The invasive Burmese python (Python molurus bivittatus or Python bivittatus) has been suspected in the drastic decline of mammal populations in Everglades National Park (ENP) over the last several decades, The park is globally recognized for its unique biotic communities, sits at the southern end of the Greater Everglades Ecosystem (GEE), a vast freshwater wetland (≈10 000 km2) encompassing most of the southern Florida peninsula. The ecological processes, functionality and restoration efforts within this distinct ecosystem are probably being substantially impaired by the disappearance of once common mammalian predators and herbivores. Declines in mammal populations in ENP appear to coincide temporally and spatially with the arrival and spread of invasive Burmese pythons, a large-bodied snake native to Southeast Asia that preys on vertebrates. Pythons were probably introduced into ENP several decades ago via releases or escapes from private ownership. Sightings and removals of pythons in ENP were sporadic in the 1980s and 1990s, and increased sharply in the early 2000s. During this time, gut content analysis of invasive pythons in ENP indicated that mammals accounted for about 75% of their diet.
Previous evidence linking pythons to mammal declines has been indirect, and there are reasons to question whether pythons or any predator could have caused the precipitous declines seen across a range of mammalian functional groups. Introduced predators, including snakes, have reduced or eliminated fauna on islands, yet there are no accounts of a lone introduced apex predator (apart from humans) removing a functionally diverse, continental mammal community. Additionally, ecological theory provides little support for the hypothesis that an apex predator could extirpate small, broadly dispersed, fecund, generalist herbivores.
In a new article in the Proceedings of the Royal Society of London B: Biological Sciences McCleery et al. (2015) test the hypothesis that pythons are driving the decline of mammal populations, and they experimentally manipulated the marsh rabbit (Sylvilagus palustris) populations in ENP.
Marsh rabbits are small (≈1 kg) lagomorphs found near fresh and brackish water throughout the southeastern USA. They are sexually active throughout the year and can produce up to six litters of three to five young annually. Through the 1980s, this species was one of the most commonly seen mammals in ENP. Despite having a wide variety of natural predators, marsh rabbits are still common in areas of the GEE outside of ENP. For these reasons, and because rabbit populations are generally resilient and capable of persisting under considerable predation pressure the authors chose marsh rabbits as a model to understand the impacts of pythons on mammals in ENP. If pythons caused the declines of marsh rabbits in ENP, they predicted that (i) pythons would be the dominant cause of marsh rabbit mortality in ENP, (ii) mammals would cause more marsh rabbit mortalities in areas of the GEE where pythons were rare or absent, (iii) marsh rabbit populations introduced in ENP would not persist, and (iv) unlike endothermic predators (i.e. mammals), the timing of python-caused mortality would vary with seasonal climate conditions.
The authors experimentally manipulated the marsh rabbit populations to determine the role of pythons in driving mammal declines within ENP. They compared the risk of mortality from different causative agents in areas with established Burmese python populations to similar areas where pythons were rare or absent. The also evaluated the influence of environmental factors on temporal variation in mortality rates from the dominant predators of marsh rabbits (e.g. pythons and mammals) in the GEE.
They captured marsh rabbits from donor populations and randomly assigned them to one of three sites: two sites in ENP and a procedural control site in the GEE, where pythons had not been observed. The purpose of the procedural control was to account for the influence of translocation on mortality events. The also established a control site where pythons were rare or absent to compare causes of mortality in ENP with an established population of marsh rabbits that was not manipulated. At the control site, they captured and released rabbits without trans-locating them.
They captured, released and radiotracked 95 adult marsh rabbits from 14 September 2012 to 19 August 2013 (coastal ENP = 15, freshwater ENP = 16, procedural control = 15, control = 49). Eighty rabbits survived the 10-day adjustment period needed to reduce exploratory movements and acclimate to the sites. Additionally, 10 rabbits were censored after the 10-day adjustment period (e.g. lost signal from predation or equipment failure), and two rabbits were alive at the end of the study, so the study documented 68 rabbit mortalities. Classification of the cause of mortality for 55 rabbits was possible and the remaining 13 rabbit mortalities were listed as being from an unknown endothermic predator. Marsh rabbits in ENP faced the greatest risk of predation from pythons, which accounted for 77% of all mortalities. And, the authors attributed only one rabbit mortality in ENP to mammal predation. This was in stark contrast to results from control sites, where no rabbits were killed by pythons and they attributed 71% of classified mortalities to mammals. Only three rabbits (8%) at the control site were eaten by snakes, all native eastern diamondback rattlesnakes (Crotalus adamanteus).
The loss of marsh rabbits and other mammals from ENP is probably causing a massive rearrangement of the ENP food web, losses in ecosystem function, and complex and unpredictable cascading effects. As prey and predators at multiple trophic levels, nutrient cyclers and engineers of vegetation, mammals are an indispensable component of the GEE. This research clearly establishes pythons as a causal agent of marsh rabbit declines, a species selected because of its theoretical resilience to predation pressure. Accordingly, pythons are a logical and likely explanation for the observed declines in less fecund mammalian prey found in ENP (raccoon, round-tailed muskrat, bobcat).
Only with the recovery of the parks mammal populations will it be possible to restore the health and functionality of this World Heritage Site. However, it seems unlikely that marsh rabbits and other mammal populations will rebound without action to manage pythons. Because pythons are capable of persisting in the environment by switching to different prey and going long periods without food controlling them is a difficult challenge. Without effective tools and a strategy for reducing the prevalence of these invasive snakes, the dire state of mammals in the Everglades will probably remain unchanged, and spread if python populations expand northward or become established elsewhere in the USA.