Iguana 11.3 b&w text 154 IGUANA • VOLUME 11, NUMBER 3 • SEPTEMBER 2004 MESHAKA, BARTLETT, AND SMITH Green Iguanas (Iguana iguana) have become a common sight in the human-altered habitats of southern Florida. Photograph by Joe Wasilewski. A young Green Iguana from Miami, Miami-Dade County. The southern Florida Peninsula. Illustration by John Binns. IGUANA • VOLUME 11, NUMBER 3 • SEPTEMBER 2004 155 If the business of biology is detective work, then answering the following questions is an attractive endeavor: Why is the Green Iguana (Iguana iguana) suddenly so ubiquitous in southern Florida? What can we expect of it in the future? Why? From an academic perspective, the Green Iguana and its relatives, the Spiny-tailed Iguanas (Ctenosaura spp., see IGUANA 10:111–118), are the only exclusively herbivorous components of Florida’s exotic terrestrial herpetofauna. As a matter of aes- thetics, the sight of an adult Green Iguana is an eyestopper. Also, these lizards are popular pets that can grow to large size, are behaviorally complicated, can be docile, and are attractive. Not surprisingly, pet trade-related releases and escapes are responsible for the mostly spotty geographic distribution of Green Iguanas in southern mainland Florida and the Florida Keys. First reported from several sites during the 1960s, breeding populations were not confirmed until sometime later. Where are Green Iguanas Now? Presently, Green Iguanas are a largely coastal species in southern Florida. They occur somewhat continuously along the coast of Colonization Success by Green Iguanas in Florida Walter E. Meshaka, Jr.1, Richard D. Bartlett2, and Henry T. Smith3 1State Museum of Pennsylvania, Section of Zoology and Botany, Harrisburg, PA 17120-0024 23101 SW 1st Way, Gainesville, FL 32601 3Florida Department of Environmental Protection, Florida Park Service, Florida 33455 Photographs by Richard D. Bartlett unless otherwise indicated. GREEN IGUANAS IN FLORIDA � An adult Green Iguana basking on a palm with the Miami suburbs in the background. Photograph by Joe Wasilewski. 156 IGUANA • VOLUME 11, NUMBER 3 • SEPTEMBER 2004 Palm Beach, Broward, and Miami-Dade counties and on Key Largo in Monroe County. Populations also are established on the lower Florida Keys. To the west, Green Iguanas are established in parts of Collier County and have been reported from Lee County. With some regularity, the species is seen in Vero Beach (Indian River County) and Tampa Bay (Hillsborough County). Although not part of breeding populations in the much colder interior of Florida, an individual was reported from Alachua County and another was seen in Highlands County. Apart from those in southern Florida, adults and hatchlings are known to occur in Cameron County in southern Texas. Green Iguana populations reach their highest densities along the warm coast, where frost-free conditions prevail through most of Palm Beach County. Almost without exception, these lizards are associated with water, such as bays, canals, ponds, and ditches — and they are especially attracted to areas with trees that extend over water and where tender edible vegetation is abundant. Population densities can be high. For example, annual reports from the Florida Department of Environmental Protection revealed that, at Cape Florida in Miami-Dade County, an astonishing 397 individuals were removed during the five-year period 1 July 1998–30 June 2003 (0, 0, 1, 12, and 384, respec- tively). During this same period, 27 individuals (5, 0, 0, 18, and 4, respectively) were removed from Hugh Taylor Birch State Park in Broward County. At Oleta River State Park in North Miami (Miami-Dade County), Park Service Specialist Laura L. Kruger began seeing iguanas within a few years of her arrival at the park in 1998 and MESHAKA, BARTLETT, AND SMITH An adult male Green Iguana at Crandon Park, Miami-Dade County. An adult Green Iguana basking at the Kampong, Miami-Dade County. IGUANA • VOLUME 11, NUMBER 3 • SEPTEMBER 2004 157GREEN IGUANAS IN FLORIDA has noticed a sizeable increase in population numbers. In the past three years, she has personally captured 16 individuals. She usu- ally sees lizards in areas surrounded by saltwater habitat and in the park’s mangrove stands. At Fairchild Tropical Garden, also in Miami-Dade County, population densities are such that Skip Snow, National Park Service, has indicated that the Hibiscus garden can no longer be maintained. Although not sampled in similar fashion, these examples and our own combined observations at these sites and at others over time corroborate the notion of sharply increasing iguana population sizes at the few sites where anyone is moni- toring the situation. Temporal and Spatial Colonization Patterns Why have Green Iguanas so suddenly become ubiquitous in an otherwise unremarkable Florida range? Efforts to separate the phenomena of increased interest in finding the species and increased dispersal rates (natural or otherwise) complicate the matter of dating newer populations. Acquiring a full accounting of which colonies are actually new and which were simply over- looked for some period verges on the impossible. If one examines the literature concerning this species in Florida, verifiable popu- lations tend to be found far more frequently in a few southern- most counties than in new counties. With the exception of Palm Beach County, much of what Green Iguanas appear to be doing is filling gaps in their current distribution. What can be said with certainty, however, is that population growth in previously known sites dramatically increases the opportunities for natural dispersal, especially along the seemingly innumerable waterways