Resetting the Internal Clock: Adaptable Butterflies’ Response to Climate Warming

Source Article: Ittonen, M., Hagelin, A., Wiklund, C. & Gotthard, K. (2022) Local adaptation to seasonal cues at the fronts of two parallel, climate-induced butterfly range expansions. Ecology Letters, 00, 1–12. Available from: https://doi.org/10.1111/ele.14085

Featured Image Caption: Wall brown butterflies are making the journey northwards from Europe in response to climate warming.Image Source: Wikipedia Creative Commons by Sharp Photography


Climate Change Can’t Change Daylengths
The lengths of days depend on latitude because of the tilt of the Earth on its orbit around the sun. This image shows how hours of daylight differ with latitude in the Northern Hemisphere on the Summer Solstice. Image Source: Brian Brettschneider

As the climate warms, habitats near the poles that were previously very cold are becoming increasingly hospitable. Many living things from plants to insects have been documented expanding the ranges where they occur toward the poles. But it remains uncertain whether this expansion might eventually be hindered by differences in daylength at higher latitudes.

Many organisms depend on sunlight cues to direct the timing of their life cycles, migratory movements, or dormant periods. If species follow the warming climate toward the poles, they may need to adapt quickly to their new local sunlight cues to survive. Are populations able to adapt quickly enough to local daylengths? Or will latitude limit poleward range shifts?

Here a wall brown larva has formed a pupa to metamorphose into a butterfly. If wall brown larvae do not pupate, they remain as larvae and survive the winter in diapause, a dormant state. Image Source: Freenatureimages.eu by Vlinderstichting-Nely Honig




Insects on a Tight Schedule

For insects, daylength commonly serves as a cue for entering diapause, a dormant state that helps insects to survive the winter. As insects move toward the poles, they will encounter longer summer days. If insects are still adapted to their original latitude after moving north, they may fail to enter diapause in time for winter because the days were still long in late summer.

To see if this was the case, researchers from Stockholm University turned their attention to wall brown butterflies (Lasiommata megera) in Sweden. This species is very sensitive to timing diapause correctly, which means that if it is expanding its range northward it may face consequences of different daylength cues. Conveniently, wall brown butterflies live near the coasts, so there happen to be two separate ranges on either side of Sweden, which gave scientists two examples to study independently to see if their findings were consistent.

Are Wall Browns Flocking North?

The researchers mapped locations in Scandinavia, Finland, and the Baltic States where the species was spotted since 1901 to see whether this species has been expanding north. The researchers used models to look for patterns in the locations of the 10 furthest north butterflies over time. They did this separately for the two wall brown ranges on the eastern and western coasts of Sweden. In a similar way, they also looked at whether the locations of the butterflies were shifting inland, which normally experiences cooler temperatures than the wall brown’s preferred seashore habitat.

The two ranges of wall brown butterflies studied live on opposite coasts of Sweden, and are separated by the inland. Image Source: PAT by Ian Macky

Their analysis revealed that as the climate warmed in the region, wall browns expanded their ranges north and inland. So, have they adapted to new daylengths at these new latitudes?

Putting the Internal Clock to the Test

To see if the butterflies were adapted to different daylengths depending on their local latitude, the researchers conducted a clever experiment. They raised caterpillars born from adults that came from butterfly populations in the south, northwest, and northeast along the Swedish coast. These caterpillars were raised in a variety of daylengths and temperatures. In response to these conditions, the caterpillars either entered diapause or formed pupae.

Finally, the researchers used models to see whether the end results of diapause or pupation could be explained by differences of source population, daylength, temperature, and other variables. They also estimated the daylength that would trigger diapause for the various populations from different latitudes.

Adaptable Butterflies Reset the Internal Clock

Caterpillars from populations further south entered diapause after shorter days than most caterpillars from northern populations, both eastern and western. This result indicated that as wall brown butterflies expanded their ranges north in response to climate warming, they rapidly adapted to local daylengths. Therefore, encountering different daylengths was not preventing, and likely will not prevent, the northern range expansion of this species.

This degree of local adaptation is slightly surprising for two reasons. First, the various populations are connected from north to south. Because the butterflies can likely still interbreed across these latitudes, one might expect that all the butterflies would remain similarly adapted to the same daylengths due to gene flow, the introduction of genes from one population to another by interbreeding. Second, as species expand their ranges, populations can initially have small numbers of surviving individuals. This can limit their ability to adapt to their local environment due to low genetic variation. If there are not a variety of traits among the individuals in the population, there may not be a well-adapted trait that can be selected for by natural selection.

The life stages of the wall brown butterfly: egg, larva, pupa, butterfly (not pictured). Image Source: Wikipedia Creative Commons by Richard South
Well Beyond the Wall Brown

It is likely that other organisms, particularly other insects, may be able to adapt quickly to their local daylengths as they spread toward the poles. Previous studies have even shown similar results with plants! As a result, we will likely see organisms expanding their ranges poleward quickly with the help of rapid local adaptation in coming decades.

This study will help scientists to make better predictions of range shifts of species, particularly due to climate change. If we can accurately predict where species are moving and how quickly, we can better protect endangered species, defend against diseases spread by insects, animals, or plants, and guard against pests or invasive species.

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Julia Bebout

Julia Bebout

I am a second year Master's student at the University of Calgary studying how competing species coexist. I graduated from Lehigh University in 2021 with degrees in Biological Sciences and Earth & Environmental Sciences. As an undergraduate, I studied paleoecology and the microbial ecology of alpine wetlands. I'm especially interested in community ecology, wetland and alpine ecosystems, and regenerative agriculture. I also love hiking, climbing, baking, and dancing! Twitter: @BeboutJulia

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