By Jessica Baggott, Evolution Ecology and Biodiversity Major, Professional Writing Minor, ’23

Author’s note: I wrote this piece in the Spring Quarter of 2022 for UWP 102B, Writing in the Disciplines: Biology. I wrote this piece partially because I have always fostered an interest in invasive species — how they enter, alter, and succeed in ecosystems. And, how we as scientists and policymakers address these threats to native ecosystems. I was also compelled to write this review because of the abundance of recent literature and the lack of another review, to my knowledge, that covered the same topics as I intended to.

I hope that readers walk away from this piece with a greater understanding of the Burmese python in the Florida Everglades — their invasion, success, and alterations to a fragile and precious ecosystem. I wish for readers to recognize the connections that I have made, combing through the literature, and I wish for them to make their own connections, too. There is no greater gift than your engagement with my work.

INTRODUCTION

Southern Florida’s Greater Everglades Ecosystem (GEE) once included over 8 million acres of 0.5-2.0 foot deep wetland from the Kissimmee Chain of Lakes just south of Orlando to the southern tip of Florida Bay [1]. Now, the GEE is estimated to be half of its historical size and is fragmented into various national, state, regional, and local parks as well as more than 12 wildlife refuges and marine preserves [2, 3, 4]. Everglades National Park (ENP), one of the federally protected regions of the GEE, only includes 1.5 million acres of this vast ecosystem [5]. However, even within the protected region of ENP, canals, pump stations, and roads have been  constructed to increase human accessibility to the Everglades, severely altering precise hydrological processes [1, 6]. These hydrological alterations, encroaching human settlements, degraded water quality, anthropogenic climate change, and the introduction of invasive species all pose significant threats to the GEE, and work in conjunction to increase negative effects on the GEE [4]. 

Perhaps the most infamous invasive species in the U.S., the Burmese python is the most well known threat to the GEE (Python molurus bivittatus). The snakes’ long lifespan, high fecundity or ability to produce offspring, as well as their generalist lifestyle which allows them to adapt their behavior and dietary habits to their environment, has allowed a small number of pythons to establish and thrive in the GEE [7]. Currently, Burmese pythons are drastically altering trophic structures as well as introducing and transmitting disease in the GEE. Furthermore, Burmese pythons have and have the potential to extend their range northward, putting other ecosystems and species at risk. A comprehensive literature review is required to inform policy decisions and assess the risk posed by Burmese pythons beyond the GEE.

Background

Native to Southeast Asia, the Burmese python was introduced into the GEE in the 1980s during a boom in the exotic pet trade and the subsequent release of the snakes into the Everglades by owners [7]. Since being first recognized in ENP in 2000, the invasive range of the Burmese python has rapidly expanded to the entirety of ENP and much of Big Cypress National Preserve [8]. However, population estimates have been hindered by the combination of cryptic python behavior (including long periods of inactivity), excellent natural camouflage, and human park management goals that include the removal of every python encountered without necessarily documenting the removed numbers [9]. These factors have caused extremely low python detection probabilities, ranging from 0.0001 to 0.0146 using visual surveys and radio transmitters [9]. Given low detection probability, population estimates range from tens of thousands to hundreds of thousands [9, 10]. Better population estimates are required for effective management strategies and to monitor changing populations of pythons [9].

Northward Range Expansion

Burmese pythons exhibit seasonal habitat preference, primarily choosing covered habitats close to water, though recent studies have found evidence that they may also be attracted to human development [11-17]. Smith et al. (2021) found that within their native range in Thailand, Burmese pythons do not avoid human dominated landscapes. Similarly, Bartoszek et al. (2021) found that in a northwest portion of ENP, within their invasive range, Burmese python hotspots were merely 515 meters from urban development on average. Researchers attributed this proximity to high quantities of readily available prey in these areas, in the form of livestock and birds attracted to the artificial lakes [11, 16]. However, egg clutches deposited in or near urban areas may exhibit lower survival rates than those in other habitats [8]. Though juveniles can travel long distances, particularly through use of agricultural canals, Pittman & Bartoszek (2021) hypothesize that in fact adult pythons with more sophisticated navigational capacities are the population driving expansion [18]. Adult sufficiency in and attraction to urban environments indicates that northward Burmese python expansion may not be hindered by human settlements. 

