As global warming causes earlier spring warmth and later fall frosts, the flowering season lengthens. This would appear to benefit pollinators dependent on flowers for food. A casual examination of longer flowering periods suggests this results in an abundance of food sources for pollinators like bumble bees. Snow melts earlier, plants begin growing sooner, produce flowers earlier, and can continue to grow longer before being killed back by frost. Bumble bees flourish and their colonies increase. Makes sense, right?
A study of three subalpine bumble bees in the Rocky Mountains, however, contradicts this simplistic assumption and instead shows that a longer growing season can lead to bumble bee population declines. The study found that the critical factor isn’t floral abundance, it’s sufficient temporal floral distribution throughout the season. Spring plants flower first and then set seed as summer plants begin to bloom. We don’t have the same plants flowering in spring, summer, and fall as most species have a particular flowering period. Ideally these flowering periods overlap and offer a continuous food supply for pollinators.
...We now have longer flowering seasons because of earlier snowmelt, but floral abundance has not changed overall. This means we have more days in a season with poor flower availability.
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Because bumble bees live for only one year, they show responses to climate change fast. Data on the three bumble bee species from the past 8 years showed their abundance was driven primarily by climate change effects on temporal distribution of flowers. Long term data for 43 years showed how climate and floral resources changed in ways that negatively affected the bees.
The three species live at high elevation in northern latitudes, a region that experiences dramatic changes as temperature increases. Ecologists consider bumble bees early responders to subtle effects resulting from climate change. Pollinator populations worldwide are known to be in decline and climate change is thought to be one contributing factor so what is found for bumble bees might apply to other pollinators. This study provides long-term data showing how climate change affects flower food sources that then affect pollinators.
"I'm afraid that this research shows conservation will be even more complicated than expected," [FSU postdoctoral researcher Jane Ogilvie] said. "In addition to the response of the target species, our findings suggest that we should be considering how a species' food resources might be responding to climate change. For bumble bees in particular, we need to make sure that they have enough flowers available during the entire season."
Scientists don’t have answers about how to resolve this problem, but it’s important to know both direct and indirect impacts from climate change and their relative importance. The study found that the indirect impact is more limiting to these bumble bee species. This knowledge helps us to focus solutions on the most significant aspect — as flowering season lengthens, there are more days of bumble bee activity with poor flower availability because gaps between flowering periods also lengthen. Conservation is complicated.
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