that crisscross the state. Why, however, do increases in population sizes continue and how can one explain the sudden and seemingly endless supply of Green Iguanas from sites that had never supported large num- bers during the 10 or so years during which we have been con- ducting searches? More directly and ironically, how has this species become so numerous that these former discards from the pet trade are now collected from south-Florida populations for sale by pet dealers? The answers to these questions potentially can serve to pre- dict the nature of future trends. Green Iguanas, like many other colonizing species, often occupy a few localized sites for very long periods while slowly building up population sizes or until condi- tions are ripe for an explosion of numbers, with the latter scenario ranging from nearly instantaneous to a period of many years. Why populations persist initially requires a favorable com- bination of characteristics, all of which are present in southern Florida (in no particular order): (1) Amenable climate exists. (2) Disturbances in association with water are common in Florida, occur in many forms, and are often accompanied by prolific growth of ornamental or native edible foliage. Perhaps these habi- tat characteristics mimic the secondary growth along rivers so favored by Green Iguanas in native ranges throughout the Neotropics? (3) Competitors for food, except other Green Iguanas, are largely absent. (4) Adults are safe from most preda- tors, and potential predators of hatchlings and juveniles are uncommon in human-disturbed situations. (5) Green Iguanas An adult Green Iguana very comfortable in brush in Broward County. are either protected by humans who like them or are hard to detect by residents who do not. (6) These lizards can persist for long periods until conditions are favorable for recruitment; although attaining maturity may take three to four years, females then can produce large clutches for many years. To illustrate the resilience of iguanas, in 1999, one of us received an adult female that had been collected in downtown Austin, Texas. According to the collector, she had been living for at least three years in a burrow (possibly of her own construction) in an extensive area of rubble from a demolished building and high- way construction. Low, weedy shrubs of unknown species grew thickly around the perimeter. On many occasions, this iguana had been seen eating foliage and blossoms from the shrubs. When crews began removing the rubble, they decided to catch the iguana. Except for having her elongated vertebral scales worn completely off, this 3-foot long iguana was in excellent condition and accli- mated immediately to captive conditions. Today, five years after being captured, her vertebral scales have regrown only moderately. Back to Florida, on Virginia Key, Biscayne Key, Cutler Bay, and in Hugh Taylor Birch State Park, Green Iguanas were not nearly as abundant before Hurricane Andrew in 1992. With the exception of Hugh Taylor Birch, they had been present for many years at these sites — but in no appreciable numbers. Within a decade subsequent to Hurricane Andrew, the Green Iguana bucket, so to speak, began to overflow. Common to these sites (with the exception of Hugh Taylor Birch) was aggressive replant- ing with a smorgasbord of plants, shrubs, and trees bearing edi- ble and apparently delectable parts. Abundant resources in a rich habitat resulted in successful reproduction and a large crop of now 8+ year old adult females that are in turn producing large numbers of hatchlings each year. This surplus is now large enough to disperse actively to previously vacant areas that also were heavily planted after the storm. However startling the pop- ulation densities, as both herbivores and ectotherms (which require far less energy for maintenance and reproduction than endothermic birds or mammals), large body size is no barrier to 158 IGUANA • VOLUME 11, NUMBER 3 • SEPTEMBER 2004 MESHAKA, BARTLETT, AND SMITH A young Green Iguana from Miami, Miami-Dade County. A sick Green Iguana on a lawn in Miami, Miami-Dade County. Although no cause is known for the condition of this individual, feral iguanas may be vulnerable to toxins used to control pests or weeds. IGUANA • VOLUME 11, NUMBER 3 • SEPTEMBER 2004 159GREEN IGUANAS IN FLORIDA enormous population densities, despite what might superficially seem like a mediocre food supply. Further accelerating geographic expansion attributable to explosions in population sizes is the ever-increasing potential for pet escapes or releases. Within the past decade, farming of igua- nas for import into this country from several Latin American countries, especially Colombia and Belize, has reached levels such that tens of thousands of baby Green Iguanas are introduced via the pet trade each year — and continuously rather than season- ally as in the past. This has resulted in close to single-digit retail prices. With this explosion in availability has come a concurrent spike in orphan iguanas, enhancing opportunities for escapes or deliberate releases of unwanted pets that have become too large, too aggressive, or too sick to keep. The Test So, Green Iguanas are thriving in their disturbed southern Florida range. Yet, the exception tests the rule. Everglades National Park, a large expanse of relatively natural wetland and upland com- munities, provides the test. The Park also was damaged by Hurricane Andrew, but was then left to revegetate on its own. Since 1970, the Park has reported only 20 sightings of Green Iguanas, with observations occurring in 1970–1977 (N = 11) and again in 1995–2004 (N = 9). Reports