Introduction
Florida has one of the greatest invasive species problems in the United States. Many factors make the state particularly vulnerable to introduced species, including Florida’s subtropical climate, status as a transportation hub, pet industry, isolation, and susceptibility to natural disasters. This is a huge threat to native species—especially reptiles and amphibians who thrive in Florida’s climate. Along with changing competition and predation, invasive species may pose a threat to human safety, domestic animals, and the economics of management. Non-native lizard species have already taken over native breeding numbers threefold in South Florida. Despite the growing problem, there is a lack of public concern or risk assessments of potential invasiveness. The general public is mostly aware of the Burmese python in the Everglades but may not understand that invasive species are a widespread problem in the state. Three species of great risk are the cane toad (Rhinella marina), Argentine black and white tegu (Salvator merianae), and Northern African python (Python sebae sebae).
Cane toad
The cane toad is a large species of toad, reaching approximately six to nine inches long (Florida Fish and Wildlife Conservation Commission, 2019). It is usually brown with no distinct pattern on its underbelly, and the parotid glands are very pronounced and can produce toxins (Makowski and Finkl, 2018). Cane toads strongly resemble the Southern toad (Anaxyrus terrestris); however, the cane toad is significantly larger (Florida Fish and Wildlife Conservation Commission, 2019). This species is native to Central and South America, ranging from the Rio Grande to the Amazon River basin (Florida Fish and Wildlife Conservation Commission, 2019). Cane toads are omnivorous, eating insects, vegetation, and small vertebrates, including native amphibians (Florida Fish and Wildlife Conservation Commission, 2019). They breed during the wet season, specifically from June through October (Makowski and Finkl, 2018). Their habitats include a variety of man-made and natural habitats, such as drainage ditches, ephemeral ponds, and other shallow bodies of water (Makowski and Finkl, 2018).
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Cane toads were initially released in the 1930s as a pest control method for agriculture (Makowski and Finkl, 2018). The most impactful releases occurred in the 1950s in Miami, starting when a pet dealer accidentally released cane toads at an airport, followed by several more intentional releases soon after (Makowski and Finkl, 2018). Since these releases, the cane toads have expanded their range to the entirety of southern Florida (Florida Fish and Wildlife Conservation Commission, 2019).
As global temperatures increase, the range of the cane toad in Florida may expand northward (Makowski and Finkl, 2018). A large threat to native wildlife is the toxins secreted by the cane toad’s large parotid glands (Florida Fish and Wildlife Conservation Commission, 2019). Red-tailed hawks and American crows have begun to flip over toads when eating them to avoid the parotid glands (Wilson and Johnson, 2017). In addition to poisonous secretions from the parotid glands, toxins in cane toad eggs can poison native tadpoles (Florida Fish and Wildlife Conservation Commission, 2019). In one study, cane toad tadpoles only outcompeted native tadpoles when other species of invasive tadpoles were present (Smith, 2005). However, there is still concern about adult cane toads outcompeting adult native frogs (Makowski and Finkl, 2018). Cane toads regularly consume small vertebrates and could affect native populations of rodents, frogs, and small reptiles (Makowski and Finkl, 2018). The current impact of cane toads on native Floridian ecosystems is so far less than the impact on Australian impacts, but those populations were started larger earlier on (Makowski and Finkl, 2018).
Argentine black and white tegu
The Argentine black and white tegu is a large, omnivorous lizard weighing up to eight kilograms and reaching over one and a half meters in length (Engeman et al., 2011; McEachern et al., 2015). They are characterized by beaded scales and a stripe of light spots or dashes. The species exhibits sexual dimorphism with males growing larger than females. The lizard is primarily terrestrial but is also an adept swimmer and active burrower. Black and white tegus are habitat generalists, living in tropical, subtropical, and temperate climates. They utilize both disturbed and undisturbed forested and open natural areas. Their omnivorous diet consists of everything from vegetation, fruits, carrion, small mammals, and reptile eggs. Rotting vertebrates are the lizard’s main food source in its native range. It mainly uses chemical cues to find food but also a vision for prey; these chemical cues are what allow the tegus to find nests and dig up eggs to eat. Tegus lay a single clutch of twenty to forty-five eggs per year in the spring starting at age three or four (Engeman et al., 2011). Some studies show that they use other species’ burrows for their eggs. Independent offspring hatch after sixty-five to seventy-five days of incubation with parental care being shown through female egg defense and male territoriality (Chamut et al., 2012).
The native range for tegus is extensive, ranging from northern Patagonia, north to the Amazon forest, and west to the Andes Mountains. They are found in the microhabitats of Gran Chaco, Caatinga, Cerrado, Pampas, and the Atlantic Forest. 7,700 live Black and White tegus were shipped from Paraguay from 2000 to 2002 for the pet trade (McEachern et al., 2015). The tegus all had defects and may have been released to establish a population for exploitation or to avoid killing unwanted animals. By 2002, a free-ranging tegu was established in Florida. These are earlier dates recorded, but these are from less credible sources. Overall, there seems to be disagreement on the origin timeline of the tegu establishment. As seen in Figure 1, Hillsborough, Polk, and Miami-Dade counties now have populations. The Florida City and Southern Glades Wildlife and Environmental Area is the current center of the populations, but the southwest is not safe from spread; this area is only six miles away from Everglades National Park, and tegus have already been seen west of the Everglades Agricultural Area (Harvey and Mazzotti, 2015). Hurricanes may increase the spread of tegus as they can damage enclosures and release pets. Argentine black and white tegus have been described as doglike because they are intelligent and can be house-broken. Trying to manage, and possibly lethally control, a species considered to be a pet may cause controversy.
