Sometimes, nature's surprises come with wings. In a new study , scientists pulled from a 35-year dataset to examine long-term population trends of the federally endangered Schaus' swallowtail butterfly (Heraclides ponceana). They found that the swallowtail's population size was positively influenced by something unexpected — hurricanes.
"This study is among the longest-running for a tropical butterfly, and it has been a privilege to get to work with such an amazing dataset," said Sarah Steele Cabrera, a doctoral candidate at the University of Florida and lead author of the study.
Schaus' swallowtail is endemic to south Florida and one of the rarest butterflies in the United States. It was among the first insects placed on the U.S. endangered species list, and since the 1980s, researchers at the Florida Museum of Natural History have been keeping tabs on those living in the Florida Keys.
In 1985, Thomas Emmel , founding director of the McGuire Center, former curator and posthumous co-author of the study, established a Schaus' swallowtail monitoring program on Elliott Key, a remote 7.5-mile-long island within Biscayne National Park. Using a strip of land originally cleared for highway construction that spans the length of the island, field scientists surveyed butterfly populations every spring, bearing the relentless heat, humidity and mosquitoes. Their dedication yielded worthwhile results.
Long-term data is necessary to fully understand the lives of insects
The ebb and flow of population trends are nearly impossible to capture in a snapshot, which is why short-term studies don't always tell a complete story. Long-term data is important to help determine the best actions needed to conserve a species.
Many invertebrates are understudied and lack long-term datasets, making it difficult to understand population trends and conservation needs. Tropical butterflies, in particular, remain poorly understood.
"You might have a drought in one year and abundant rain in the other year, and that can cause dramatic shifts in both occupancy and abundance, especially in insects, which produce a lot of offspring and respond relatively quickly. So, the benefit of having a long-term study is you can help minimize that noise in the system," said Jaret Daniels, senior author of the study and curator at the museum's McGuire Center for Lepidoptera and Biodiversity.
Insects have relatively complex life histories, further complicating the situation. Each stage of an insect's life can fulfill a different ecological role. They live in different places, eat different things and are eaten by different things. What benefits the insect in its larval stage may be harmful as an adult. With such complex needs, making conservation decisions can be difficult if you don't have all the information you need. That is why databases like iDigBio and GBIF, which provide access to historical data, are so important.
These butterflies haven't just adapted to hurricanes, they also benefit from them
Steele Cabrera was looking over the Schaus' swallowtail dataset to see how the population had fared over the last few decades. Upon first inspection, there wasn't any noticeable trend she could pick out. The number of Schaus' swallowtails on Elliott Key appeared sporadic, jumping up and down with seemingly no rhyme or reason. Alongside co-lead author Michael Belitz, she began looking at weather variables to see if anything might line up. After looking at maximum wind speeds, the pattern finally become clear. "The trend of increased Schaus' swallowtail populations after strong hurricanes was immediately apparent. We were really excited since the data backed up what we had seen in the field," Steele Cabrera said.
When a hurricane blows through the Florida Keys, strong winds strip trees of their leaves, and storm surge floods the land with seawater. "The immediate impacts are detrimental for the butterfly," said Steele Cabrera. Like clockwork, however, Schaus' swallowtails seem to bounce back better than before in the following years.
Akin to many other insects, Schaus' swallowtails are specialists, meaning they require specific plant species on which to lay their eggs and feed. Their survival relies on two known larval host plants: torchwood and wild lime (Amyris elemifera and Zanthoxylum fagara).
Both are fragrant and belong to the citrus family, ranging from large shrubs to small trees. They occupy the forest's understory, and after a hurricane, they explode with fresh leaf shoots, which are the preferred food of Schaus' swallowtail caterpillars. The caterpillars can technically feed on older leaves too, but fresh foliage is much more tender and easier to digest. Mature leaves can be tough and even inedible for smaller caterpillars.
"Schaus' swallowtail and its host plants live in dense forests where not a lot of light reaches the ground," said Steele Cabrera. After the hurricanes temporarily knock out their bigger, taller competitors, the host plants flourish. Large trees are toppled or stripped of their leaves and shorn of branches. When additional sunlight hits the forest floor, it triggers the rapid growth of new leaves, replacing foliage lost in the storm. All that new growth, in turn, means more butterflies.
Habitat loss, climate change and sea level rise threaten endemic species
Historically, Schaus' swallowtails were much more abundant and were distributed throughout the Keys and parts of the Florida mainland. But habitat loss and urban development have severely impacted the butterflies, forcing them out to small areas of protected land and causing their populations to dwindle.
"The development of greater Miami and Homestead took out potentially viable tropical hardwood hammock habitat. There are some small pockets of it left, but nothing contiguous enough to withstand or support a population of this butterfly," Daniels said. Had it not been acquired by the National Park Service, Elliott Key would have likely seen the same fate.
The butterflies are now thriving on protected land, but a new, far more daunting challenge lies ahead—one that threatens both humans and butterflies. While they benefit from some periodic disturbance, larger storms might be too much of a good thing.
"This is a challenging organism when it comes to climate change," Daniels said. "With storm intensity potentially increasing, it's very worrisome that one major storm, another Hurricane Andrew, could really devastate this butterfly."
Other species in the Keys have already succumbed to increasingly powerful storms and encroaching seas. The Key Largo tree cactus recently claimed the grim title of the first species in the United States to become locally extinct due to sea level rise.
As a precautionary measure, researchers have begun reintroducing Schaus' swallowtail to unoccupied conservation lands, with the aim of establishing self-sustaining populations as part of a five-year draft recovery plan. The more spread out the population is, the less likely it is for the species to be wiped out by a single storm.
The study was published in the journal Biological Conservation .
