Immigration became the subject of increasingly ugly rhetoric during Donald Trump’s 2024 presidential campaign. He portrayed migrants as alien or dangerous, described them as animals, and used invasion analogies that made associations with vermin, infestations, and other undesirable biological phenomena. That rhetoric followed Trump into office: Among the executive orders he issued on his first day back in the White House was one about immigration titled “Protecting the American People Against Invasion.”
But the use of analogies between different types of human migration and biological invasions is not just a common tactic of racists and opportunists. In conservation biology, such analogies work in reverse, with plant, animal, fungal, or microbial species compared to people on the move. Such analogies are used in science with the goal of clarifying a pattern or process. Yet the use of human migration as a metaphor for biological invasions has been a highly contentious practice within the discipline for decades. In the current political climate, heightened tensions around migration and immigration enforcement have brought back this old debate.
The debate roars (somewhat sedately) within the pages of peer-reviewed journals. Biological invasions and human migration offer some “complex parallels that are potentially fruitful to explore,” argued a group of scientists in a February article published in Biological Reviews. Respondents countered a few months later in the journal BioScience that the comparison “conflates two globally occurring phenomena in a scientifically flawed way and risks the misappropriation of scientific concepts for ideological and political agendas.”
Laura Meyerson of the University of Rhode Island, an author of the BioScience rebuttal and one of three editors-in-chief of yet another journal, Biological Invasions, explained the distinction between the two phenomena: “The frameworks in invasion science were designed for introduced plants, animals, microorganisms, etc. but not for people,” she wrote to me in an email. “Biological invasions are species introduced by people from outside their native range that establish, spread and have impacts. Invasion science recognizes people as introducing organisms but does not include people as introduced organisms.”
The use of human migration as a metaphor for biological invasions has been a highly contentious practice within the discipline for decades.
These inter-journal arguments unfolded as inflammatory and dramatic human migration-related events were ongoing in the U.S., including heavy-handed federal response to protests in Los Angeles against excesses by Immigration and Customs Enforcement agents, showing that the issue is not just academic. Thinking through the function of metaphor in science — and exploring how this particular analogy has played out in the history of conservation science — can reveal whether it’s a good one, scientifically speaking, and whether it advances or weakens the discipline.
When scientists use metaphors or analogies, they are comparing one thing to another based on salient, or relevant, features that those things have in common. No one expects the things to be identical. But the comparison opens up the possibility of politically motivated hijacking.
For example, in the past, proponents of invasive species control and native plants have been charged with xenophobia by critics. A 2003 paper by University of Tennessee, Knoxville ecologist Daniel Simberloff argued that such charges are unfair. Regardless of how people use this analogy, Simberloff thought we should take them at their word when they say their motivation is to prevent environmental harm. And yet, he also described how the Nazis campaigned for elimination of non-native plants and explicitly linked this to their more notorious campaign of eliminating non-Aryan people. Simberloff also noted that early 20th century gardening writers were likewise often racist in their nativism.
In a 2021 paper, researcher Jonathan Davies described how the nationalist press in the U.K. co-opted the issue of invasive species in Britain after 2015, when migration began to be seen as a crisis in Europe. He wrote that “abject nature” was used as a metaphor to serve xenophobic and anti-immigration ends in the lead-up to the Brexit vote to leave the European Union. This March, University College London biologist Tim Blackburn’s popular science article about invasive and “alien” species arriving on British shores risked — as lay readers observed in the comments — fanning xenophobic flames that have smoldered since before Brexit.
Last year, an international group of biologists attempted to provide a common framework of terminology to use across the discipline, advising caution around the use of various terms that carry ideological connotations, including xenophobic ones that might offend and confuse. Still, while we may deplore such co-optation, from a scientific perspective, it doesn’t flat out invalidate the analogy.
Perhaps the most relevant point noted by the authors of the common framework is that imprecision is fatal to science. Analogies that implicitly or explicitly extend themselves too far or that seize on irrelevant factors for comparison only serve to weaken their value as scientific models.
In an insightful 2018 paper, Cynthia Taylor and Bryan Dewsbury of the University of Rhode Island warned: “The metaphors we rely upon may uphold and reinforce outdated scientific paradigms, contributing to public misunderstandings about complex scientific issues.” They critiqued the use of military metaphors in invasion biology, for example, largely on the basis of their polemic use. “Invasion metaphors are performative,” they wrote, and such metaphors “blur ‘facts’ with ‘values,’” promoting militaristic responses to invasive species.
Analogies that implicitly or explicitly extend themselves too far or that seize on irrelevant factors for comparison only serve to weaken their value as scientific models.
Socially, culturally, and economically freighted concepts drawn from human experiences of immigration — such as the search for work or opportunity, the refuge from oppression or war, the pain of exile, the loneliness of exclusion, or joy of family reunification — make them poor analogies for conceptualizing the problems of invasion science.
Most problematic of all for the analogy’s scientific and ethical validity: Humans consist of just one species. Comparing the arrival of different populations or communities — distinguished perhaps by economic status, skin color, language, or citizenship — is a fundamentally different exercise than assessing the impact of a new species on an existing biome, food web, or ecological niche.
The ease with which conservation or invasive species biologists can slip into these analogies reflects the ease with which we forget such a fundamental truth about ourselves. With very little genetic variability across human populations, subspecies of humans do not exist anywhere on this planet. That throughout the history of conservation science, researchers have so easily and so often fallen for this particular fallacy reflects not a scientific impulse, but a tragic social one.
Carlyn Zwarenstein is a journalist based in Toronto who writes about biology and power. She is the author of “On Opium: Pain, Pleasure, and Other Matters of Substance” and is working on a book about convergent evolution.