How extirpations, colonizations and introductions of bird species typically alter the regional diversity, distinctiveness, and body size distribution of regional bird biota

Much research in recent decades has been dedicated to investigating the effects of the anthropogenically-driven factors on the Earth’s biodiversity, evaluating which taxa are most at risk, and researching how to mitigate these effects. Species assemblages are shifting in response to human activities, with large-bodied species historically most at risk due to human overexploitation as well as intrinsic life history traits. To investigate how avian communities are changing because of the Anthropocene, I analyzed the data of 10820 avian species in 105 regions from 1815 to 2015. I measured median body mass on a regional scale both as an indication of faunal assemblage change and as a way of determining if large-bodied species are still predominantly at risk. I found that regions are becoming more diverse within themselves, and more similar between themselves, with no change in median body mass, and a less diverse distribution of body masses. Although extirpations were predominantly larger-bodied species, arriving (introduced and colonizing) species were also significantly larger than surviving species. Through an increase in regional diversity, driven by arriving species, bird assemblages are changing in body mass distribution but not median mass despite the continuing extirpation of large-bodied species. There is still potential functional loss in assemblages as the largest species are lost and not replaced by similarly-sized species. Conservation efforts should focus on replacing lost ecosystem functionality by rewilding extirpated species or replacing with similarly-sized species if they have become globally extinct.