Besides suitable habitat, the range of ectotherms such as the Burmese python is typically limited by climate and/or the possession of behavioral adaptations such as retreating into underground refugia during winter months [19]. Though a conservative estimate allows Burmese pythons to survive for short periods of time at 5 °C,, temperatures must be above 16 °C in order for them to maintain digestion [19]. In isolation, these requirements make further expansion of the Burmese python in more northern parts of Florida extremely unlikely without the additional development of hibernation behaviors [19]. However, other researchers have found evidence of rapid adaptation for increased thermal tolerance after an extreme cold event in 2010 that caused high python mortality [20]. Adaptations included the maintenance of an active digestive system and changes in gene expression related to regenerative organ growth and behavior [20]. This rapid evolution by natural selection may permit Burmese pythons to expand their range northward into more temperate climates. 

However, there have been no studies in the last decade examining Burmese python’s potential for northward expansion, despite advances in climate and habitat models, tracking, and a greater understanding of Burmese python cold physiology. What studies do exist were inconclusive and results varied greatly: Rodda et al. (2009) and Pyron et al. (2008) provided oppositional potential range estimates. Rodda et al. (2009) concluded that the potential Burmese python range could include most of the southern U.S., from California through North Carolina. In contrast, Pyron et al. (2008) only included southern Florida and extreme southern Texas as the potential range of Burmese python expansion. Previous studies examining potential Burmese python range primarily agreed with Pyron et al. (2008) and all but two directly refuted the range suggested by Rodda et al. (2009) [19, 23-25]. Furthermore, climate change is projected to decrease the frequency and intensity of cold events in North America, allowing tropical species historically found at or near the equator, such as the Burmese python, to move poleward [26]. A literature review examining potential northward expansion of tropical organisms as a whole, with brief mentions of the Burmese python in Florida, posits that Burmese python range expansion is likely given the evidence for rapid adaptation for cold tolerance presented by Card et al. (2018) [26]. However, a complete understanding of the adaptive capacity of species, ecosystems, and biomes to climate change still remains lacking [26]. 

In addition to rapid adaptation to cold temperatures, Burmese pythons have shown evidence of hybridizing with another closely related invasive species, the Indian python (Python molurus) [27]. Hybridization has increased the population’s genetic diversity and allowed Burmese pythons to mitigate the founding and bottleneck effects — loss of genetic diversity due to a small founding population size or environmental effects [27]. Additionally, Hunter et al. (2018) found evidence of multiple paternity—the insemination of a female by more than one male during a single reproductive event—in Burmese pythons, also increasing python diversification rate. These behaviors allow for pythons to increase genetic diversity and will likely increase fitness, increasing the probability of northward expansion. 

Burmese Python Presence (1979–2016), Conyers & Sen Roy 2021.

Disease 

The invasion of the Burmese python in the GEE has introduced at least one pathogen, a lung parasite known as Raillietiella orientalis. Lacking coevolution with North American hosts, the spread and severity of this pathogen has increased in native species. This parasite now affects 13 species of native snakes and has extended beyond the python range into north central Alachua County, Florida, approximately 170 miles from the northernmost point of the GEE [28-30]. Researchers observed higher infection intensity, prevalence, and body size of R. orientalis in native snakes than in Burmese pythons, as native snakes do not share evolutionary history with R. orientalis and therefore are immunologically naive [29]. Infection by R. orientalis may be lethal or sublethal, and may be the cause of population decline of the pygmy rattlesnake [29, 31]. Additionally, R. orientalis’ native snake hosts have the highest rate of competence, or are most likely to transmit a resultant infection to a new host or vector after being exposed to a parasite. Furthermore, as R. orientalis’ native snake hosts are three of the most abundant snakes in North America [29], the parasite has a high likelihood of continued expansion throughout North America and possibly beyond [29]. Since the snakes of North America have not coevolved with R. orientalis, infections will be more severe and may cause population wide declines potentially resulting in devastating trophic cascades. The negative effects of the introduced parasite compound with those of Burmese python predation create weakened native populations more susceptible to parasitism, disease, and other stressors. More research is needed to ascertain the complete range of R. orientalis, expansion rate, intermediate hosts, sublethal effects on native snakes, and impact on populations. 

In addition to introducing a novel pathogen, Burmese pythons are competent hosts of at least one native pathogen and are suspected to be competent hosts of more [28, 32]. As a competent host to native pathogens, the Burmese python likely acts as a reservoir for these pathogens, and increases transmission to native species and humans [28, 32]. However, Burmese pythons are also able to change disease transmission through alteration of host communities via predation. Such is the case with the endemic Everglades Virus (EVEV), which can cause inflammation of the active tissues of the brain, known as clinical encephalitis, in humans. Decreased mammal diversity as a result of Burmese python predation was found to increase blood meals on amplifying hosts—hosts in which infectious agents multiply rapidly to high levels—increasing EVEV infection in mosquitoes [12]. Thus, it is possible that Burmese pythons could increase disease prevalence in humans as well, though contact with infected hosts is required for spread and therefore human disease may be driven by different factors than those in the mosquito-rodent cycle [12]. Understanding of the complex relationship between Burmese python predation on host species while also acting as hosts themselves remains lacking for many other important diseases, and presents an opportunity for future research. Additionally, studies should be conducted to estimate human risk as a result of the Burmese python altering host communities.

