Impact of landscape fragmentation and climate change on body size variation of bumblebees during the last century

Body size is a key parameter of organism fitness. While the impact of
climate change on body size has received increasing attention, the
long-term consequences of landscape fragmentation are still poorly known.
These two major global threats may potentially induce opposite trends: the
decrease of body size in warmer environments (e.g. individuals developing
faster) or the selection of larger individuals in fragmented habitats
(e.g. large individuals more capable of reaching distant patches). We
assessed the relationship between temperature and landscape fragmentation
with mean body size during the last century, within four European regions
(Austria, Belgium, England and above the Arctic circle in Scandinavia) and
among queens of five bumblebee species. At the regional scale, we first
analysed the variation over time of body size and the two hypothesised
drivers, temperature and landscape fragmentation. Then, at the local
landscape scale, we tested whether body size varied according to these
drivers irrespective of the region. At the regional level, we observed a
statistically clear increase of queen body size corresponding to an
increase of landscape fragmentation (i.e. in Belgium and England). There
was no increase of size when fragmentation did not increase (i.e. in
Austria and above the Arctic Circle). Temperature also increased through
time in all regions. At the local landscape scale, we found that all
species were impacted by changes in both climate and landscape
fragmentation but show different trends. The body size of the two largest
species significantly increased at landscape level with higher
fragmentation while body size of the two smallest species decreased with
higher fragmentation. We highlight that, in a context of global changes,
landscape fragmentation can also be a major driver of body size clines.
Depending on the dispersal abilities of species, larger species could be
positively selected for and overcome landscape fragmentation.