Two ongoing TrEnCh projects in the mountains of Colorado are repeating historical field observations and field and lab experiments as well as measuring historical specimens to examine ecological and evolutionary responses to recent climate change.
Museum specimens of Colias meadii that tell the story of responses to decades of climate change.
In the late 1950s, Charles Remington, a Yale biology professor, journeyed to Colorado to explore the ecology of high-elevation insects. As curator of Entomology at the Peabody Museum, Remington was immediately taken by Colias butterflies, a small, yellow, mountain specialist with unique strategies to combat the cold - primarily, dark pigmentation on the undersides of their wings that helps them absorb solar radiation when they need to warm up.
Remington conducted pioneering studies of the butterfly’s ecology, evolutionary history, and genetics based at the Rocky Mountain Biological Lab (RMBL) in Gothic, Colorado while amassing a massive specimen collection. Remington inspired extensive subsequent Colias research. We trace our resurvey project through Remington’s student Ward Watt, whose mechanistic research focused on metabolic enzymes and pigmentation, and Watt’s student Joel Kingsolver, who focused on responses to complex and variable natural environments. The experiments and specimens have become an important legacy, offering insight into physiological and genetic changes in the butterflies following decades of climate warming.
In collaboration with the Kingsolver Lab at UNC, we repeated historic field and laboratory experiments, and examined Remington’s and other museum specimens, to understand the ecological and evolutionary responses of Colias butterflies to climate change. These types of “before-after” or “resurvey” studies take advantage of historical data to monitor actual biological shifts in response to observed climate change.
Butterflies are highly sensitive to environmental temperatures due to their small size and reliance on external sources of heat (ectothermy). Butterfly body temperatures need to reach a relatively high and constrained range to fly, as flight is essential to surviving and reproducing via activities like finding mates, foraging for food, and laying eggs.
The Colias butterflies we work with live in the mountains of Colorado, where air temperatures are often below those required for flight. To warm up, the butterflies close their wings and orient them towards the sun to absorb solar radiation. On the underside of their wings they have dark pigments that allow them to absorb more radiation to increase their body temperature. Our research focuses on how many dark pigments a butterfly has on the underside, its “wing absorptivity.”
Dark wings are not always an advantage. High elevations can be extremely sunny, and it’s possible for butterflies to overheat. Overheating decreases the number of high quality eggs a female can lay.
Our research has allowed us to develop models that can predict how butterflies will respond to current and future warming, based on their initial wing coloration and its evolution. We can estimate important traits, like flight time, flight duration, and egg survival, based on wing patterns and climate conditions.
We wanted to know what happens to wing absorptivity as mountain ecosystems get hotter due to climate change.
Do darker wings become a disadvantage?
Will Colias butterflies change their pigmentation in response to warmer temperatures or will populations suffer?
Learn more about the framework and our models below:
We included a model to estimate the microclimate conditions that butterfly larvae, pupae, and adults experience at each site - now and in the future, given climate warming scenarios.
We raised butterflies in the lab to measure the developmental rates of larvae and pupae. We then built this model, which can estimate development rates and when adults will emerge given thermal conditions. Developmental temperatures alter wing absorptivity via plasticity (quantified by lab rearing).
Butterfly body temperatures are estimated by balancing heat losses to and heat gains from the environment. The largest source of heat in most cases in the absorption of solar radiation, which depends on wing solar absorptivity. Body temperatures are closely tied to how long butterflies can fly and egg viability, based on field and lab experiments.
We estimate population growth rates as the product of fecundity and survival. Colias butterflies lay eggs singly on hostplants, which results in the time they have available to fly being a strong predictor of the number of eggs they are able to lay. We first estimate flight activity time as how long a butterfly is able to achieve temperatures suitable for flight. We translate flight time into daily rates of egg production by multiplying by rates of egg laying (oviposition) and the proportion of time spent foraging observed in the field.
Estimates of fitness (population growth rates) of Colias as a function of wing solar absorptivity allow us to estimate natural selection gradients on both the mean and plasticity of wing absorptivity. We incorporate the selection gradients and heritability in simple quantitative genetic models to predict the evolutionary changes in the next generation.
Modeling details and code are available via our latest papers: Kingsolver and Buckley 2017, 2018; Buckley and Kingsolver 2019.
Predictions for the evolution of Colias reaction norms for wing absorptivity with & without plasticity. The Colias models suggest that plasticity in wing absorptivity can facilitate evolution, particularly at lower elevations with long seasons, by reducing temporal variation in the strength and direction of evolutionary selection. They also suggest selection for greater plasticity (steeper reaction norm slopes), particularly at high elevtions.
Projections for Colias wing absorptivity in 2040 from our models including evolution and plasticity. We project that many areas will evolve lighter wings (red) in response to thermal stress that the scenario without evolution and plasticity. However, other areas are predicted to evolve relatively darker wings (blue) due to temporal changes in selection and evolutionary lags.
We found selection for darker wings at high elevations across the butterflies’ distribution to capitalize on warming by increasing flight time. We also found, however, that seasonal and annual variation in climate causes the strength and direction of selection to fluctuate.
Our models show that plasticity facilitates evolution, particularly at lower elevations with long seasons, by reducing temporal variation in the strength and direction of evolutionary selection. Phenological shifts (e.g., timing of maturation) caused by environmental effects on developmental rate can also reduce variation in selection.
We used lab and field experiments, as well as museum specimens, to test our models and measure their predictive power. Ultimately, our experiments confirmed some of our predictions but also highlighted how the interactions of multiple responses (e.g., plasticity and evolution) complicate the shifts we see in wing absorptivity.
