Over 2,200 dams and weirs have been torn down across European rivers since 2020, opening waterways that had been fragmented for decades and giving native fish and aquatic creatures a chance to roam freely once again. Yet a sweeping analysis of 45 global studies reveals an uncomfortable truth: while the removal of these barriers helps native species recover, it simultaneously rolls out a welcome mat for invasive species that had been held in check by those same obstacles.
Nearly a quarter of all freshwater species now face extinction, a crisis that sparked the dam-removal movement. Rivers choked with artificial barriers lose their natural flow, trapping sediment and nutrients while isolating populations of fish and plants. Atlantic salmon and other migratory species became landlocked, unable to reach spawning grounds. The damage was severe enough that governments and environmental organizations worldwide invested billions in barrier removal to restore rivers to their natural state. In the UK alone, over 70,000 artificial barriers have been recently mapped—many aging structures built for mills, water supply, or flood control that no longer serve their purpose.
The science appeared straightforward: remove the barriers, restore the flow, save the species. And for native species, the results are genuinely encouraging. Research shows that native freshwater species generally benefited from barrier removal and demonstrated stronger long-term recovery than invasive species after reconnection. That is where the story should have ended—except it didn't.
Invasive freshwater invertebrates, particularly species like the New Zealand mud snail, showed substantial population increases following barrier removal. This is what conservation scientists now call the "connectivity conundrum": the same pathways that allow native species to recolonize newly accessible habitats become highways for invasive species to spread rapidly across regions where they had previously been contained.
The costs of aquatic invasions are staggering. Non-native species cost global economies more than US$400 billion annually. In rivers and lakes, invasives are typically introduced through human activities—sometimes intentionally through fish stocking for angling and aquaculture, sometimes accidentally via boats, fishing gear, and equipment moving between waterways. Once established, these species can remain hidden for years before exploding across a catchment. They feed on native species, outcompete them for food and space, and introduce diseases. Barriers, whether placed deliberately or by accident, had at least slowed their spread.
The research team analyzed thousands of observations of fish, plants, and freshwater invertebrates from studies conducted primarily in the United States, where dam-removal programs have been underway for decades. The data showed clear patterns: native species responded differently than invasive species, though outcomes varied by location, species type, and years since removal. The encouraging finding is that native species generally recovered faster and more robustly than invasive species could spread—but "generally" is not a guarantee.
The challenge now is clear: restoring river connectivity is essential for native biodiversity, yet it demands careful management of the invasion risk that comes alongside it. Europe's commitment to removing barriers—with more than 1.2 million still remaining in European rivers—shows the scale of the opportunity. But as thousands more barriers come down, the stakes for getting invasive species management right have never been higher.