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Florida's Creeping Crawlers
by Howard Youth
Like the millions of people who migrate to Florida to vacation or live, iguanas—and many other reptiles—also find Florida's warmth and exotic greenery very inviting. Since their first appearance in the 1960s, after escaping from zoos or being freed by pet owners, green iguanas (Iguana iguana) seem to be right at home in the Sunshine State, even though they originally hail from places well to the south.
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| Green iguanas are not native to Florida. (Jessie Cohen/NZP) |
"If it snowed in Miami, we wouldn't have an iguana problem anymore," says Kenneth Krysko, a herpetologist with the Florida Museum of Natural History at the University of Florida in Gainesville. "In the Keys, you see multi-million-dollar homes but notice something doesn't look quite right," he says. "Then you see that all the beautiful flowering plants have chicken wire around them. A big male green iguana gets more than five feet long and he's got to eat."
Krysko patrols the state, documenting new reptile invaders that have arrived from all corners of the globe. With his noose-fitted fly rod and bucket of crickets for bait, he's been reeling in not only green iguanas, but a real menagerie.
Over the last 40 years or so, Florida's population has grown quickly, as have international travel and trade, including the pet trade. At the same time, the number of new reptiles popping up all over the state has grown. They arrive in plant shipments, in crates, or in travelers' cars. Or they escape from pet dealers or owners, or are released on purpose. From small to goliath, these scaly new arrivals have turned the heads of scientists, gardeners, pet owners, and home owners.
Today close to 40 exotic, or non-native, reptile species
breed in Florida, and new species keep showing up, despite
long-standing regulations that prohibit introductions.
More than 30 of these are lizards, ballooning the state
lizard list from the original 16 native species to about
50 and growing. Krysko and other scientists are just
starting to narrow their focus on the ecological impact
of the state's new "herps"—introduced
reptiles and amphibians—and they have found that
some may pack a real wallop. Some, like the iguanas,
also leave many residents crying, "Not in my backyard!"
Iguanas by the Pool
From Palm Beach to Key West, the iguana population seems
to be growing beyond control. Krysko, half-joking, promotes
marketing the introduced iguanas as prime table fare.
"There are some great recipes out there for iguanas,
so why don't we go out and get [the reptiles], humanely?"
he says, adding, "There's a little meat on the
tail but a large portion on the legs." Iguana meat—which
like so much other exotic game is touted to taste like
chicken—already features prominently on rural
tables throughout the lizard's native range in Mexico,
Central and South America, and the Caribbean islands.
Despite its popularity in the tropical countryside,
iguana has yet to become an important part of Miami
restaurant cuisine.
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| Green iguana. (Jessie Cohen/NZP) |
It's hard to know how many iguanas now live and breed in southern Florida, but on Key Biscayne alone, Krysko's colleagues hauled out more than 1,000 in 2003. Many homeowners find the sight of the large, scaly lizards distasteful, if not because of their appearance, then due to their habits. To date, no one has documented these fast-growing omnivores' impact on native plants, but landscapers bemoan the loss of ornamental hibiscus and other flowers. "Any pretty flower, they're going to eat it," says Krysko, who has noosed and wrestled individuals out of Miami's Fairchild Tropical Botanic Garden, where ten to 20 may be found in one tree. "And they defecate everywhere—on people's docks, in their pools…everywhere," adds Krysko.
Krysko and his colleagues also noticed some troubling behavior by the introduced black spiny-tailed iguana (Ctenosaura similis), a stripy Central American species that now proliferates in the Miami area and in a few residential areas in southwestern Florida. The scientists recently documented this lizard eating a state-endangered plant, the Curacao bush (Cordia globosa). They also found black spiny-tailed iguanas inhabiting gopher tortoise (Gopherus polyphemus) burrows and eating many of the same plants sought by this declining and protected native species.
Brown Versus Green
These days, the most commonly seen lizard in southern
Florida is not the green iguana, but a far smaller—and
even more prolific—introduced animal, the brown
anole (Anolis sagrei). Tan to chocolate-brown
in color, depending upon temperature and mood, brown
anoles rarely grow to be more than eight inches long.
