The red rocks of the Papago Buttes rise around the Desert Botanical Garden, forming a barrier that hides the urban core of downtown Phoenix. Walking trails are lined with San Pedro cactuses and clusters of low-lying prickly pear, and amid the tranquil setting lives a robust and diverse population of butterflies found nowhere else in the city.
Arizona is home to more than 65 species of butterfly, and while populations have declined over the last four decades — victims of climate change, drought and urbanization — the botanical garden in central Phoenix stands out as an important refuge for the insect pollinators.
A new study from the University of Arizona found that urban green spaces can preserve surprising levels of biodiversity for butterflies and specifically identified botanical gardens as biodiversity hotspots. The research may give scientists a clearer roadmap to expanding biodiversity across urban areas.
“We looked at the number of butterfly species at botanical gardens and asked, ‘is that fairly represented of all the butterfly species found in urban areas?’” said lead study author Kathleen Prudic, an entomologist in the UA’s School of Natural Resources and the Environment in the College of Agriculture and Life Sciences. “When you think of species distributed across space, not everybody is everywhere all at once, but it turns out at botanical gardens that might be the case.”
Researchers found that the species richness of butterflies is significantly higher in botanical gardens when compared to areas of the same size across Southwest cities.
Comparing gardens with urban neighborhoods
The study focused on five cities: Tucson, Phoenix, Albuquerque, El Paso and Palm Desert, California. Each city averages less than 11 inches of precipitation annually and, with the exception of Palm Desert, each has a population over 500,000 residents.
UA researchers partnered with scientists at the Desert Botanical Garden and used publicly available datasets through INaturalist, a community science program, as well as an Ebutterfly data source.
The process involved identifying and correlating areas similar in size to each city’s corresponding botanical garden. Using city maps, researchers randomly drew rectangles around areas that matched the garden’s footprint. They used observations that occurred within that rectangle to calculate a single value of butterfly richness and diversity.
The researchers repeated this rectangle drawing and calculation to quantify the richness and diversity of butterfly species for 1,000 “typical” regions of each metropolitan area. Botanical gardens that demonstrated observed species richness diversity higher than 75% of those samples were classified by the team as “butterfly hotspots.”
While botanical gardens make up less than 1% of metropolitan landscapes, these urban green spaces have disproportionally high butterfly species richness and diversity compared to the much larger surrounding city area, researchers found. Species richness in these gardens scored in the 86th percentile or above.
“Many butterflies have seen record population declines in recent years, and these pockets of urban green space can serve as important refuge for pollinators navigating city landscapes,” said Prudic.
Drought, rising temperatures threaten butterflies
Western butterfly populations are declining at an estimated rate of 1.6% percent per year, according to a report Prudic co-authored in 2021. The report included more than 450 butterfly species, including the western monarch, whose latest population count revealed a 99.9% decline since the 1980s.
This decline in population is linked to habitat fragmentation caused by development and climate change. The study suggested one of the biggest factors has been abnormally warmer fall temperatures.
In Arizona, researchers say, fall temperatures have increased by about one-fifth of a degree Fahrenheit per decade since 1895. This abnormally warm autumn weather is believed to alter migration patterns and hibernation for butterflies.
Prudic says the biodiversity study focused on the Southwest because of its arid climate.
“We thought there would be more likelihood of the impact of botanical gardens because there’s not many places in the city with reliable water,” she said.
Some species of butterflies, like the western monarch, are well adapted to finding new habitat when urbanization threatens their land. But Prudic says climate change is affecting how butterflies find that new habitat because there is less access to water, which affects the plants the insects rely on for food and shelter.
“There’s a lot of wild spaces still left, but those wild spaces aren’t becoming great habitat, they are drying out,” she said. “For an insect, in particular, hot and dry is really bad because it’s difficult for them to thermal regulate.”
People can create butterfly habitats at home
As climate change is predicted to cause the Southwest to warm and dry, Prudic believes humans will rely on a greater share of watershed that would further hinder the species habitat. She says creating more green spaces to share with urban wildlife could give insect pollinators, like butterflies, a better chance of survival.
In dense urban corridors like downtown Phoenix, habitat and access to water is nearly obsolete, as is the likelihood a diverse population of butterflies. But in more suburban areas of the city, the potential for more butterflies may hang in the balance of gardening techniques and land use.
The Desert Botanical Garden works to educate the public on what plants can be used in their own gardens and yards for optimal water consumption. By saving water and increasing habitat, species adaptation would likely improve as sprawl continues further from the city center.
“It’s a way to get people thinking of their urban ecosystems and supporting native plants and insects and thinking about how things work in their own space,” said Natalie Melkonoff, who coauthored the study and is the plant and insect ecology program manager at the garden. “So anything we can do here to help grow native plants and expose more people to them can have an impact.”
In recent years the Desert Botanical Garden has made access to native plants more readily available for the community and works to educate the public on native vegetation and water use. Melkonoff calls the botanical gardens a powerhouse of knowledge for plants and gardening.
“People come here all the time who just moved here and want to know what to do in their yards and see what they can do with native vegetation and low water use options,” she said. “A lot of people come here from places that look very different and if we can show them an alternative that’s good for our climate, and also good for supporting wildlife, native plant life and biodiversity in urban area then it’s a win-win.”
Ornamental grass requires four times as much water as drought tolerant foliage like cactus and succulents, according to the Southern Nevada Water authority. In 2021, Nevada passed a law banning ornamental grass, a step water officials believe will save 73 gallons of water per year for every square foot of grass removed.
The removal of invasive plant species and re-introduction of desert-dwelling ones has the potential for increased biodiversity across the city and Southwest. Melkonoff says it’s an opportunity for species richness to thrive in areas where they have been forced out of due to urbanization and a lack of vegetation and water.
“We are a hotspot, but we can also create a whole bunch of hotspots around an urban area,” Melkonoff said. “And there’s no reason the whole urban area shouldn’t be a hotspot.”
Jake Frederico covers environment issues for The Arizona Republic and azcentral. Send tips or questions to email@example.com.
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