Data from: Unravelling the complexities of temporal biotic homogenisation and heterogenization: Avian assemblage dynamics in Britain

Biotic homogenization is a process whereby species assemblages become more
similar through time.  The standard way of identifying the
process of biotic homogenization is to look for decreases in spatial
beta-diversity. However, using a single assemblage-level metric to assess
homogenization can mask important changes in the occupancy patterns of
individual species. Here, we analysed changes in the spatial
beta-diversity patterns (i.e., biotic heterogenization or homogenization)
of British bird assemblages within 30km x 30km regions between two periods
(1988-1991 and 2008-2011). We partitioned the change in spatial
beta-diversity into extirpation and colonisation resultant change (i.e.,
change in spatial beta-diversity within each region resulting from both
extirpation and colonisation). We used measures of abiotic change in
combination with Bayesian modelling to disentangle the drivers of biotic
heterogenization and homogenization. We detected both heterogenization and
homogenization across the two time periods and three measures of diversity
(taxonomic, phylogenetic, and functional). In addition, both extirpation
and colonisation contributed to the observed changes, with
heterogenization mainly driven by extirpation and homogenization by
colonisation. These assemblage-level changes were primarily due to
shifting occupancy patterns of generalist species. Compared to habitat
generalists, habitat specialists had significantly (i) higher average
contributions to colonisation resultant change (indicating
heterogenization within a region due to colonisation) and (ii) lower
average contributions to extirpation resultant change (indicating
homogenization from extirpation). Generalists showed the opposite pattern.
Increased extirpation resultant homogenization within regions was
associated with increased urban land cover and decreased habitat
diversity, precipitation, and temperature. Changes in extirpation
resultant heterogenization and colonisation resultant heterogenization
were associated with differences in elevation between regions and changes
in temperature and land cover. Many of the ‘winners’ (i.e.,
species that increased in occupancy) were species that had benefitted from
conservation action (e.g., buzzard (Buteo buteo)). The ‘losers’ (i.e.,
those that decreased in occupancy) consisted primarily of previously
common species, such as cuckoo (Cuculus canorus). Our results
show that focusing purely on changes in spatial beta-diversity over time
may obscure important information about how changes in the occupancy
patterns of individual species contribute to homogenization and
heterogenization.