Beautiful Invasions, Part 1: The Rule of Tens

The biosphere does not belong to us; we belong to it. — Edward O. Wilson

The summer my husband and I married, we drove to Seattle from Denver, where we were living at the time. We drove through Wyoming and Montana on the way there — you can almost forget Idaho exists — and back home as newly married people through Oregon, California, Nevada, Utah, mountain bikes on the roof of our car. Along vast stretches of the highway were profusions of happy yellow blooms. They struck me as pretty; only Justin thought to ask what it was, a question that back then didn’t occur to me. Pretty was enough. But I looked it up, and as I have done so often and continue to do, I learned something from Justin’s instinct for curiosity: the plant is called scotch broom.

Scotch broom is native to North Africa and parts of Europe. It was brought to the United States in the 1800s as an ornamental plant for people’s gardens — I’m not the only one to think: pretty. Later, when in the early 1900s automobiles were introduced and their prevalence grew from there, roads and highways were built, clearing swaths of land across the country: over rivers, through and around mountains. Through agricultural areas, connecting cities. And so: scotch broom. It has a deep and sturdy root system; where the land was clear cut for the highway, the edges tended to erode, falling back into itself, which of course defeated the purposes of the automobiles and their drivers. Scotch broom seemed just the thing to hold this crumbling earth in place. Unfortunately for these would-be road preservers, it is toxic to livestock and has now spread well across the entire region. With each plant producing tens of thousands of seed pods each year, each of which can survive in the ground for more than fifty years, it’s nearly impossible to eradicate once established, and can outcompete the things that used to grow there. Well-intentioned people either didn’t consider the implications of their actions or didn’t have the information they would have needed to do so effectively.

Invade (v): to enter for conquest or plunder

Invasive (adj.): of a non-native organism: growing and dispersing easily usually to the detriment of native species and ecosystems

Ecosystem (n): the complex of a community of organisms and its environment functioning as an ecological unit

To invade, then, requires some intention. Simply to enter a space is not to invade. Arrival alone could be done to something — to a plant, certainly. But no, invasion requires arriving with the purpose of conquest and plunder. Humans aside, organisms don’t really do this on any large scale. A cowbird might take over the nest of a smaller songbird but it won’t take over the forest. Animals may fight with their own kind for territory, but they won’t kill off everything that doesn’t directly serve their purposes in an effort to make a living there. Any individual being wants the same thing any other individual wants: to eat, to be safe, to reproduce. To survive, their biological imperative.

And yet — species do invade, as a group. Become invasive, as it were. They are brought to a new area, sometimes intentionally, sometimes accidentally, and for whatever reason, they thrive and overtake. The ways in which they arrive can vary nearly as much as the species themselves. Migrations used to be limited to the wind and the waves or perhaps the droppings of birds, but these happened slowly, both in terms of time and space. And though invasions long predated this, the term “invasive” as it relates to ecosystems wasn’t coined until the 1950s, and many current definitions require that they must be human-mediated. Some scientists think even human-mediated invasions have been occurring for the last 20,000 years. We carry seeds on the soles of our shoes and in the treads of our tires, release our pets when they no longer suit us even if they’re pythons, bring rats and innumerable marine creatures in the ballast or holds of our ships used for travel or global trade. We plant things in our gardens and then move away, take firewood with us on road trips, forgetting the pests that may be too small for us to see, but not too small to wreak havoc on a forest. Humans are resourceful and creative and not always aware of the impacts of our actions.

Not all organisms brought to an area from somewhere other than their native range become invasive: the “tens rule,” coined in the 1990s estimates that 10% of species brought to an area will either escape or be released. Of those escapees, 10% will establish viable populations in the wild, outside of human care, and of those, 10% will become invasive. If you’ve been doing that math on this, you’ll notice that this means that only .1% of species that are brought somewhere will become invasive.

This “rule” isn’t exactly supported by empirical evidence, but as a high-level stand-in perhaps it still serves its purpose. The ratio itself may be wrong, or at least highly contextual and not applicable to all circumstances — the percent of species capable of establishing viable populations in the tropics, for example, is drastically different than in the arctic. J. M Jeschke and P. Pyšek from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin, for example, did a review of 102 empirical tests of the tens rule from 65 publications and found that the real numbers are closer to 25% for each of those stages for plant and invertebrate animals, and a whopping 50% for vertebrates.

It’s also possible that the measure is wrong — Daniel Simberloff, one of the preeminent invasion biologists, says the measure is based on the percent of organisms that cause significant economic damages, instead of basing the measure on ecological impact. There are many different economic models used to estimate damages, but they are all broadly based either on what it costs to remove, manage, or keep tabs on, or losses in production. Because human societies tend to only value some things about ecosystems — the ones that provide us tangible benefits — this economic measurement is necessarily an underestimation. But the concept remains illustrative. It also makes the problem seem much smaller than it is, even aside from the economic-ecological divide. One tenth of one percent doesn’t sound like much, but when considered on a global scale, and among all the ways and vectors that species are introduced, you begin to see the trouble. Humans move around a lot, and we always bring things with us.

No species is inherently invasive: what is destructively so in one ecosystem originated somewhere, is native somewhere. In that place, it may have co-evolved with natural predators or diseases, with resource limitations or other parts of the system keeping it in check. It could be the food chain — something that eats it in the place where it originated is missing from its new locale, it could be the climate — without a hard freeze, a winter die-back might not occur, allowing a population to explode. It could be a local toxin, a fungus, a change in reproductive opportunities, anything. These are just examples, ecosystems and evolution are infinitely inventive. It’s only in the absence of these factors keeping it in check, a change in context, that allows something to become invasive. The inverse of course is what maintains the balance of the tens rule (or 25% rule), the other 99.9%. More likely than a perfect lack of threats is the introduction of new threats that prevent a species from establishing and thriving in a new place. After all, most species don’t bother evolving protections against things that don’t threaten them. We can’t know about most of these, we can’t know everything, see everything, and so the problems that don’t happen go unnoticed.

Read part II here

Burdick, Alan. Out of Eden: An Odyssey of Ecological Invasion (Farrar Straus Giroux, 2005)

Elton, Charles, The Ecology of Invasion by Animals and Plants (Springer, 1958)

Courtney A. Hofman, Torben C. Rick, Ancient Biological Invasions and Island Ecosystems: Tracking Translocations of Wild Plants and Animals, (Journal of Archaeological Research, Springer, 2013)

Jeschke, J. M., Pyšek, P. Invasion biology: hypotheses and evidence. Tens rule, Chapter 13 (Berlin: Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), 2018)

Merriam-Webster, Dictionary, Online 2022

Slater Museum of Natural History, University of Puget Sound, Bracken Fern

St. George, Zach, The Journeys of Trees: A Story about Forests, People, and the Future (W. W. Norton Company, 2020)

Wilson, E.O, Half-Earth: Our Planet’s Fight for Life (Liveright, 2016)