Butterflies shed light on how some species respond to global warming
With global warming and climate change making headlines nearly every day, it could be reassuring to know that some creatures might cope by gradually moving to new areas as their current ones become less hospitable. Nevertheless, natural relocation of species is not something that can be taken for granted, according to Jessica Hellmann, Associate Professor at the University of Notre Dame Department of Biological Science in Notre Dame, Ind. By studying two species of butterfly, she and her team have found evidence suggesting that a number of genetic variables affect whether and how well a species will relocate.
Dr. Hellmann and her team have conducted a series of studies in which manipulating the temperature of the butterfly larvae's environment revealed how the two species might respond to global warming. She will discuss the team's work at the 2010 American Physiological Society's Intersociety Meeting in Westminster, Colo., August 4-7.
Duskywing and Swallowtail Butterflies: Coping with Change
The Notre Dame team studied the larvae—or caterpillar phase—of two butterfly species, the Propertius duskywing butterfly (Erynnis propertius) and the Anise swallowtail butterfly (Papilio zelicaon). These butterflies, both cold-blooded insects, were chosen because of their ecological differences but they live in the same ecosystem, allowing Dr. Hellmann to compare their responses in a single study.
The duskywing is a small butterfly that does not easily fly great distances and stays close to the West Coast of the United States. Because it does not fly great distances, the genetic makeup of the group does not spread very far. The species is also characterized by the fact that its larvae consume only the new leaves of oak trees, making it highly specialized. The Anise swallowtail, on the other hand, is a much larger butterfly, and can fly greater distances with greater ease. Its genes are more likely to be spread out over a larger range as its flies between the Rocky Mountains and westward to and around California. The swallowtail larvae eat an assortment of plants, which also helps to spread genes across its range.
The researchers performed a number of experiments between butterfly larvae from the northernmost ranges of their habitat (Vancouver Island, Canada; "northern larvae") and butterflies from the central part of their habitat (California and southwest Oregon; "central larvae"). They exposed each group of larvae to conditions simulating the other group's summer and winter climates and fed each group food grown in the other group's location, all with a special focus on how the northern larvae responded. According to Dr. Hellmann, understanding how populations at the edge of a species' range respond to warming will provide insight on whether the species will shift with climate change.
The team theorized that northern members of a species whose genes are more spread out, like the swallowtail's, might be pre-adapted to rising temperatures and could perhaps even thrive as the northern climate gets warmer. Conversely, species like the duskytail, whose genes are not as spread out, could be locally adapted to climatic conditions at the edge of the range and northern populations might reduce under climate change.
Either way, it boils down to whether the species in Vancouver would respond positively to their climate becoming more like California's. So far, the answer for both species is "no," for different reasons in each species.
"In summer conditions, the duskywing larvae grew bigger, faster, and they survived better, which suggested that they liked it warmer, but winter was another story," said Dr. Hellmann. "In the warmer winter, they increased metabolism and burned through energy faster. This suggests that they were adapted to the cooler winters of Vancouver."
As for northern swallowtails in central conditions, "They just didn't care," Dr. Hellmann said. "They didn't respond to warming at all. They didn't do better or worse. This means that assumptions about warming possibly benefiting species [with more spread out genes], particularly at the northern edge of the range, are not appropriate."
The Genetic Connection
The team has begun studying the genetic explanation for how the two species respond to warming. They are investigating what genes are responsible for the individualized responses, and will use genomic tools to learn which genes are involved when the species is experiencing climate change, said Dr. Hellmann. "We will also try to determine which genes these butterflies are synthesizing when they experience climate warming. We want to know if northern and southern members of the same species are expressing their genome differently or the same."
The answers may explain the differences between various populations of the same species—northern vs. central—and why some species might not be inclined to relocate as the climate heats up.
"Expecting creatures to pick up and move north makes sense theoretically," Dr. Hellmann said. "But the reality is that genetic and physiological interactions are so complicated, it's hard to imagine how it will play out for all species everywhere."