Dramatic changes to the Swedish community composition of bumblebee species and their relative abundances have emerged in the last 70 years (1940s, 1950s and present), new research from Sweden shows. The study, funded in part by the STEP ('Status and trends of European pollinators') project that has clinched EUR 3.5 million under the Environment Theme of the EU's Seventh Framework Programme (FP7), is published in the journal Proceedings of the Royal Society B – Biological Sciences.
Researchers from the Swedish University of Agricultural Sciences and University of Lund also report that the average seed yield of red clover has shrunk since 1921, while the variation in yield has increased two-fold. They propose that the dependence red clover has on some species in order to pollinate has had an adverse effect on the stability of seed yield.
The team points out that the decline of the species richness of flower-visiting insects has fuelled concerns that the ecosystem service they offer by pollinating crops and wild plants is also in jeopardy. 'The relative commonness of different species with shared ecological traits can play a pervasive role in determining ecosystem functioning, but information on changes in abundances of pollinators over time is lacking,' the authors write.
'It is worrying to see evidence that previously common bumblebee species have become rare and even red-listed,' explains lead author Riccardo Bommarco, a professor at the Swedish University of Agricultural Sciences. 'It is possible that such changes in community composition precede extinctions. In our efforts to conserve species and manage ecosystem services [it] appears important to promote not only species-rich, but also more evenly composed communities of service-providing organisms.'
It should be noted that scientists investigated pollination and seed production of red clover (Trifolium pratense) in the period from 1940 to 1960. Red clover is considered a key forage crop that depends significantly on pollination by bumblebees for seed set. The Swedish researchers compared their data, targeting relative abundances of bumblebee species collected from 2008 to 2010 in 44 red clover fields across the Nordic state, with detailed historic records from that 20-year period.
Their findings indicate that two bumblebee species, namely Bombus terrestris and B. lapidarius, swelled by 40?% in the 1940s to dominate presented communities with 89?%. The team adds that other species, including B. hortorum and B. pascuorum, have shrunk in relative commonness, down from 20?% to 2?%, of the bumblebees investigated in a flowering clover field. Another species, B. distinguendus, also declined from 11?% to 0.7?%; it should be noted this species is on the near-threatened list in Sweden.
'Our results suggest a need to develop management schemes that promote not only species-rich but also more evenly composed communities of service-providing organisms,' the authors write.