Above: The HSA is in the southern portion of the White Admiral butterfly’s range, suggesting that this species may be diminished by winter warm spells. The White Admiral overwinters as a caterpillar. Credit: K. McCarrel
This is the fourth article in a series about climate change impacts on various ecosystems. To read the other articles in this series, click here.
Throughout children’s stories, it always seems to be the villain who is casting spells on the story’s hero. In The Little Mermaid, Ursula casts a spell on Ariel to make her lose her voice. In Sleeping Beauty, Maleficent curses Princess Aurora to make her fall asleep. And in Harry Potter, Voldemort casts a spell on Harry with the intent to kill.
Witches and wizards use spells to hinder and damage the story’s hero. Climate change is no different.
In this case, our heroes are the butterflies.
The spells climate change typically uses to impact our heroes are cold spells and heat spells. For the most part, cold spells amount to simply an inconvenience for HSA butterflies, as they have evolved with cold weather. Cold in the winter has no impact, and cold in summer may slow butterflies down. If heat spells happen during the summer months, the butterflies and caterpillars may experience some stress, but overall, insect development is accelerated and butterflies are positively impacted. Conversely, if the heat spells occur during the winter there will be dire consequences.
Butterflies are most abundant in tropical regions, where they don’t have to contend with sub-zero temperature and snow during such a large part of the year. Butterflies in the HSA have a unique challenge: winter. Our butterflies have evolved with these conditions, developing an overwinter state like hibernation to stay safe until spring.
In eastern North America, butterfly climate ecology seems to be understudied. In one of the few local studies, researchers in Massachusetts found that northern butterfly populations are rapidly declining, particularly those that overwinter as eggs, and climate change is the only evidenced cause (Breed et al., 2012). In a long-term project out of Britain, they also found that the overwintering stage is the most important period when considering climate change impacts (Long et al., 2017).
Most species overwinter on or near the ground, where there is usually a layer of snow for the better part of the season. The snow insulates the overwintering egg, caterpillar, chrysalis, or butterfly, keeping it out of reach of brief warm spells. Less snow cover and longer warm spells are putting our butterflies at greater risk than usual. The warm temperature signals to the butterflies that spring is here, and so they awaken. The return to cold temperatures causes the butterfly to use more energy and lose body mass, making it more susceptible to disease and starvation (Harvell, 2002; Long et al., 2017). Reduced snow cover also removes water sources for the overwintering eggs and chrysalises (Breed et al., 2012).
An HSA species, the Baltimore Checkerspot, is one species that is being caught in the curse of climate change. Baltimore Checkerspot populations in the southern portion of its range are dwindling, and researchers are pointing at climate change. A study out of Washington, D.C., ran trials on captured butterflies at various stages to determine how different temperature patterns influence survival. They found that the most detrimental event was when there was a warming period followed by a return to cold temperatures during the overwintering chrysalis stage (Abarca et al., 2019).
Two other local species of Tiger Swallowtails are interesting cases. Canadian Tiger Swallowtail and Eastern Tiger Swallowtail are very closely related but cover rather different areas of North America. Surprisingly, the more northern species, Canadian Tiger Swallowtail, falls for the winter warm spells more often and to more detriment than the southern Eastern Tiger Swallowtail (Scriber et al., 2012). This is likely because northern species, the Canadian Tiger Swallowtail, have such limited spring and summer warm temperatures in their native range that they take advantage of any indication of an early spring. Eastern Tiger Swallowtails are southerners and are slower to emerge to the warmth as they are used to longer summers (Scriber et al., 2012). Tiger Swallowtails are showing us that butterflies that have evolved in (previously) cold environments, like the HSA, are the most at-risk of overwinter die-off.
When overwintering butterflies perish at the hands of the climate curse, there will be some profound impacts rippling across the landscape.
Firstly, caterpillars and butterflies play an important role in food webs across the HSA. Caterpillars are perfect little sacks of protein and fats needed for birds during migration and chick-raising. Butterflies are also large, easily caught food sources for the aerial insectivores like swallows and kingbirds. When the butterflies fail to make it through the winter, bird populations will also change throughout the following spring and summer.
Another important impact will be the loss of important pollinators. Butterflies and bees are the most abundant native pollinators across all our ecosystems. Although bees are more efficient pollinators (Rader et al., 2016), they do not travel as far across the landscape as butterflies (Schmitt, 1980) — though butterflies pick-up less pollen, they transfer it farthest. There are many flowers that have adapted to butterfly pollination rather than bee pollination, like coneflowers, thus relying on butterflies to transport their genetic material across the landscape. Without the far-reaching butterfly pollen transport system, some plant species may start evolving separately, splitting what was a single species into many species.
Lastly, and perhaps most importantly, butterflies are a flagship species of conservation across Canada. Butterflies represent the beauty and grace that is the environment. These species engage almost everybody to experience and enjoy nature, which in turn, spurs conservation and stewardship. Without the butterflies, it could be seen that all other species (especially insects) become overlooked and left to the wayside. Without butterflies, collective conservation may never happen.
The story of climate change is not without its own magic, though it is certainly not a fairy tale! Our hero Butterflies are still working to find a counter-curse to one of climate changes most damaging spells, the winter warm spell. Maybe this one will work: “Abracadabra, alakazam, make it cold, once again.”
Abarca, M., Larsen, E.A., Ries, L. (2019). Heatwaves and novel host consumption increase overwinter mortality of an imperiled wetland butterfly. Frontiers in Ecology and Evolution, 7:193, doi: 10.3389/fevo.2019.00193
Breed, G.A., Stichter, S., Crone, E.E. (2012). Climate-driven change in northeastern U.S. butterfly communities. Nature Climate Change Letters, doi: 10.1038/NCLIMATE1663
Harvell, C.D., 2002. Climate warming and disease risks for terrestrial and marine biota. Science, 296(5576): 2158-2162, doi: 10.1126/science.1063699
Rader, R., Bartomeus, I., Garibaldi, L.A., Garratt, M.P.D., Howlett, B.G., Winfree, R., Woyciechowski, M. (2016). Non-bee insects are important contributors to global crop pollination. PNAS, 113(1): 146-151, doi 10.1073/pnas.1517092112
Schmitt, J. (1980). Pollinator forging behavior and gene dispersal in Senecio (Compositae). Evolution, 34(5): 934-943.
Scriber, J.M., Maher, E., Aardema, M.L. (2012). Differential effects of short term winter thermal stress on diapausing tiger swallowtail butterflies (Papilio spp.). Insect Science, 19: 277-185, doi: 10.1111/j.1744-7917.2011.01477.x