Males often perch head-down low on tree trunks and other
vantage points, flashing their yellow-rimmed scarlet
throat fans, or dewlaps. Diamond-backed females lurk
in nearby vegetation. Many Floridians bemoan the loss
of the once-abundant native green anole (Anolis
carolinensis), which used to be all over their
backyards but now seems to have vanished in the wake
of the introduced brown anoles.
Native to Cuba and the Bahamas, brown anoles now also inhabit virtually all of peninsular Florida, coastal cities in Louisiana, Texas, and Georgia, some Caribbean islands where they are not native, and also sites in Hawaii and Taiwan. Jason Kolbe, a graduate student at Washington University in St. Louis, and his colleagues traced the origins of recently established brown anole populations in Taiwan, Hawaii, Grand Cayman, and Grenada not to the brown anole's native islands, but to Florida. "The real key was the mixture of genetic variants present within Florida populations that do not occur together in the native range," says Kolbe, "and that these mixtures also exist in Hawaii and Taiwan." Obviously, the lizards have not traveled from Florida on their own. "My best guess is that it's a combination of various factors," says Kolbe. "They could be coming and going on exotic plants and can survive days to weeks during shipment. The pet trade is likely a source as well."
In their study, the results of which were published in Nature in September 2004, Kolbe and his colleagues wrote that Florida has weathered at least eight brown anole invasions. The scientists believe that the well-entrenched anoles spring from "blending genetic variation from different geographic source populations and producing populations that contain substantially more, not less, genetic variation than native populations." They believe that this genetic variation may make brown anoles "particularly potent sources for introductions elsewhere." Isolated introductions seldom come on so strong, because they suffer from genetic bottlenecks without infusions of new blood.
Herpetologists note some morphological differences between Florida brown anoles and those in Cuba, such as larger size in some areas of Florida. But Kolbe thinks it's far too soon to entertain thoughts of a new Florida brown anole species already existing, although there might be one in the making. "The trick there is that we'd have to stop introducing them, but we're still introducing them all the time." If the peninsular population were isolated, says Kolbe, a unique Florida anole species might evolve, given enough time.
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| Green anole. (Jessie Cohen/NZP) |
The brown anole is one of Florida's earliest known reptile imports, first found in the late 1800s on Key West, probably after stowing away on cargo ships. After several more introductions, brown anoles formed satellite populations that eventually merged, then really kicked into gear from the 1940s through the 1970s. Since then, anoles likely filled in the gaps by "vehicular rafting"—hitching rides on the undercarriages of cars and in ornamental vegetation trucked north—and via introductions from Cuba. They have even been seen catching rides on windshield wipers. One observation that points to vehicular rafting came in 1996, when University of Tennessee Ph.D. candidate Todd Campbell found brown anoles on northbound interstate highway rest areas but not on those serving southbound traffic across the highway.
When he made his rest-stop observation, Campbell, now an assistant professor and ecologist at the University of Tampa, was studying brown anole colonization in a different area—on green-anole-inhabited islands, or piles of sediments made from dredging channels, off of Cape Canaveral. In 1995, he introduced brown anoles to three of these islands, and also studied three control islands inhabited only by native greens. While green anoles were found at all levels and types of vegetation at the start of the study, by 1998 brown anole populations, which started with either 40 or 80 individuals, "increased dramatically and became dense in all habitat types within the study period," wrote Campbell in a summary of his study published in the Anolis newsletter in 1999. Meanwhile, green anole populations on brown-inhabited islands plummeted. What's more, greens on islands with browns only occurred in the most densely vegetated areas, and they had shifted to higher perches, but on the control islands without browns, green anoles were still skittering all over.
Why are browns outcompeting greens? "A lot of factors are operating here at the same time," Campbell says. "Among them is this 'double whammy' of [brown anoles] eating another lizard's prey items and its hatchlings. This can knock a species down pretty quickly," he says. "Green anole hatchlings have to run a gauntlet of brown anoles all the time. They live near the ground and are directly exposed." Campbell stresses that no one knows for sure, but he wouldn't eliminate the possibility that, to survive, young green anoles learn to avoid areas frequented by brown anoles. They may carry this avoidance behavior into adulthood, when they choose to perch higher than brown anoles. While adult green anoles occasionally feed on young brown anoles, browns occur at far higher densities: "Brown anoles can reach densities of one lizard per square meter," says Campbell. "That's 10,000 lizards per hectare [2.47 acres]."
