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.
Citation