Spring has been coming early to the Rocky Mountains, and while warm weather may be good news for humans, it’s bad news for flowers and butterflies. A long term study by Carol Boggs (Stanford University) and David Inouye (University of Maryland) shows that early snowmelt causes a decrease in flower populations, which in turn means reduced food for the mormon fritillary butterfly. In years where the snow pack melted early, spring frosts often damaged newly emerged plants, and the butterfly’s preferred flowers became scarce. Caterpillars were also casualties of this early spring thaw; butterfly larvae emerged before the danger of frost had passed and were often victims of sudden freezes. The Rocky Mountains are warming faster than other parts of the country, so the effects of early snow melt are expected to worsen in coming years.
Mormon fritillary caterpillars have an unusual life cycle that makes them especially vulnerable to climate change: their eggs hatch in late summer before snow fall and caterpillars hibernate under the snow pack until spring. Early snow melt can be a false alarm for caterpillars: if they come out of hibernation before the danger of frost has passed, they may perish in a sudden cold snap. Adult butterflies are similarly affected by spring frosts. Females depend on nectar to produce eggs, and early frosts can kill newly sprouted flowers. In years where flowers are scarce and butterflies are abundant, females must compete for food and produce fewer eggs, resulting in smaller butterfly populations the next spring.
Boggs has been studying the mormon fritillary for 34 years at the Rocky Mountain Biological Laboratory (where Jessica and Heidi do their field work). She and Inouye wanted to use this long term data set to test whether early warming had an effect on butterfly populations. The researchers combined flower surveys with butterfly population counts from 1980 to 2005, and used this data to build a mathematical model to see whether the number of flowers and the snow melt date in a given year could predict how many butterflies emerged in the spring. They found that the number of flowers per female was a good predictor of butterfly populations the next spring. However, flowers alone didn’t explain the population declines: early snow melt also decreased butterfly populations in the following year through caterpillar mortality. Together, these two factors explained 84% of the variation in butterfly populations. The study showed that climate can affect a species in two ways, indirectly through it’s food chain and directly through temperature stress.
Boggs and Inoye were able to detect this pattern because their data set spans three decades. Long term studies like this one are the only way to accurately detect the effects of climate change. Becasue there can be substantial variation in temperature from year to year, it may take decades to understand how slow shifts in climate affect wildlife populations. The Rocky Mountains are a particularly important location for climate change research because they are a warming hot spot. The american west is warming 70% faster than the global average, and temperature data from Rocky Mountain Biological Laboratory shows that the average temperature has increases 0.72 degrees F per decade. While that sounds small to a human, it can mean big problems for butterflies and flowers.
If you want to follow the weather at the Rocky Mountain Biological Laboratory, you can find weekly updates here
To read the full paper:
Boggs, C., & Inouye, D. (2012). A single climate driver has direct and indirect effects on insect population dynamics Ecology Letters DOI: 10.1111/j.1461-0248.2012.01766.x