Can the green anole survive the onslaught? "Floridians
grew up with the green anoles all over the outsides
of their houses. Now they see only brown anoles. But
usually the green anoles are just higher up, not gone,"
says Campbell. Thickly vegetated gardens provide urban
havens for green anoles, and at least for now, outside
the most urbanized areas, brown anole densities are
lower and many green anole populations continue to thrive.
A Potential Nightmare in the
Mangroves
When the staff of NBC's "Today" show called
and asked Campbell to appear on national television,
they didn't want to hear about small, feisty lizards.
Campbell was brought on the air to discuss some of his
other work—specifically his efforts to locate
and capture Africa's longest lizard, the Nile monitor
(Varanus niloticus), on Cape Coral, a suburbanized
peninsula just southwest of Fort Myers.
Nile monitors grow up to seven feet long and can weigh up to 30 pounds. They can inflict serious injuries with their jaws and lash attackers with their formidable tails and claws, although if found in the wild they usually flee rather than confront people. The thriving Cape Coral monitor population likely originated either from animals that escaped from pet breeders or some that were intentionally released by stressed pet owners or by dealers hoping to establish stock in the wild that could later be recaptured.
Like many other successfully introduced species, Nile monitors are habitat and diet generalists. Armed with formidable claws and muscular tails, they readily dig in soil to find buried eggs or to enter animal burrows. Strong swimmers, they spend much of their active time at the water's edge or in water, chasing prey or escaping Campbell and other dangers—hatchlings are vulnerable to large raptors, alligators, dogs, cats, and even adult Nile monitors. Over the past year and a half, Campbell and his colleagues have captured and euthanized more than 80 monitors. They checked the animals for introduced parasites ("They were incredibly clean," Campbell told me), examined the lizard's reproductive tracts for eggs and other signs of breeding, and pored over the stomach contents.
Items discovered on the monitors' menu were varied, and sometimes a bit surprising. "We found a lot of cockroaches, mangrove tree crabs, snails, clams, members of five frog species including the Cuban treefrog [an introduced species], native glass lizards, birds [black feathers—perhaps grackles], fish, some mammal hair, a whole pygmy rattlesnake, and a wasp nest," says Campbell. "We also found a whole heck of a lot of brown anoles and their eggs. That's a lot of [brown anole] biomass available to a [Nile] monitor and sure enough, they eat them. And they eat a lot of them." Campbell cites a similar example on the Pacific island of Guam, where introduced brown tree snakes (Boiga irregularis) decimated endemic wildlife and continued to flourish, feasting on two prolific introduced lizard species.
Nile monitors favor wetlands but they also have been popping up in the suburbs, chasing after pond goldfish and potentially posing a threat to small dogs and cats. The monitor's catholic tastes concern conservationists like Campbell. He, Krysko, and their colleagues Kevin M. Enge, Kraig R. Hankins, and F. Wayne King recently published the first paper on the species' ecological status in Florida, which appeared in Southeastern Naturalist in 2004. In it, the authors ponder the lizard's potential impact on native fauna and plot out areas where monitors have been found. They write: "Presently, the natural history of V. niloticus in Florida has not been documented, but information gathered from studies in Africa indicates that this exotic species may pose a serious threat to native wildlife, particularly if it expands its range."
"They're not limited by anything and they're definitely breeding," says Campbell, "We've caught only 80. There are probably in the high hundreds or thousands out there. They are currently highly localized in the Cape Coral area, but we've also had sightings in some troublesome places, such as nearby Pine Island. If they breed there, they would be very difficult to eradicate."
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| Nile monitors have already invaded the habitat of the largest burrowing owl population in Florida. (Jessie Cohen/NZP) |
The effect on local wildlife is a concern. "The monitors are already right in the heart of the largest burrowing owl [(Athene cunicularia)] population in the state," says Campbell. So far, there is no evidence that these raptors have become monitor prey. Other wildlife potentially at risk are gopher tortoises and large concentrations of pelicans and herons, including those at nearby Sanibel Island. Campbell and his colleagues worry that, given beach access, monitors might start digging up sea turtle nests.
