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
Charles J. Sharp, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
Climate Change Can’t Change Daylengths
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?
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.
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.
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.