European bison imported from Poland now roam Denmark's Baltic island of Bornholm in places where the animals haven't lived for thousands of years. Meanwhile, in a far corner of Siberia, scientists are attempting to reconstruct an ecosystem that was lost many thousands of years ago along with the woolly mammoth by introducing bison, musk oxen, moose, horses, and reindeer to a place they call Pleistocene Park.

These efforts to "rewild" the landscape have become increasingly popular in some corners, but researchers writing in the Cell Press journal Current Biology on February 8 say that scientific evidence supporting the potential benefits of this form of restoration is limited at best. As history has shown, the introduction of species into new places is often met with unexpected, negative consequences for the environment.

"Implementation of rewilding in the field is already occurring," says David Nogués-Bravo of the Center for Macroecology, Evolution, and Climate at the University of Copenhagen. "However, scientifically we are in the dark about the consequences of rewilding, and we worry about the general lack of critical thinking surrounding these often very expensive attempts at conservation. Practitioners mustn't assume that scientists are able to predict the full consequences of introducing novel species to dynamic and ever-evolving ecosystems."

Nogués-Bravo and his colleagues say that before implementing rewilding in the field as a major conservation approach, more basic research is needed about the consequences of modifying ecosystems. They argue that, for now at least, conservation efforts should focus instead on protecting biodiversity and on reducing major threats to the environment, such as deforestation, climate change, and invasive species.