If they can’t stand the heat… will butterflies simply head north?

Caterpillar study reveals likely impact of global warming

August 2010: 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.

ADAPTABLE: The swallowtail caterpillar behaved in exactly the same way regardless of temperature - 'they just didn't care' says chief researcher Dr Hellman

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.

The Notre Dame team studied caterpillar phase of two butterfly species, the Propertius duskywing butterfly and the Anise swallowtail butterfly, which live in the same ecosystem making it possible fo the team to compare their responses.

How would larvae respond to a change in climate?
The duskywing is a small butterfly that does not easily fly great distances and stays close to the West Coast of the United States. Its comparative lack of mobility means the genetic makeup of the group does not spread very far, and the species has also become characterised by a very specialised diet. Its larvae consume only the new leaves of oak trees. In contrast, the larger Anise swallowtail flies large distances with ease and 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 experimented with two groups of butterfly larvae, the first from the northernmost part of their range (Vancouver Island) and the second from the central part (California). Each group of larvae was exposed to conditions simulating the other group's climates.

The team theorised that northern members of a species whose genes are more spread out, such as 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, the real question was whether the larvae in Vancouver would respond positively to their climate becoming more like California's. So far, the answer for both species is no - although for different reasons.

‘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.'

Assumptions about possible benefits are not appropriate
As for northern swallowtails in central conditions, ‘they just didn't care', reported Dr Hellman. ‘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 team is now going on to find the genetic explanation behind how the two species respond to warming. They are investigating which genes are responsible for the individualised responses, and will use genomic tools to learn which genes are involved when the species is experiencing climate change.

‘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,' said Dr Hellmann.

The answers may explain the differences between various populations of the same species 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,' she adds. ‘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.'