Given that climate change has been linked to phenological advances in numerous species of flora and fauna worldwide, new research is being done to determine what, if any, unique challenges mutualistic ecological relationships face as global temperatures continue to rise. Bartomeus et al. (2011) seek to determine to what extent wild bees of northeastern North America have undergone phenological advances due to climate warming and whether their host plants have are advancing in compatible ways. Researchers found wild bee phenology to have advanced in the last few decades paralleling global warming. However, they also determined that the the ten species of bees studied have kept pace with the phenological advances of their host-plants in the region. Nevertheless, further research must be done to determine if climate change will eventually cause phenological mismatches between bees and their host-plants.
Bartomeus, I., Ascher, J., Wagner, D., Danforth, B., Colla, S., Kornbluth, S., Winfree, R. 2011. Climate-associated phenological advances in bee pollinators and bee-pollinated plants. PNAS 51.
Phenological advances have been recorded in many plant and animal species over the last fifty years as a result of climate warming. One potential threat to food production caused by phenological advance is what is known as a phenological ‘mismatch,’ when a mutualistic relationship in an ecosystem is destroyed because the species in question undergo different advances as temperatures increase. Generalist species of wild bees are a critical pollinators in North America yet until Bartomeus et al. no major study had been launched to determine phenological advances in key species. Using historical data from museums over a 130 year period for ten species of wild northeastern North American bees, researches sought to determine phenological advance by comparing the day in April over time bees were sited with the historical mean temperatures for the month over the time period studies. The results demonstrate a mean advance for the ten species between 10.4 and 1.3 days in April.
There were many challenges in the formulation of this study and the collection of data. Unlike many other fauna in North America, there exists no standardized scheme for monitoring bees, thus researchers had to collate data from a variety of institutions collected in disparate ways. The ten species chosen for the study were selected for the richness of data and for the widespread presence of all ten species in the geographic area being studied. Furthermore, Bartomeus et al. argue little is known about the relationship between bee development and environmental triggers, with the few studies completed thus far showing North American bees to have complex relationships to winter and spring temperature changes. Finally, despite the results shown by this research project, the authors argue for the need of more complex, physiological models to predict phenological advances in bees and their host-plants into the future, beyond the data confines of this paper.
Having graphed historical data on collection days in April for the ten bee species in question and mean April temperatures against the year of collection, Bartomeus et al. demonstrate that the bees in question have been appearing earlier and earlier as temperatures increase. Roughly 69% of phenological advance has occurred since 1970, mirroring the comparatively steeper increase in mean temperatures since that time. This data leads researchers to conclude phonological advance appears to be gaining in severity as average temperatures increase. To see if there was threat of a mismatch between the bees and their host-plants, Bartomeus et al. used four published studies on phenological advance in native plant species in the region under analysis which flower during the period in which the bees are active. In two of the studies, the researchers found bees to be advancing faster than host-plants by 3% to 33%, however the other two studies suggest there is no clear pattern of divergence. To further complicate these mixed results, the authors argue not enough data exists to determine whether the demonstrated divergence is a result of actual biological differences or simply methodological error. Thus, further research is needed to gain a clearer insight into threats of phenological mismatch in natural pollinators in North America.