Southwestern deserts are a surprising hotspot for butterfly evolutionary diversity

March 23, 2021

Butterflies are doing well in the desert, surprisingly. (Unsplash/Robert Murray)

The deserts of North America are an unexpected hotspot for butterfly biodiversity, new research indicates, underscoring the importance of conserving these regions.

For the study, published March 23 in iScience, scientists compared how closely butterflies in habitats across the continent are related to their neighbors, and found that these patterns didn't always match those of flowering plants. While deserts are relatively low in plant and butterfly species, the desert butterflies were more distantly related to each other than the researchers expected.

"Due to the unique amount of evolutionary history being harbored in these areas, I think it calls for greater study to really understand how climate change could affect these areas," said Michael Belitz, a doctoral student at the Florida Museum of Natural History at the University of Florida and second author of the paper. He noted that deserts may face rising temperatures and droughts in the future.

Compared with knowledge of vertebrates and flowering plants, relatively little is understood about insect biodiversity, Belitz and his colleagues wrote in the study. However, many kinds of insects around the world are declining because of climate change, habitat loss and other threats. 

Monarchs are of particular concern because of a variety of threats including climate change and reductions in the milkweed plants they rely on. U.S. lawmakers introduced a bill March 17 to help fund monarch conservation projects.

To preserve any insect, it's important to understand where and why different populations thrive. But butterflies are an ideal group to explore these questions because the vibrantly colored insects are comparatively easy for scientists and collectors to find and study. Butterflies are also important pollinators and can be indicators for how their ecosystem is responding to climate change and other stressors. 

"Their development is tied to the climate, so changes in the climate can be seen through changes in butterflies, as well," Belitz said.

Most biodiversity studies concentrate on tallying the different species in a given area, says Chandra Earl, a postdoctoral researcher at the Bernice Pauahi Bishop Museum and first author of the paper. She and her team instead assessed the butterflies using a measure called phylodiversity, which takes into account the evolutionary relationships between different species.

"If you have two groups of species in an area that are more distantly related, you have a higher phylodiversity, and if you have the same number of species in an area but all the species are more closely related, then you have a low phylodiversity," Earl said.

She and her team consulted field guides to determine the geographic ranges of butterfly species found across Canada, Mexico and the United States. The researchers then gathered genetic data that was already available for around 1,300 species and took DNA samples from an additional 140 species in museum collections. This allowed the team to determine how closely related the 1,437 species, or about three-quarters of the continent's known butterflies, were to each other.

Overall, the total number of species and phylodiversity were both highest in the warm, wet areas, including the tropical forests of Mexico, and lowest across the Canadian arctic. 

Intriguingly, the American Southwest and Baja California had a higher amount of phylodiversity than would be expected based on the number of species in these regions.

"There aren't necessarily a lot of species in warm deserts, so these areas aren't going to pop up when you're just counting the number of species," Earl said. "But when you take into account the ... evolutionary distance between them, it does pop up."

The researchers were also interested in how closely butterfly diversity was tied to plant diversity, because butterflies and caterpillars depend upon flowering plants for food. To their surprise, the butterflies showed a "strikingly different" pattern of phylodiversity compared with the plants, particularly in the deserts.  

The researchers aren't sure yet why there are so many different butterfly lineages in the deserts. The insects may be able to thrive despite the lower plant diversity because butterflies can explore and feed from many different plant species, Belitz says. 

The biodiversity hotspots that are often targeted for conservation tend to be defined based on plant species, Earl says. 

"Plant biodiversity hotspots are important, but there are a lot of other diversity hotspots out there ... The requirements for biodiversity between flora and fauna are so vastly different, you can't just focus on one thing," she said. "If you lose all the diversity in these desert areas, then you lose a good amount of diversity in butterflies."

The study, "Spatial phylogenetics of butterflies in relation to environmental drivers and angiosperm diversity across North America," published on March 23 in iScience, was authored by Chandra Earl, Bernice Pauahi Bishop Museum; Michael W. Belitz, Vijay Barve, Narayani Barve, Douglas E. Soltis, Pamela S. Soltis, Akito Y. Kawahara and Robert Guralnick, University of Florida; Shawn W. Laffan, University of New South Wales; Julie M. Allen, University of Nevada, Reno; and Brent D. Mishler, University of California, Berkeley.

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