First spotted around Cape Coral in 1990, the monitor population has, like those of brown anole and other introduced animals, grown rapidly after an initial lag. "Its fecundity, eclectic diet, and ability to travel over land and across water would allow the species to disperse widely in Florida….Extensive canals in southern Florida would provide ideal dispersal corridors," reads the Southeastern Naturalist paper. Beyond the canals lie Everglades National Park and Big Cypress National Preserve, then the Florida Keys.
But how far north might they move? Many tropical species, like green iguanas, appear to be limited by Florida's occasional cold snaps. This may not slow the Nile monitor. "This species could survive cold weather in northern Florida by using underground refugia, as it does in temperate regions of Africa," Campbell and colleagues write. Campbell adds, "They could get pretty far north from here. I wouldn't have any doubt that they could survive in all of Florida and at the southern edges of all southeastern states from Georgia to Texas."
As is the case with other exotics that have crept beyond easy control, the Southeastern Naturalist paper recommends that "any individuals that appear elsewhere will need to be immediately eliminated."
Everglades Wildlife: Feeling
the Squeeze?
Introduced snakes have not wound up in the sights of
Florida's keen-eyed herpetologists nearly as often as
lizards, but two have become firmly entrenched.
From the mid-1990s through 2004, 113 Burmese pythons (Python molurus), including one measuring up to 14 feet long, have been seen on or collected from the wetlands and main park road of Everglades National Park. In January 2005 alone, 15 more turned up. Based on the varied sizes and locations, park officials believe the snakes are breeding.
The pythons, like the monitors, likely originated from growing pets cast off by panicked owners. Park biologists fear that the local "mangrove" form of fox squirrel (Sciurus niger), the wood stork (Mycteria americana), and other threatened species might soon become prey items. So far, animal remains found in park pythons include those of pied-billed grebe, white ibis, house wren, opossum, raccoon, cotton rat, and the black rat.
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| Burmese python. (Jessie Cohen/NZP) |
Park wildlife biologist Skip Snow is also a bit concerned about visitor and employee safety. Among the world's largest constrictors, Burmese pythons can reach 20 feet or longer. "These are large predatory animals. While not known to do it, the python has the tools and techniques to kill people, as supported by foolhardy pet owners who didn't handle feeding time correctly," he says. Snow believes, however, that the pythons will pose more of a road safety threat, perhaps causing unsuspecting motorists to swerve off the park road at night when they spot what looks like a log lying across the pavement. (On cool nights, pythons may haul out onto the road to soak up heat radiating off the pavement.)
Far smaller and far more widespread is the bizarre and rarely seen Brahminy blind snake (Ramphotyphlops braminus). At most, it grows to seven inches long—the shortest snake living in North America—and resembles a scaly, dark earthworm. This burrowing animal has traveled across the state in potted plants and can now be found in a variety of areas, especially under logs, trash, and other debris; thought to be Asian in origin, it now also occurs in Mexico, Central America, Africa, and Hawaii. Many herpetologists consider the Brahminy blind snake to be one of the most widespread reptiles in the world. It eats termites and other insects and in turn has been eaten by cane toads and Puerto Rican crested anoles (Anolis cristatellus), another introduced anole species breeding in Florida. Don't try to find a male—only females occur. Reproducing by a process known as parthenogenesis, females produce offspring genetically identical to themselves.
Florida's list of 40-plus exotic reptiles and amphibians will likely continue to grow. While laws prohibit the release of exotic animals into the wild, none are currently in place to prohibit the import of popular and potentially invasive pets such as monitors, pythons, and iguanas. In flower pots, cars, or on crates, they continue their creep across Florida, while scientists keep tabs on them, reel them in, and try to figure out how to keep them under control.
—Howard Youth has been fascinated by Florida's introduced reptiles since the 1970s, when as a boy he spent vacations reaching around palm trees in often-vain attempts to capture brown anoles.
Sidebar—Toxic
Toads and Tyrannical Treefrogs
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