Who is to blame for mammal declines in the Everglades?

Who would want to harm a cute, cuddly, little bunny? Burmese pythons, it turns out. That’s the result of an experiment recently conducted in Florida’s Everglades. In recent years, the mammal communities of the Greater Everglades Ecosystem – and, in particular, within Everglades National Park – have seriously plummeted. The obvious culprits when entire taxa are suddenly in serious decline are invasive species.

And the Florida Everglades might be thought of as the invasive species capital of Earth. There are more non-native lizard populations there than native ones, for example. Particularly worrisome are the Burmese pythons, large apex predators who will eat just about anything. Estimated to number some 150,000 individuals now, the introduced serpents have firmly established themselves in the southern portion of the Everglades ecosystem, covering most of the Everglades National Park. Corroborating evidence for the python’s culpability in the disappearance of the area’s mammals comes from analysis of the gut contents of captured pythons: some 75% of their diet appears to be comprised of mammals.

But the only evidence linking the pythons to mammalian declines has been, at best, indirect or correlational. And, from the perspective of ecological theory, “[t]here are reasons to question whether pythons or any predator could have caused the precipitous declines seen across a range of mammalian functional groups.” That’s according to a group of wildlife officials and researchers led by Robert A. McCleery of the University of Florida Department of Wildlife Ecology and Conservation.

The reason to doubt the pythons is that marsh rabbits reproduce like… well, like rabbits. They breed throughout the year, producing up to six litters of three to five young annually. Theoretically, they should be making enough new bunnies each year to allow the population to withstand even a voracious predator like the Burmese python.

To put the pythons to the test, the researchers rounded up a bunch of those cute, adorable little bunnies from parts of the Everglades to which the pythons had not yet spread and dropped some of them square in the middle of python territory. If the pythons gobbled them up, that would pretty obviously implicate the snakes in the rabbits’ decline. If the rabbits managed to recolonize and sustain a small population, then perhaps it would not be fair to assign all the blame to the invading reptiles.

In all, the researchers captured, released, and radio-tracked 95 adult marsh rabbits between September 2012 and August 2013. Some of the rabbits were relocated to python territory, others were released at a control site with no pythons (to account for the effects of relocation), and still others were captured, assessed, and released without being relocated (to account for the effects of the capture and handling itself).

Eighty of the 95 rabbits survived the 10-day adjustment period after release. An additional ten of them were lost due to radio malfunction. That left 70 rabbits for the researchers to keep track of. By the end of the study, all but two of them had died. The primary cause of mortality within python territory – explaining 77% of the rabbit deaths – was the Burmese python. Outside of the national park, mammals such as bobcats and coyotes were the rabbits’ primary predators. The lack of mammalian predation within the national park can be explained by the overall decline of mammals there, either directly, because the pythons are eating them too, or indirectly, because they’re competing with pythons for the same food.

Even with such an insatiable predator, ecological theory suggests that the remarkably fecund rabbits should be able to survive. It could be that they simply haven’t evolved to account for a predator as ravenous as the Burmese python. Indeed, the last time massive snakes were present in the eastern US – at least, the last time prior to the recent introduction – was some 16-21 million years ago.

The loss of marsh rabbits and other mammals from Everglades National Park is therefore likely to result in a massive rearrangement of the food web, with unpredictable consequences. If pythons can wipe out a population of mammals as reproductively adept as rabbits, it is reasonably to suspect they’re also responsible for the decline of slower reproducing mammals as well, like raccoons, muskrats, and bobcats.

Introduced predators, including snakes, have certainly been implicated in plenty of species extirpations or extinctions on islands. But humans have been the only non-native apex predator ever known to severely reduce or eliminate a mammal community on the continental mainland. At least, until now. – Jason G. Goldman | 18 March 2015

Source: Robert McCleery et al. (2015). Marsh Rabbit Mortalities Tie Pythons to the Precipitous Decline of Mammals in the Everglades. Proceedings of the Royal Society B: Biological Sciences. DOI: 10.1098/rspb.2015.0120.

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