Revisiting the case for assisted colonisation under rapid climate change

•1. Climate change is driving the rapid reorganisation of the world's biota as species shift their ranges to track suitable conditions, but habitat fragmentation and other barriers hinder this adaptive response for species with limited dispersal ability. Active translocation into newly suitable areas has been suggested as a strategy to conserve species otherwise unable to expand their ranges; however, assisted colonisation has not been widely adopted because the deliberate introduction of non‐native species poses invasion risks and runs counter to traditional conservation approaches. •2. We use the future of forest ecosystems in Great Britain as a thought experiment to argue that mass‐scale assisted colonisation will likely be required not to conserve threatened species, but to maintain functional ecosystems. As the climate changes, existing forest plant and animal communities of northern Europe will increasingly die out in their current locations, but in Great Britain, their replacement with range‐expanding species from further south will be limited to a subset of mobile species able to overcome the ocean barrier. As a result, British forests will come to lack many important component species unless these are actively translocated; will have reduced resilience and adaptive capacity; and may eventually collapse. •3. Policy implications : Maintaining functioning ecosystems in a hotter world will require mass‐scale assisted colonisation, so appropriate conservation policy, legislative frameworks and regulating bodies must be urgently developed. Conservationists must shift focus from the prevention of species extinctions to the maintenance of functioning ecosystems; from trying to prevent change and maintain the biotic communities, we have to trying to shape the biotic changes that are now inevitable. We must shift from reactive to proactive approaches to facilitate the emergence of robust novel ecosystems.