With global warming, small and localized populations in southeast Florida could reproduce and spread with better conditions. This is especially worrying, as the population is very close to Everglades National Park already. Climate change also increases the likelihood of natural disasters and drastic temperature changes. This would increase the potential for escaped pet tegus. Argentine black and white tegus can also survive drastic climate events brought on by climate change that other species cannot. They practice winter dormancy in underground burrows called brumation. Tegus brumate for an average of 137 days and could “wait out” hard winters (McEachern et al., 2015). By using other species’ burrows, tegus could additionally threaten native species, such as gopher tortoises and burrowing owls. They have been documented predating on the nests of Florida red-bellied cooters and American alligators. Figure 2 shows the first evidence of this in a camera trap study. Populations are approaching a nesting area of federally threatened American crocodiles in the east and American alligators in the west and south. The native diet of the species supports the threat towards crocodilians; tegus are the main predators of spectacled caiman eggs in the Venezuelan Llanos. Gut studies have shown that tegus have preyed on the native cotton rat, a similar species to the endangered Key Largo woodrat (Harvey and Mazzotti, 2015). The varied diet of tegus poses a threat to federal and state-listed reptiles, birds, and even mammals.
Northern African python
The Northern African python is a large snake with dark blotches or spots; their scales are entire and sometimes fused. They are considered a giant snake species, ranging from three to six meters long. They prefer evergreen forests or moist savannahs in their native range and tend to live near rocky areas and water. Their climate niche ranges from fifteen to thirty-five degrees Celsius (Reed and Rodda, 2009). The python also thrives in disturbed environments. This could include agricultural areas, canals, farm fields, or towns. When living near humans, they can adjust their natural diurnality to nocturnality to avoid conflict. They predate on a variety of endothermic prey through constriction—mainly rodents or abundant domestic animals. The python has also been observed consuming lizards, fish, crocodiles, and frogs. Mating can occur during varying seasons or be year-round depending on the location; about twenty to fifty eggs are laid in hollow trees, termite nests, or mammal holes (Reed and Rodda, 2009). The females show maternal care by coiling around the eggs and incubating them. This is a very rare parental care attribute in snakes and is only seen in pythons. The eggs incubate for fifty-seven to sixty-one days or longer in lower temperatures (Branch and Patterson, 1975). For this species, there may be potential for hybridization with the invasive Burmese python. Genetic diversity studies show that Burmese pythons and Indian pythons are likely to have been hybridizing in the wild for years, and hybridization with the Northern African python has been observed in captivity (Hunter et al., 2018). This could cause hybrid vigor, or the increased diversity, fitness, and fecundity of future populations. Burmese pythons are arguably the most damaging invasive species in Florida currently. Their range is extensive, and breeding with the more constrained in-range Northern African python would only increase their spread.
Northern African pythons natively inhabit the coasts of Kenya and Tanzania, across much of central Africa to Mali and Mauritania, and north to Ethiopia and Eritrea. Their range can be seen in Figure 3. There are uncertainties in this data denoted with question marks. Before the species was subdivided, feral rock pythons were seen in the Everglades in the 1990s (Murphy and Henderson, 1997). The earliest recorded Northern African python in Florida was seen around 2001 to 2005, and they are now in the counties of Miami-Dade and Sarasota with an established population in the former (Krykso et al. 2011). Credible sightings of different age classes show evidence of a breeding population. Their spread is thought to be due to released or escaped pets, as is with most of Florida’s invasive species. The export of Northern African pythons is lower than other snakes at an average of 1,056 per year, but the damage to Florida has already been done (Luiselli et al., 2011). Further, African pythons may be able to inhabit populations north of their climate limit.
The range for the Northern African python is expected to increase with global warming. Breeding populations are currently restricted to warmer climates due to egg incubation, but with increasing temperatures, they could move further north. When the temperature is low, and thus the incubating female is too cold, python eggs either don't hatch or hatch deformed. Because of this, warmer temperatures brought on by climate change would increase reproductive success for the python. The pythons are restricted to a small area in Florida, but they are already threatening humans and native fauna. Attacks of Northern African pythons on humans are rare but existing. In 1979, a thirteen-year-old boy was constricted to death by the species in South Africa (Branch and Hacke, 1980). Rare as these stories are, they create myths and animosity for the species that muddle science and management. Native animals are the ones truly at risk. Wading birds such as wood storks are known to be prey, and many more are suspected to be (Johnson and McGarrity, 2017). As well as eating the Florida equivalents of their prey in Africa, the pythons spread diseases and parasites. African pythons have been shown to host several ticks: Amblyomma nuttalli, Amblyomma marmoreum, Amblyomma sparsum, Aponomma exornatum, Aponomma flavomaculatum, and Aponomma latum (Burridge, 2001). After being transported to the U.S., the ticks host-switch to native species; This has occurred with at least two species in Florida already. Some of the species are known or suspected to transmit mammalian heartwater disease, Q fever, and reptile haemogregarine parasites. Unfortunately, there is an inadequate amount of data on disease ecology to understand more specific impacts.
Conclusion
Florida is a hotspot for invasive species and is currently a threat to native populations. The high amount of invasive reptiles especially poses a threat to reptiles and amphibians. The cane toad has the potential to affect herpetofauna populations through its poison secretions and its consumption of vertebrates. The Argentine black and white tegu will affect future populations of crocodilians by eating their eggs. The African pythons are known for carrying diseases and parasites that could be transmitted to different taxa. Not only are invasive species a threat to native wildlife, but they also pose a risk to humans and domesticated animals. Climate change is likely to worsen the effects of invasive species in Florida as the temperatures rise, allowing easier establishment and movement of invasives. As native populations decline, invasive species will begin to fill their niche, making them harder to remove and potentially harming the ecosystem.
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