Species Contributions to Biotic Homogenisation and Differentiation

Aim Homogenisation (decreasing beta-diversity) among biological assemblages is often interpreted as being caused by already-widespread species increasing. The link between individual species level trends and homogenisation between assemblages, however, has not been fully addressed, with most studies focused solely on either assemblage level or species level changes. Here we aim to test the widely held hypothesis that homogenisation is driven by the decrease of localised species and increase of those already widespread using species contribution to beta-diversity. Location North America, Europe, South Africa. Time Period 1970 to 2019, 1966 to 1996, 11,700 years ago to present, 2011 to 2021, 1960 to 2016. Major Taxa Studied Birds, plants, benthos and mammals. Methods Here, we consider individual species contributions to spatial beta-diversity and how these change over time. We focus on the relative contributions of localised and widespread species across five case studies to determine which are contributing most to homogenisation. Results Species occurring in around half of sites provided the greatest contributions to beta-diversity at a given time, but not through time. The most widespread species (> 0.75 of sites) contributed little to beta-diversity change, with this most apparent in highly nested assemblages. In contrast, localised species (initially in < 0.25 of sites) contributed most to both homogenisation (when declining) and differentiation (when increasing) regardless of nestedness. Main Conclusions This challenges the hypothesis that widespread generalist species are the main drivers of homogenisation, underlining the importance of rare species and of nestedness to patterns of beta-diversity change. Conservation interventions to increase localised species occurrence would do more to limit homogenisation than attempts to control already-widespread species or prevent others becoming widespread, especially when assemblages are highly nested.