Visiting Invasive Species Brewing in South Florida can give you a pretty good idea of the kind of wildlife you’re going to encounter in the larger Miami area. In fact, patrons may step over curly-tailed lizards on their way in for a beer. Inside the establishment, taxidermy and mounted skeletons—everything from green iguanas to Burmese pythons and tegus—line the walls. The brewery’s name and the creatures brandished on their T-shirts and swag offer the perfect glimpse into just how pervasive nonnative species have become in the area.
Now, a new study confirms just how alien the urban reptile and amphibian community of Miami-Dade County has become—even in a period of just five years.
“Whether you’re in more native habitat or human-modified habitat, almost all the herps you see in South Florida are nonnative,” said Christopher Searcy, an associate professor of biology at the University of Miami.
Searcy and his colleagues first set out to characterize the urban reptile and amphibian fauna in Miami in 2017, publishing the results of the study in Biological Invasions in 2019. In the years since that publication, he noticed the makeup of reptile species in Miami changing right under his eyes. To confirm what he was seeing unfold around him, he and his colleagues repeated the same process of surveying reptiles and amphibians in Miami-Dade County. They published their more recent results in Ecology and Evolution.
Just as in their previous study, the team surveyed the reptiles and amphibians in 15 human-modified parks in the county and in 15 parks that were more like native ecosystems.
The richness of nonnative species increased in ratio to the richness of native species. And the abundance of nonnative species substantially increased. The team found that 92% of the reptiles and amphibians they spotted in all parks were nonnative. The remaining 8% were mostly green anoles, which in South Florida are sometimes actually hybrids between the native (Anolis carolinensis) and the nonnative Cuban green anole (A. porcatus).
Surveys revealed that in the past five years, nonnative species increased in abundance by 33% on average across all parks. But they were spreading more quickly in human-modified parks than in more native ecosystems. The best indicator species of native parks in the recent survey was the southern black racer (Coluber constrictor priapus).
The species that were the best indicators of nonnative parks were bark anoles (A. distichus), Peter’s rock agama (Agama picticauda), northern curly-tailed lizards (Leiocephalus carinatus) and Cuban knight anoles (A. equestris).
Over the past five years, the species making the biggest inroads were rock agamas and curly-tails. Curly-tails, thought to have been introduced in West Palm Beach to the north of Miami-Dade County, have increased in abundance 22-fold in the past five years, while rock agamas, thought to have been introduced around Homestead in the south of Miami-Dade County, have increased 36-fold.
Feeling right at home
Searcy speculated these two species are taking off because the concrete jungle somewhat resembles their native ecosystems. “Because they are rock-loving species, and so much human habitat is cement, that just ends up being really successful habitat for them,” Searcy said.
Temperature also might play a role, according to preliminary work Searcy is now conducting. Curly-tails and rock agamas can both tolerate hotter temperatures, which may give them a leg up in the city affected by higher urban temperatures than surrounding wild areas—rock agamas especially are often found perching on the hottest rocks. But he’s looking to confirm this with more analysis.
Red-eared sliders (Trachemys scripta elegans) and yellow-bellied sliders (T. s. scripta) were the most common nonnative turtles. Overall, 80% of the individual turtles they saw in the city were nonnative.
One drawback of the study was that surveys only occurred during the daytime, meaning the researchers may have missed nocturnal herps like geckos and some amphibians.
The daytime surveys didn’t reveal any native amphibians—only alien species like Cuban treefrogs (Osteopilus septentrionalis), greenhouse frogs (Eleutherodactylus planirostris) and cane toads (Rhinella marina).
The old invasion
Other more established nonnative species, like green iguanas (Iguana iguana) and brown basilisks, increased at a standard rate, and the researchers only found a couple Burmese pythons (Python bivittatus) and Argentine black and white tegus (Salvator merianae) in urban parks—those invasive species are mostly causing a problem in the wilder areas outside of cities.
The surveys revealed other species just beginning to appear and which may begin to cause problems in the future. They found rainbow whiptails (Cnemidophorus lemniscatus) in one park and black spiny-tailed iguanas (Ctenosaura similis) elsewhere. Searcy suspects the latter may become a problem, as they have the potential to expand their current numbers in the city due to a generalist diet, their tolerance to live in high densities and their large size.
Philosophically, having so many introduced species in a novel environment like a city may not be entirely a bad thing, Searcy said. But some of these species may not stick to the city.
“If you could ensure that they would stay there, then maybe [it wouldn’t be] a big deal,” Searcy said. But this is often wishful thinking, he added.
Most of these populations originate from released pets, he said. Surveys like Searcy’s could help managers figure out which recently introduced species are most successful and potentially limit their, or similar species’, introduction elsewhere.
But for species like curly-tails or Peter’s rock agamas in Miami, that ship likely sailed several years ago.
“Ideally, we would not have any more nonnatives get introduced,” Searcy said.
