The invisible species held the answer.
For over a century and a half, ecologists inherited from Darwin a paradox they could not dissolve: does a newcomer species succeed by resembling its neighbors, or by differing from them? A team of researchers drawing on 340 years of Swedish lake records has now shown that both answers are correct — and that which one applies depends on the invisible, the absent, the species that could be present but are not. In measuring this 'dark diversity,' science has found that the gap between what is and what could be holds more explanatory power than what is alone.
- A 160-year-old contradiction at the heart of ecology — whether similarity or difference helps a species colonize new territory — has resisted resolution because researchers kept finding evidence for both sides simultaneously.
- The breakthrough came not from adding new observations, but from accounting for absence: the 'dark diversity' of species ecologically capable of living somewhere yet missing from it.
- Using 340 years of Swedish lake fish introduction records, researchers discovered that species-poor but ecologically saturated lakes favor similar newcomers, while species-rich but ecologically underoccupied lakes favor dissimilar ones.
- Two lakes with identical species counts can behave in opposite ways — the invisible roster of absent-but-possible species is the hidden variable that makes the difference.
- The framework now points toward practical application: as climate change and habitat disruption redraw the map of where species can survive, dark diversity offers conservation planners a way to anticipate and guide ecosystem transformation.
For more than 160 years, ecologists wrestled with a puzzle Darwin posed but never resolved: when a species arrives somewhere new, does it succeed by resembling the locals or by being different from them? Both theories were logically sound, both were supported by evidence, and neither could be declared the winner. The contradiction simply persisted.
The resolution came from an unexpected direction. Wen-Gang Zhang and Meelis Pärtel, working at the University of Tartu, proposed that the answer was never hidden in what was present — it was hidden in what was absent. They built their case around the concept of dark diversity: the species that could theoretically survive in a given habitat but simply aren't there, whether because they never arrived or because they once did and disappeared. The ratio of actual species to potential species — what the team calls community completeness — turned out to be the missing variable.
To test this, they drew on 340 years of Swedish lake fish records, tracking which introduced species established themselves and which did not. The pattern that emerged was clear. In lakes where most of the ecologically possible species were already present, similar newcomers tended to succeed. In lakes where only a fraction of the possible species actually lived, ecologically distinct newcomers had the advantage. Two lakes with the same number of fish species could behave in entirely opposite ways, depending on whether those species represented most of what could live there or only a small slice of it.
Published in the Proceedings of the National Academy of Sciences, the findings do more than settle an old debate. As climate change shifts where species can survive and human activity continues to fragment habitats, the dark diversity framework gives conservation planners a way to anticipate how ecosystems will respond — and where intervention might matter most.
For more than 160 years, ecologists have circled around a puzzle that Charles Darwin himself posed but never fully resolved: when a species arrives in unfamiliar territory, what determines whether it will take root or fade away? Darwin offered two competing theories, each logically sound, each supported by observations—and for generations, researchers found evidence for both, unable to say which was right.
The first theory held that newcomers thrive when they resemble the species already living there, sharing similar needs and fitting neatly into the existing ecological fabric. The second insisted the opposite: that outsiders succeed precisely because they are different, exploiting resources the local community ignores and avoiding direct competition. Ecologists kept finding cases where each rule applied, and the contradiction persisted, unresolved.
Now a team of researchers working across institutions, including Wen-Gang Zhang, a visiting doctoral student at the University of Tartu, and Meelis Pärtel, a professor of botany there, has proposed an answer—one that doesn't dismiss either of Darwin's predictions but instead explains when each one holds true. Their work, published in the Proceedings of the National Academy of Sciences, hinges on a concept called dark diversity: the species that could theoretically survive in a given place but simply aren't there.
The insight is subtle but powerful. Every habitat can support only so many species—a limit set by climate, soil, water, and other physical conditions. The actual species living in that place represent only a fraction of what could live there. Dark diversity is the gap between potential and reality, the invisible roster of organisms for which conditions are suitable but that have never arrived, or arrived and vanished. By measuring this gap—what researchers call community completeness—the team found they could finally untangle Darwin's knot.
To test the idea, they drew on an extraordinary dataset: 340 years of records from Swedish lakes documenting which fish species had been introduced and which had successfully established. This historical depth and precision allowed them to observe not just outcomes but patterns across many different ecological contexts. What emerged was striking. In lakes where the total number of species that could theoretically live there was small, but where most of those species were actually present, newcomers that resembled local fish succeeded more often. In lakes where many more species could theoretically thrive, but only a fraction actually did, newcomers that were ecologically different from the residents succeeded more often.
The key was that simply counting the species already present told you almost nothing. A lake with ten fish species might look identical to another lake with ten fish species, yet behave entirely differently depending on whether those ten represented 80 percent of the possible pool or only 20 percent. The invisible species—the dark diversity—held the answer.
Pärtel explained the implications: dark diversity offers a new theoretical framework for understanding how species distribute themselves and how communities assemble. It reconciles the conflicting evidence that had puzzled ecologists for over a century and provides a way to predict which newcomers will establish themselves under new conditions. As climate change reshapes where species can live, as human activity fragments habitats, and as species ranges shift in response to warming, this framework becomes more than academic. It offers conservation planners a tool to anticipate how ecosystems will respond, to guide where protection efforts should focus, and to forecast how biodiversity itself will transform in the decades ahead.
Citações Notáveis
Dark diversity offers a new theoretical framework for understanding species distribution and community formation, helping reconcile previously conflicting results.— Meelis Pärtel, Professor of Botany, University of Tartu
As climate change and human activity reshape biodiversity, dark diversity offers a new way to assess how ecosystems respond and guide conservation efforts.— Meelis Pärtel
A Conversa do Hearth Outra perspectiva sobre a história
So for 160 years, ecologists couldn't agree on whether species succeed by being similar to what's already there or by being different?
Right. Darwin proposed both ideas, and both seemed to work sometimes. It created this frustrating stalemate—you'd find evidence for one rule, then find a case where the opposite rule applied.
And dark diversity solves this by looking at species that aren't there?
Exactly. It's about understanding the full potential of a place, not just what you can see. A lake might have ten fish species, but if it could theoretically support fifty, that's very different from a lake where ten is nearly the maximum.
Why does that distinction matter for whether a newcomer succeeds?
Because it tells you whether the ecosystem is full or has room. If it's full and complete, newcomers that fit the existing pattern do better. If it's incomplete, with lots of unused ecological space, newcomers that are different thrive because they're filling gaps.
So the answer to Darwin's puzzle was that he was asking the wrong question?
Not wrong—incomplete. He needed to know not just which species were present, but which ones could be. The invisible species turned out to be the key.