occurred on or very near heavily traversed trails, along canals, or in campgrounds. In all cases, animals were near or very near water. Mean estimated total length of these individuals was 94.3 ± 19.4 cm (61.0–121.9 cm, N = 16), with size estimates not available in four instances. Could the second wave of reports corroborate the notion of a human-mediated boom in the Green Iguana population after Hurricane Andrew, now spilling into the Park? Of the nine reports during 1995–2004, all but two came from the north end of the Park or in association with adjoining canals. The excep- tions were a single individual found along the road just north of the West Lake boat ramp and boardwalk and a single individual found in the Flamingo campground; both likely locations for vehicle-related dispersal from other locations. Reasons for the first pulse of sightings in the 1970s and the subsequent hiatus in the 1980s are unknown, but the opportunity for Green Iguanas to disperse into the Park appears to have increased since the nearly 20 year hiatus in sightings. This phenomenon is most easily explained by connections to the Park via canals that pass through Florida City and Homestead, both of which most certainly sus- tain populations, and by the post-Hurricane Andrew boom in quality habitat. Yet, despite the increased opportunities for dis- persal into the Park, both pulses of sightings represent failed col- onization attempts with no evidence of successful reproduction. We suggest that the reason for this failure is because the Park is, in many ways, what urban and suburban southern Florida is not — namely natural habitat occupied by a suite of well-adapted native predators and potential competitors. In sharp contrast to the nearly ideal conditions experienced by Green Iguanas in the developed areas of Miami-Dade County, the Park has an abundance of predators (including the American Crocodile, a known predator of iguanas), a dearth of suitable fully-insolated nesting sites in areas of open-canopy and well- drained soils, and a food supply that is less than ideal for iguanas, which prefer the more palatable ornamentals so beloved by South Floridians. As long as human intervention with the Park’s water- shed does not alter any of these three factors in any substantive South Floridians often encounter feral Green Iguanas basking in open areas. Photograph by Joe Wasilewski. An accumulation curve of sightings of Green Iguanas in Everglades National Park from 1970–2004. 160 IGUANA • VOLUME 11, NUMBER 3 • SEPTEMBER 2004 MESHAKA, BARTLETT, AND SMITH Geographic distribution of Green Iguanas (Iguana iguana) in Florida. Only those counties with established colonies are indicated; Highlands and Alachua counties are not shaded because those records appear to represent waifs. IGUANA • VOLUME 11, NUMBER 3 • SEPTEMBER 2004 161GREEN IGUANAS IN FLORIDA fashion, the southern Everglades should continue to resist colo- nization by iguanas. The Future? Urban and suburban areas of southern Florida effectively provide a glimpse of the not-too-distant future, when continued growth of the human population and the inevitable alterations of the few remaining natural areas will assure that Green Iguanas will mimic the exceedingly successful Brown Basilisk (Basiliscus vittatus), which has already become an ubiquitous inhabitant of waterside properties, but is held in check solely by penetrating frosts in regions to the north. For Green Iguanas, this translates into a secure coastal existence north to Martin County along the east- ern coast and to near Sarasota, Sarasota County, on the west. Why? Because human-mediated changes will increase opportu- nities for lizards to disperse, the number of wayward pets will increase. Also, the climate is largely amenable and further habi- tat modifications to their liking are likely. Iguanas are innately highly fecund, and, with few exceptions, human attitudes regard- ing their presence range largely from indifference to enjoyment. Only in the ever-shrinking natural areas of the state does the Green Iguana seem ill-suited. As for every other of the 40 exotic species of amphibians and reptiles that are successfully reproducing in Florida, humans are the cause of the Green Iguana’s very existence in Florida and their subsequent success. The evident prosperity of this large herbivore effectively underscores the huge role humans can, even if inad- vertently, play in the good life enjoyed by so many exotic species in the hospitable climes of southern Florida. References Bartlett, R.D. 1980. Non-native reptiles and amphibians in Florida. Indigo, News Bulletin of the Florida Herpetological Society 1:52–54. Bradley, J.T. 1977. Freeze Probabilities in Florida. Bulletin 777. Institute of Food and Agricultural Sciences, University of Florida, Gainesville. Greene, H.W., G.M. Burghardt, B.A. Dugan, and A.S. Rand. 1978. Predation and the defensive behavior of Green Iguanas (Reptilia, Lacertilia, Iguanidae). Journal of Herpetology 12:169–176. King, F.W. and T. Krakauer. 1966. The exotic herpetofauna of southeast Florida. Quarterly Journal of the Florida Academy of Sciences 29:144–154. Meshaka, W.E., Jr., B.P. Butterfield, and J.B. Hauge. 2004. The Exotic Amphibians and Reptiles of Florida. Krieger Publ. Co. Malabar, Florida. Wilson, L.D. and L. Porras. 1983. The ecological impact of man on the South Florida herpetofauna. University of Kansas Museum of Natural History, Special Publication (9):1–89. Green Iguanas using arboreal perches may be difficult to see, explaining why few area citizens are aware of their growing numbers, especially in urban and suburban yards and parks. Photograph by Joe Wasilewski.