It was a relatively inexpensive winter for heating in central North America, with mild temperatures reducing the need to warm our houses, but the mild, fluctuating temperatures will cause butterflies to suffer severe energy shortages this spring.
A new study published in the journal PLoS One by researchers from Western University and the University of Notre Dame reveals that these kinds of winters are energetically expensive, even for butterflies that can shift gears to reduce their energy use.
The research was conducted on overwintering caterpillars of the Propertius Duskywing butterfly, which lives on Garry Oak trees. Like all insects, the warmer the temperatures the caterpillar experiences, the higher their metabolic rate and the more fat they burn throughout the winter. Since caterpillars don't eat over winter, they have to conserve as much energy as possible during winter so they can use the change to metamorphose into butterflies in the spring.
"The energy reserves the caterpillars collect in the summer need to provide enough energy for both overwintering and metamorphosing into a butterfly in the spring," explains the study's lead author Caroline Williams formerly a Western graduate student and now a postdoctoral fellow at the University of Florida. "When it's warm, butterflies use a lot more energy than when it is cool, which is exactly the opposite for a mammal like humans."
Emerging butterflies that have burned through more of their energy have fewer reserves for surviving and laying eggs. Warmer, variable winters, therefore, may decrease the number of butterflies.
But it's not all doom and gloom for caterpillars exposed to warm winters. The researchers found that caterpillars are able to turn down their metabolism to save energy in more variable environments. The team used caterpillars from two locations: a site in Oregon which experiences warmer and more variable temperatures than a second location on Vancouver Island, which is cooler and more stable. The researchers reared the caterpillars under each set of conditions, and discovered that the caterpillars can reduce their sensitivity to temperature if they have been exposed to variable conditions in the past.
"By reducing their thermal sensitivity in response to variable conditions, the caterpillars are less susceptible to temperature fluctuations, which means they can save more energy for metamorphosis and reproduction in the spring," explains Brent Sinclair, a biology professor in Western's Faculty of Science, who directed the study. "This sort of response had been predicted by theory, but it is the first time we've observed it in nature. It does show that the butterflies are able to respond rapidly to changing thermal environments, and is somewhat heartening as we consider their ability to cope with changing winters due to climate change."
But the researchers found a limit to the energy savings the caterpillars could make. To test whether this would have an effect in many different temperature conditions, the researchers combined the lab data with historical weather data from both Oregon and Vancouver Island to calculate the caterpillars' energy use over the past 30 winters. It was found that the caterpillars in Oregon -- warmer, more variable conditions -- would use more energy even though they were compensating for the tougher conditions by lowering their metabolic rates.
"The caterpillars are doing all they can to reduce their energy use," says Williams. "And even still, they find the warm, variable winters tougher to handle. As much as we might like warm winters for ourselves, it's important to understand how they will affect all the species around us."
Sinclair adds, "The costs and benefits of a weird winter on insects are a bit of a double-edged sword. On the one hand, an expensive winter might be damaging to populations of pest insects, like mosquitoes or agricultural pests – and we like that. On the other hand, we have beneficial insects, like ladybugs that eat aphids, or species that we care about for biodiversity, like butterflies, that could be negatively affected because they have less energy in the spring.
“We can expect more weird weather like we experienced this winter thanks to climate change. We can use events like this and experiments simulating warmer and more variable winters to tell us how plants and animals will respond in the future."
The team also included Katie Marshall and Heath MacMillan from Western and Jason Dzurisin and Jessica Hellmann from Notre Dame. The work was funded by the Natural Sciences and Engineering Council of Canada, the Canadian Foundation for Innovation and the Ontario Ministry of Research and Innovation and the U.S. Department of Energy.