A Warmer Climate May Spell Doom For Western Butterflies

Apr 21, 2021
Originally published on April 15, 2021 5:29 pm

Wide open spaces, like much of Wyoming, are known to be strongholds for pollinators like butterflies. They often contain critical habitat and food resources, far away from the disturbance of human civilization. But it turns out even those areas are under threat.

"I think in the past we thought wildlands, we could just set them aside away from humans and they would be fine because our impact is direct, right?" said Katy Prudic, a researcher from the University of Arizona who studied the impacts global warming is having on butterfly numbers.

According to Prudic, butterflies and many other species are showing signs of being impacted indirectly by humans.

"When we think about the impacts of warming, or drought, or fire, or these things that are related to climate change, then we need to be thinking about managing wildlands with intent and with a mind towards that they'll be affected in strange ways that are a little unpredictable, that aren't about direct human impact," she said.

This indirect impact is not good.

"So what we found is that on average, there's an almost two percent annual decline of butterfly populations across the west," she said.

That's overall, but there is a bit of variation between species, with some doing better than others. The researchers were able to compare data on each species' characteristics, like wing size and what they ate as caterpillars, and determined that they weren't associated with declines. Prudic said it's clear that declines are happening in areas with fall warming and drought.

"This was a surprise for us, because most of the literature has really focused on spring changes in terms of timing," said Prudic. "Spring, it's been the focus of the work because that's when everything emerges, and then things have to be ready, the plants have to be available for the butterfly to lay eggs on and the caterpillars to eat and those sorts of things."

This is important because fall is when butterflies are building fat reserves for winter hibernation.

"We think the plants are senescing, or dying back sooner because they're under drought conditions. And we also think their quality is decreased by the droughts, the ones that aren't senescing, so that they're not providing great nutritious food for the caterpillars and the adult butterflies as they prepare for hibernation," Prudic said.

According to Prudic, this pattern is applicable across the west, not just in the area she and her team studied. But University of Wyoming researchers say there isn't enough historical data on the state's butterfly population to determine that for sure.

"We don't have a lot of past data, most of the information on butterflies is in the mountains because that's where people want to be when they collect butterflies, and so we don't have a lot of information for the rest of Wyoming," said Lusha Tronstad, an invertebrate zoologist with the Wyoming Natural Diversity Database housed at UW.

Tronstad and her graduate student Madi Crawford have been trying to establish baseline data for the state's butterflies for the past few years. It's currently home to three butterfly species of concern, the Regal Fritillary, the Monarch, and the Great Basin Silver Spot.

"Wyoming's rangelands are really good for butterflies. Unlike some other states, we do have rangelands instead of crops," said Crawford, "So we have a bigger variety of flowers that these butterflies can choose from, so Wyoming may be a really good resource for these insects to use."

But no matter how much we have, it's all threatened by increasing temperatures.

"In general, if our summers continue to get warmer and drier, some plants are gonna have a hard time adjusting to that change quick enough to be able to produce a good enough amount of flowers for these insects to use," Crawford said.

Tronstad agrees and adds that if one struggles, they both do.

"Plants and pollinators are very closely tied, and one cannot survive without the other," she said.

So protecting butterflies means protecting the plants they're so tightly linked with too because changing their home ranges isn't really an option for these tiny creatures.

"Remember, it's an arduous journey. They're tiny animals, they're usually adults for only a couple of weeks. Many of them, given the opportunity, a lot of them do find new habitat," said Prudic. "It's hard to locate though in a really complex landscape where that habitat is, and they may be rare little pockets, and when you get there, everybody might be there, and so there's a lot of competition then for those plants."

She suggests giving butterflies similar protections as some birds.

"I think sort of riffing off the Migratory Bird Act would be a great place to start," suggested Prudic. "So we know that that's been really successful in increasing wetland bird population sizes, and so if we can start incorporating into those management plans plants and habitat associated with the butterflies that are locally there, that would probably go a long way."

Butterflies aren't just pretty. They perform many roles in the ecosystem as important pollinators and prey for other species.

"I often refer to them as eggs with legs when they're in their caterpillar stage because they are incredibly nutritious. Becoming a butterfly is a really energetically costly endeavor. Basically, a caterpillar has to melt down and rebuild everything again. It's almost like a second embryogenesis, and so they have a lot of nutrients that they're storing for that process," said Prudic. "A bird or a mammal or a tiny rodent or another insect [that] often eats butterflies, in ecosystem services talk we say 'they move carbon through the system,' which, in other words, they're good snacks for other things."

Individuals can help protect butterflies by lowering their climate impact. Other options are to join citizen science projects which help track these insects, plant native flower gardens, and learn more about your local native butterfly species.
