For 340 years, fish have been moved between Swedish lakes—some introductions thriving, others vanishing without a trace. Now, a team led by University of Tartu researchers Wen-Gang Zhang and Professor Meelis Pärtel has cracked open one of ecology’s oldest mysteries using this remarkable archive: Darwin’s naturalization conundrum. Why do some species flourish in new environments while others fail? For over 160 years, scientists have debated whether newcomers closely related to native species have the advantage—or whether distant relatives, with different resource needs, are more likely to succeed. The answer, it turns out, lies not just in the species present, but in those that are absent: the so-called 'dark diversity.'
Dark diversity refers to species that could live in a habitat based on its conditions but don’t currently occur there. Together with the species that are actually present, they form the full species pool—the complete set of organisms that could theoretically inhabit a place. By measuring community completeness (the proportion of that pool already filled), the researchers uncovered a unifying principle behind Darwin’s puzzle. In lakes with low species richness but high completeness—where most of the suitable species are already present—newcomers closely related to locals were more likely to establish. They shared adaptations to the specific environment, giving them a foothold. But in lakes with high potential diversity yet low completeness—ecosystems far from saturated—distantly related species succeeded more often, exploiting unused resources and avoiding competition.
The 340-year Swedish fish dataset proved pivotal, offering rare clarity by including both successful and failed introductions. "The study showed that dark diversity offers a new theoretical framework for understanding species distribution and community formation," says Pärtel. This framework doesn’t just resolve a historical debate—it transforms how we predict biodiversity change. As climate shifts push species into new territories and human activity reshuffles ecosystems, understanding not just who’s there, but who could be, becomes essential.
The implications extend far beyond lakes. Conservationists can use dark diversity to assess ecosystem resilience, identify missing species for restoration, and anticipate which invaders might take hold. In a world of accelerating change, this approach offers a more nuanced lens for protecting biodiversity. The study, published in Proceedings of the National Academy of Sciences, doesn’t just illuminate the past—it equips us with a tool to navigate the ecological uncertainties of the future.