Weirs and dams act as barriers within river ecosystems – removing them affects how aquatic wildlife and invasive species move. Bardhok Ndoji/ShutterstockAlmost a quarter of all freshwater species are threatened with extinction. The removal of human-made barriers from rivers, such as dams and weirs, is a popular way to restore water flow and sediment transport to its natural state and allow fish and other aquatic wildlife to move more freely.There are more than 1.2 million barriers in European rivers. But more than 2,200 dams, weirs and other barriers have been removed between 2020 and 2025.This may help native species move more freely, but we’ve been researching how non-native invasive species respond to the removal of barriers along rivers. In a new study, our team analysed data from 45 studies that monitored freshwater communities before and after river barriers were removed. We found that native species generally benefited from barrier removal and showed stronger long-term recovery than invasive species. However, invasive freshwater invertebrates increased substantially following removal, suggesting that reconnecting rivers can also have unintended consequences.Our paper highlights what conservation scientists increasingly call the “connectivity conundrum”, where restoring connectivity can benefit native biodiversity while simultaneously increasing invasion risk.More than 70,000 artificial barriers have recently been mapped across rivers in the UK. Many are ageing structures built to power mills, supply water or control flooding. Where these structures no longer serve a useful purpose, they are increasingly targeted for removal as part of efforts to restore river connectivity and improve conditions for native wildlife.Dams and weirs break rivers into fragments. This prevents fish from reaching their spawning grounds, reduces genetic diversity in isolated populations of plants and animals and disrupts the movement of sediment and nutrients through the system. Removing barriers can help restore rivers to their natural conditions and aid species recovery.For decades, conservationists have highlighted the damage these barriers can cause, particularly for migratory fish such as Atlantic salmon. As a result, governments and environmental organisations have invested heavily in removing barriers and reconnecting rivers. The narrow-clawed crayfish is an invasive species. Aleron Val/Shutterstock However, alongside habitat fragmentation, rivers face another serious threat. Invasive species are plants and animals that have been introduced by humans to a new area which they do not naturally occur, where they then establish, spread and can have detrimental ecological or socio-economic consequences. These species cost global economies more than US$400 billion (£300 billion) every year.Highways to invasionRivers and lakes are particularly vulnerable to invasive species. Many are introduced through human activities, whether intentionally through fish stocking for angling and aquaculture, or accidentally on boats, fishing gear and other equipment moving between waterways. Once established, these species can remain unnoticed for years. The natural connectivity of rivers then acts like a highway, helping them spread rapidly throughout a catchment and increasing the risk of ecological and socio-economic damage.This has led to considerable efforts to control non-native species introduction and spread as well as to manage their impacts. Often, barriers have acted as accidental or intentionally placed roadblocks for invasive species.If these barriers are removed, invasive species may gain access to habitats they could not previously reach. In a worst-case scenario, they may enter these newly reconnected habitats more quickly than native species can recover. Once established, these invasive species can feed on, compete for food/space with and spread disease to native species within restored habitats. Read more: Paddleboards, swim kit and fishing gear pose a biosecurity risk – here’s why The connectivity conundrumTo better understand these risks, we gathered evidence from studies conducted across the globe. Most were from the US where barrier removal programmes have been underway for decades. Together, the studies contained thousands of observations of fish, plants and freshwater invertebrates.We wanted to know whether native and invasive species responded differently once rivers were reconnected. They did, although patterns varied considerably depending on the species, location and time since barrier removal.While responses varied among species, direction to the barrier, and the number of years since the removal, native species generally showed stronger recovery than invasive species. This is encouraging news and demonstrates the conservation benefits of barrier removal.Invasive invertebrate species, such as the New Zealand mud snail, showed substantial population increases after barriers were removed, however. These animals do not receive the same public attention as charismatic fish, such as salmon or trout, but they can have major effects on river ecosystems. Our research also revealed how few studies examined invasive invertebrates. None focused on responses of native invertebrates. Understudied groups of species need greater representation in future research.Native species generally showed stronger long-term recovery than invasive species. However, many studies only monitored rivers for a few years after barrier removal. These short-term studies may overlook important long-term ecological responses. Species recovery, movement and reproduction can take much longer. Plus non-native species may only be introduced to the river after the barrier removal event. These “new” invasions were not considered in our analysis but pose a substantial threat as populations can rapidly grow and spread after introduction.Rivers differ greatly in their physical and ecological characteristics, so there is no universal answer to whether a barrier should be removed. A barrier that is harming native fish may also be slowing the spread of an invasive species. Removing it could deliver major conservation benefits or create new ecological problems.By adopting a whole-ecosystem approach as efforts to reconnect rivers continue, restoration planning can consider both biodiversity recovery and invasion risk.Ellen Dolan receives funding from the Department of Agriculture, Environment, and Rural Affairs (DAERA), UK. Jaimie Dick and Ross Cuthbert do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.