Climate change impacts bee fitness by mismatching with flowers
Climate change is usually in the news because of more frequent floodings, storms or hunger crisis. Yet, it is an important threat for biodiversity, too. Not only polar bears are losing their habitat, also bumblebees are displaced to the north because of global warming. There are also other complications coming from this: you may have noticed that plants flower earlier every year. Especially in early spring and late fall the changes are most notable. You may wonder why I’m telling you this. Well, it leads to the subject I don’t know if you ever wondered about the timing between bee species and plants. Pollinators and the flowers they forage on should occur at the same time, especially if the relationship is very close. A bee species oligolectic on bell flowers should be active in late spring and summer, not already in March, when bellflowers don’t bloom.
This means that if plants flower earlier, bees should follow. But this isn’t as easy as it sounds. Increasing temperatures may have a different effect on the activity of bees and the flowering periods of plants. There are some indicators that this is the case. Therefore, there is the danger that the close relationship between bees (or other pollinators) and flowering plants gets disturbed. A recent study assessed the consequences for three solitary species: Osmia cornuta, O. bicornis and O. brevicornis. All three are Mason bees, the first two though are polylectic, while the third is oligolectic on the cabbage family.
One of the problems of field work is that many factors influence your observations. If you want to isolate the influence of a single parameter, you have to go from the observation in the natural setting to a more artificial, but controlled environment. In this study, both bees and plants were in tents. The two polylectic species got all types of flowers to meet their love of a variable diet, while O. brevicornis got oilseed rape and mustard to meet its special taste. Assuming that climate change produces a mismatch between flowers and bees, the scientists made three different set-ups:
- “Perfect synchronisation”: both bees and flowers were together in the tent from the very beginning
- Mismatch of three days: the bees were in the tent, but the flowers came in only three days later.
- Mismatch of six days: the bees were in, but the flowers followed only six days later.
In the two mismatch scenarios the bees had to starve until the flowers came in. All three species were observed separately, to assess if the mismatch had specific effects on each species. The main question of this research was the impact of the lack of foraging on the reproductive success. Therefore, both males and females were in the tents. Interestingly, the mating was completely independent of the presence of flowers. But later on, when it came to nest building, provisioning and egg-laying there were effects. Six days without flowers were too long, the survival was only low in this group. But if the mismatch was only of three days, the effects were more subtle and species specific:
- Osmia cornuta produced less female, but more male offspring. As it’s the females that sustain the population, this results in a smaller population in the next generation.
- Osmia bicornis provisioned less nests, but with more cells. These may be more susceptible for cleptoparasites, as they remain open for a longer period.
- Osmia brevicornis layed less eggs than in the tents with “perfect synchronisation”. Which again results in a smaller population in the following year.
Already a short mismatch, therefore, may have a large effect on solitary bee populations.
This study is interesting and I totally share the conclusion of the authors that the consequences of climate change on species interactions have to be considered more seriously. However, there are some caveats: as @wurflenii commented on Twitter, it is not clear why the bees had to starve completely in the mismatch scenarios. As they were going for the reproductive success, a little bit of sugar water may have increased survival in the 6-days-mismatch group and still showed the effects on reproduction (as they couldn’t feed on pollen). Also this “short” mismatch may not be so short: six days in the life of a bee are much longer than for a human (considering the total life-time). And in their habitat, they would most probably find some nectar even if their preferred pollen sources were still not flowering.
In addition, I wonder why the bees were introduced first in the tents. Most plants flower earlier; wouldn’t it be more logical to think that the bees arrive only at the end of the flowering or when it’s already over? From honey bees, there are anecdotal reports that colonies couldn’t use goat willow pollen for their spring development because they arrived “too late”. In fact, this plant is a good example for anticipated flowering.
Then, I would assume that the effects on oligolectic species are larger than for polylectic bees. The latter just provide their nests with other pollen, an oligolectic bee like Andrena vaga has no resources left if the willows already finished their flowering period.
Consequences of climate change
Anyway, this is a very interesting publication. It highlights the consequences of climate change from an unusual, understudied perspective. Decreasing pollinator populations are a big threat for food security and biodiversity. That global warming affects our pollinators in such an intrinsic part of their biology as it is reproduction should enlarge our efforts to stop it.