The yucca and the moth: How extreme weather impacts the timing of biological events

Whether it is flowers sprouting in the spring, cicadas mating in the blistering heat of summer, or caterpillars hatching to feed on their favorite host plant – across the natural world, the timing of biological events is so important it spawned its own scientific discipline: phenology. 

Much of phenology is rooted in the seasons, but in a changing world, researchers are increasingly discovering disruptions to the intricate balance underlying the timing of organisms and their ecological interactions. 

In a new study published in Nature Climate Change, a team led by University of Arizona ecologist Daijiang Li took a closer look at how extreme weather events impact phenological processes, specifically the flight period of butterflies and moths and the flowering time of plants across the United States. 

Despite dramatically affecting biological systems, extreme events such as heat waves, cold snaps, heavy rainfall and droughts are often overlooked in studies, as researchers focus more on impacts of gradual shifts in average climate conditions. As a result, there is a critical knowledge gap when it comes to the ecological impacts of extreme weather events on organisms, populations and ecosystems under current and future climate change.

 "Most studies have focused on climatic variables or seasonalities, such as annual temperatures or precipitation averages," said Li, who is an assistant professor in the U of A Department of Ecology and Evolutionary Biology. "But as individuals, we respond to and make our decisions in response to immediate environmental stress, rather than a 10-year trend, right?"

As scientists strive to improve forecasts of how plant and animal phenology responds to global change, it is important to include extreme weather events, Li and his co-authors say. Li's team found that while average temperature has an important role in driving the phenology for plants and insects, the effects of extreme weather are as important as other commonly studied variables.

"Our results provide overwhelming evidence that extreme weather events and the climate context in which they occur are crucial to understanding the phenological response in plants and insects," Li said.

The research also showed that in addition to affecting plants and insects directly, extreme weather events also interact with climatic variables, Li said.

"For example, extremely hot and extremely dry periods can have a compounding effect on the phenology," he said. "These variables don't occur in isolation. Rather, they interact with each other and interact with climatic variables in driving the phenology of plants and insects as a whole."

The researchers looked at data collected by citizen scientists through iNaturalist, an app popular with nature enthusiasts that allows users to identify plant and animal species in the field, take photos and upload them to a database. They combined the iNaturalist data with daily weather data recorded from 1980 to 2022. 

"We looked at about 581 species of angiosperms (flowering plants) and the flying adults of 172 Lepidoptera species (butterflies and moths) from 2016 to 2022," Li said. "We also looked at the spatial distribution patterns, how extreme weather events affect the phenology of those plants and insects, and also whether the plants and insects have similar or different responses to extreme weather events."

Of particular concern are disruptions to the delicate relationships between certain species that depend on each other for survival, such as the yucca plant and yucca moth, Li said. The small, whitish moth blends well with the white flowers of the yucca plant, where it spends most of its adult life. The moth pollinates the flowers, and in return, the yucca provides the moth larvae with food.  

"If larval stages of insects are too responsive to early spring heat waves, they may be more susceptible to mortality when temperatures return to normal or during cold snaps," Li said. "Similarly, if spring is warmer than usual, plants are going to flower earlier, making them more vulnerable to late frost during growing season."

Finely tuned dependencies among species are susceptible to disruption, whether caused by long-term shifts in climate or short-term extreme weather events. If the emergence of pollinators, such as butterflies or moths, is out of sync with plant flowering, the pollinators won't be able to find food, and flowers will go unpollinated. 

While plants and insects often show similar phenological responses to extreme weather, responses can also be quite different, the researchers found. For example, extreme cold spells in the spring prompted adult butterflies and moths to unexpectedly emerge and take to the skies earlier than normal, while the beginning of flowering time in plants was largely unaffected. 

"Those kinds of phenological mismatches are going to have consequences for both the plants as well as for the insects, causing both populations to suffer," Li said. 

Li and his co-authors – Michael Belitz, Lindsay Campbell and Robert Guralnick at the University of Florida in Gainesville – hope that by including extreme weather events in climate models, researchers will be better able to predict mismatches and their consequences to the ecosystem, and develop effective mitigation and resilience plans in the face of climate change.

"This work has the potential to be relevant across multiple sectors, from conservation to food security," said Campbell, an assistant professor at the Florida Medical Entomology Laboratory. 

"We are still just beginning to more fully understand how extreme weather impacts insects and plants, which ultimately impacts us and our food security," said Guralnick, curator of biodiversity informatics at the Florida Museum of Natural History. "This work is a step towards that goal."