Intentional and unintentional changes to avian and mammalian diversities in the U.K.

Rewilding is emerging as a promising restoration strategy to tackle challenges of global change and maintain the persistence of natural ecosystems and their biodiversity. However, it has also been criticised for lacking a consistent definition and insufficient knowledge about its possible outcomes. Here, we explored the effects of rewilding on filling functional gaps created by native species extirpation. We contrasted rewilding with three other mechanisms for change in community composition – species introduction, unassisted colonisation, and extirpation – with regard to their impacts on avian and mammalian diversity changes in the UK. We show that: (i) while rewilding increases functional diversity most on average, introduced/naturalised birds contribute more functional uniqueness to native functional space than other groups of birds; and (ii) functional diversity change caused by ‘rewilded’ species was species-dependent and idiosyncratic. Our results suggest that although rewilding can expand or infill native functional trait space to some extent, such effects of specific programmes may need careful assessment.