Further disease spillback is mediated by elevated rates of mosquito feedings on Burmese pythons [32]. The mosquitos that prefer feeding on Burmese Pythons also feed on a range of other species, including mammals, birds, reptiles, and amphibians [32]. Additionally, mosquito ranges extend beyond that of the Burmese python [32]. Thus, through both preferential feeding by mosquitoes on Burmese pythons and large mosquito range, the introduction of the Burmese python into the Everglades has increased disease spread beyond the python range.

Predation

The Burmese python has more than 40 prey documented in the Everglades, including a wide range of mammals and birds, and occasionally American alligators [33]. Given their appetite and potentially large population numbers, Burmese pythons are able to exert control over species populations. The decline of particular species relative to others can then cause ecosystem-wide cascades. Pythons have been found to cause severe mammal population declines through predation in their invasive range including 99.3%, 98.9%, and 87.5% decreases in observation frequency of raccoons, opossum, and bobcats respectively [33, 34]. Additionally, pythons have caused a complete local extinction of marsh rabbits, once one of the most commonly seen animals in ENP [33, 35, 36]. When reintroduced to ENP, marsh rabbits were able to establish a breeding population five months after translocation, but by 11 months after reintroduction, 77% of deaths were attributed to Burmese pythons and the population was unable to reestablish [35]. This disproportionate predation makes the reestablishment of this and other similarly affected species impossible as long as the python persists. Similarly, an analysis of anthropogenic stressors and those posed by pythons found that the strongest predictor for marsh rabbit occurrence was distance from the epicenter of python invasion [36]. These results indicate that pythons have profound effects on ecosystem composition through predation and are able to cause trophic cascades, damaging the ecosystem. Additionally, as is the case with Marsh Rabbits, species may be unable to reestablish in the core invasion area, even with translocation efforts. This demonstrates that without removal of Burmese pythons from the GEE, biodiversity and community composition of the GEE may be irreparably damaged.

Large, highly fecund species with wide habitat breaths were found to be the least susceptible to increased pressure from pythons, so the decline of a highly fecund and habitat generalist such as the marsh rabbit is especially concerning [37]. Using trait relationships, researchers predicted exclusively negative responses in occupancy probabilities to the presence of Burmese pythons regarding five unobserved species of concern: the everglades mink, feral hog, gray fox, red fox, and Key Largo woodrat [37]. Though rodent populations were previously thought to be resistant to the effects of pythons, declines in these populations have also been observed, and due to their lack of evolutionary history, one species, the Eastern woodrat, has even been suggested to be attracted to python scent [34, 38]. These results and research conducted on mammal resilience to pythons have shown that there is little evidence of resilience among mammals within the core invasion area, which only further contributes to the homogenization of the ecosystem [34]. Additionally, it is likely that loss of diversity and competition will allow other invasive species to establish more easily [34]. The results show the need for continued monitoring of species to analyze trends, research on response to novel predators, and the mechanisms for negative responses of native species to Burmese pythons. Furthermore, these results suggest that removal or significant population reduction of Burmese pythons may be the only way to curb their negative impacts. 

CONCLUSION

The purpose of this review was to examine the effects of the Burmese python in the GEE through predation, introduction and alteration of disease transmission, and potential range expansion. It is evident from this review that the Burmese python, through predation trophic alteration, has had severe effects on the native fauna of the GEE. Ultimately, it is the lack of coevolution between the Burmese python and native fauna that have led to the acute and persistent problems in the GEE. Burmese python establishment in the GEE has proved to be extremely detrimental to an ecosystem already facing considerable anthropogenic stressors. Given this, special attention should be paid to curb further Burmese python expansion to avoid similar ecological catastrophes due to the Burmese python. Further studies should be conducted regarding native resilience and recovery as populations eventually enter the third stage of invasion. Additionally, studies should be conducted to better quantify python density as to frame future understanding of ecosystem dynamics. The Burmese python is a prime example of many regarding invasive species across the globe. So, it is not only critical to better understand these aspects of python success and native fauna response, but the results may be applicable in the broader effort to manage invasive species. 

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