactic process of life's dissemination? What if the spread of life goes from one inhabited planet to another? Perhaps the biospheres could merge and co-evolve, or perhaps one might take over and lead to the extinction of the other.

The idea gains even more traction when we introduce agency. Imagine a technologically advanced civilization, an emergent property of its own mature biosphere, actively and intentionally spreading its native life—or even engineered ecosystems—to other planets. That’s practically what we’ve begun doing for the Moon and Mars already in our solar system, and in the not too distant future we may choose to extend our biosphere more actively to Mars (and may even terraform that world by integrating more of our life with it). Here, the parallels to terrestrial invasive species become stark: a new species (the transplanted biosphere or its core components) arrives in an ecosystem (a barren or nascently-living planet) and potentially outcompetes, transforms, or even eradicates any pre-existing life.

Viewing biospheres through this lens forces us to confront some rather profound questions.

Does a biosphere have a right to exist? Is there a galactic carrying capacity? And if we, as products of Earth's biosphere, eventually develop the capacity for interstellar travel and terraforming, what ethical framework would guide our actions as potential agents of our own biosphere's invasion or propagation across the cosmos?

I think framing a biosphere as a unit in a larger ecological system challenges us to think differently. It compels us to view ourselves as an emergent expression of Earth’s biosphere—not separate from nature, but an extension of it. A species capable of reflection, stewardship, and even restoration. And maybe our biosphere, through us, can choose when and how to spread to other worlds in a way that won’t threaten a larger cosmological homeostasis.

We may not be a perfect species, but neither are we a plague (even if Agent Smith thinks of us as a virus). We are participants in a grand planetary experiment—one still unfolding. And if we are to carry the story of our life into the wider cosmos, we really need to learn the difference between invasion and integration.

Because the future of life, here or elsewhere, may depend on how well we understand our own nature—and how willing we are to grow.

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