The combined heat from climate change and urbanization is likely to reduce the number of eastern swallowtails and other native butterflies in Ohio and promote the spread of invasive relatives, a new study led by a Case Western Reserve University researcher shows.
Among 20 species monitored by the Ohio Lepidopterists society, eight showed significant delays in important early lifecycle events when the two factors were combined—a surprising response that may render the eight unfit for parts of the state where they now thrive.
Butterflies serve as important indicator species for how the broader ecosystem might be impacted, the researchers say. They are pursuing further studies to learn whether the negative impacts on multiple species add up to cause disruptions to the environment.
Other studies have shown that butterflies respond to higher temperatures due either to climate change or turning farmland and forests into asphalt and concrete by first appearing—and reaching peak numbers—earlier in the year.
"But when you combine the effects, it sort of throws a wrench in how you predict species' responses," said Sarah Diamond, an assistant professor of biology at Case Western Reserve and lead author of the study, now online in the journal Ecology.
The findings, the researchers say, may be useful to predict effects of environmental changes over the next decades and develop strategies to respond, and can likely be applied worldwide
Diamond worked with Heather Cayton, Tyson Wepprich, Clinton Jenkins, Rob Dunn and Nick Haddad of the department of biological sciences at North Carolina State University; and Leslie Ries, of the biology department at the University of Maryland.
Researchers analyzed 13 years of butterfly monitoring records by lepidopterists society volunteers at 83 locations statewide. The monitoring sites include locations in parks and preserves throughout Ohio.
Nearly 230 Society monitors recorded species numbers from the first week of April through the first week of November. Their data was compared with temperature records.
Temperatures were taken from the nearest National Oceanic and Atmospheric Administration meteorological stations, which avoid the warming effects of urbanization.
Sites in southern Ohio ranged 2 to 3 degrees Celsius warmer than northern sites on a given day. That difference reflects the amount of warming climate change models predict for the Midwest later this century, enabling the researchers to see how the change may alter butterfly lifecycles.
The lepidopterists' records showed that butterflies generally emerged up to three weeks earlier and reached peak numbers sooner in southern Ohio than mid- and northern Ohio.
Biologists believe that advancing their lifecycle events a couple of weeks enables butterflies to take advantage of earlier flowering induced by warmer temperatures. The bigger payoff is a species may add another generation in a year, increasing its population and ability to compete.
Urban heat tips pattern
To estimate the heat from urbanization, researchers calculated the percent of impervious surface within 1 kilometer of each monitoring site, using the National Land Cover Database. Research by others shows the percentage of impervious surface is correlated with specific increases in surface and air temperatures.
When this heat was added to the already warmer temperatures in greater Cincinnati and Dayton, seven species delayed their initial appearances. Three of them, plus one other species, delayed their peak numbers.
"Butterflies need warmth from the environment to develop," Ries said. "As their environment gets warmer, they have more and more energy but at extremes, it's too hot and they die.
"Before it becomes lethal, too much heat can slow growth," she said. "That's why we see the delay."
The delays may leave a species with fewer resources to feed and lay eggs or may expose them to great risk of predation, resulting in a smaller next generation, the scientists said. Such a scenario may lead to loss of local populations.
The eastern tiger swallowtail, pearl crescent and red admiral, all noted for their beauty, are among the eight species that delay and are more likely to suffer in the future in Ohio.
Invasives, such as the cabbage white and European skipper, are more likely to thrive. These species are called "weedy," meaning they can feed and lay eggs on a wide variety of plants. They were largely unaffected by the combined heat.
The researchers studied Ohio butterflies after Ries found the state has the most intensive monitoring records in the United States. While many other states are doing monitoring, most volunteers only go out about 7 times a year, Ohio volunteers go out 15 to 30 times per year. "That really helps us track shifts in timing, something that is more difficult to do with less intensive protocols" Ries said.
"If our data can be used for some enlightenment, we're happy to have it used," said Jerry Weidman, who chairs the society committee in charge of monitoring.
Volunteers generate the records. They choose a section of trail and record the species and numbers they see within 7.5 feet of either side, Weidman explained. They monitor weekly from the beginning to end of the main butterfly flight season
"One lab could never do something of this scope," Diamond said. "Citizen scientists can help us do things on a scale never thought possible."
The researchers believe their findings are applicable nationally and globally. Patterns of temperatures varying by geography, and over time under climate change, coupled with gradients of rural to urban habitats, are widespread.
Diamond and colleagues are now setting up growth chambers to study butterfly physiology in the lab. They will simulate environmental conditions, including climate change and urbanization to see what temperatures or other factors trigger changes in growth and behavior, and when. From that, they hope to develop predictions to help conserve each species.
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