Environments are everchanging, forcing animals to deal with daily and seasonal variations, such as changing temperature or rainfall. Such variations can shift the times when different species are active and can affect what foods are available, causing direct and indirect competition with other animals for limited resources. Environmental shifts can have different effects on an animal depending on its life stage. Yet, these important life-stage-specific responses to environmental change are often unaccounted for in ecological studies, not because scientists do not recognize their importance, but because long-term monitoring can be challenging. However, these types of studies are desperately needed if we are going to predict the vulnerability of a given species to climate change. Jane Ogilvie at the Rocky Mountain Biological Station, USA, and Paul CaraDonna at the Chicago Botanic Garden, USA, asked how bumble bees at each life stage respond to changes in weather, pollen and nectar availability and how the abundance of previous life stages affect the following generations.
From 2015 to 2021, Ogilvie and CaraDonna monitored a community of seven bumble bee species every week between April and September in the Colorado Rocky Mountains, USA. They recorded the numbers of worker bees, emerging queens and males, while noting whether the insects were foraging, just flying through, looking for new nesting sites, or whether the males were searching for unmated females. The pair also recorded the temperature and precipitation at each site, to find out how the weather conditions influenced the activity of workers and male bees. As queens can only come out of hibernation once the ground is completely free of snow, Ogilvie and CaraDonna recorded the date when the snow melted completely, to look for connections between how long each winter lasted and queen bee winter survival. Finally, because bumble bee species visit various different flowers, the pair tallied and identified the flowers at each site and counted how many times workers of each species visited each kind of flower to see how changes in pollen and nectar availability influenced bumble bee population sizes.
Ogilvie and CaraDonna discovered that bumble bee species respond differently to changes in weather and food availability. For example, longer winters reduced the number of queens that survived to spring, presumably because they burned through their winter energy stores, starving to death. In addition, only three species in their study – Bombus flavifrons, Bombus insularis and Bombus mixtus – showed an increase in the number of males flying during warmer, drier weather. Yet, the workers of all species but one – the nest parasite B. insularis, which doesn't produce workers of its own – seemed unaffected by the weather. They also found that worker-bee numbers increased when there was more pollen and nectar available and, with more workers collecting pollen and nectar, the colony had enough food to create more queens and males to venture off and establish new colonies. And, for some species, such as B. mixtus, the number of bumble bee workers from the year before determined how well the overwintered queens did the following year.
This investigation by Ogilvie and CaraDonna has implications far beyond studies of social insect pollinators. The study emphasizes the need for researchers to account for life-stage-specific sensitivity to shifts in weather conditions and food availability if we are to have any hope of predicting how a given animal will fare in a changing world.
