– Every year when summer starts to sizzle and the rest of South Florida heads inside, a dedicated crew of citizen scientists ventures into the buggy heat to do an increasingly difficult job: count butterflies.

For the last quarter of a century, the North American Butterfly Association has tallied the nation's population of butterflies three times a year, including the Fourth of July butterfly count now occurring in Florida through July. But across the nation, and particularly in Florida, finding butterflies to count is getting more difficult as habitats continue to vanish and climate change makes butterflies' brief lives ever more perilous.

"Every single day there are fewer butterflies in the United States than there were the day before. You don't have to be a genius to figure it out," said geneticist Jeffrey Glassberg, NABA's president and founder.

Nowhere is that more evident than among more specialized butterflies, like the many imperiled in South Florida, that live on an ecological razor's edge balanced between their dependency on particular plants and fragmented populations. Of the 160 to 180 species found in the state, about two dozen are in trouble, among the highest concentration of threatened butterflies in the U.S. Now effects from climate change — increased temperatures or sea rise that threaten host plants — may be driving them closer to the edge.

"Once upon a time I wasn't happy unless we [counted] 50 species," said wildlife biologist and butterfly expert Mark Salvato, who for the past 15 years has organized counts. "Last year, I had 32."

The count is modeled on the National Audubon Society's popular Christmas Day Bird Count and started in the 1970s by the California-based Xerces Society, which works to protect pollinators. NABA took over in 1993, a year after the organization was formed.

Because of its warm weather, South Florida was always considered a mecca for rare butterflies. When an amethyst hairstreak, a rare tree-inhabiting bluish purple butterfly, turned up in Castellow Hammock Park near the Redland in 2004, it drew butterfly enthusiasts for days.

"You expect that with rare birds, but that was one of the first butterflies," Salvato said.

The protection of butterflies has not always ended well, in part because what drives them to extinction can sometimes be complicated. The Miami Blue once inhabited much of the South Florida coast but can now only be consistently found in the Marquesas, islands west of Key West.

In 2014, the U.S. Fish and Wildlife Service added two butterflies, the Bartram's hairstreak and Florida leafwing, to the endangered species list. But critics have said the designation does little in reality to protect the butterflies and their habitat on private land.

"The Schaus' swallowtail has been listed for a long time and it's no better off," Glassberg said. "People have this misguided but understandable view that if someone takes a meadow and puts in a parking lot, that the butterflies just move somewhere else. But that's not true. The butterflies are removed from the planet. You've just decreased the population of the world's butterflies."

Salvato said both the number and diversity have declined. Because they react so quickly to changes, butterflies have been used to detect effects from climate change, which can occur faster than with mammals or birds.

Rising temperatures prompted the furry brown three-spotted skipper, which looks more like a moth and feeds on grasses, to head north to Gainesville. A decade ago it was not seen outside Miami-Dade County, Salvato said. The Fulvous hairstreak, another brown butterfly that feeds on Brazilian pepper, is now showing up north of Melbourne.

Salvato said, "I'm hoping somebody can get to the biology of these butterflies and understand what caused them to blink out in their historic range."