Featured Image: Invasive Burmese pythons have transported parasites from their native range in Asia, causing infections in Florida’s native snakes. They also compete with native snakes for prey and have caused severe mammal declines. Photo by Skeeze on pixabey
Reference: Miller, M.A.; Kinsella, J.M.; Snow, R.W.; Hayes, M.M.; Falk, B.G.; Reed, R.N.; Mazzotti, F.J.; Guyer, C.; Romagosa, C.M. (2018). Parasite spillover: indirect effects of invasive Burmese pythons, Ecology and Evolution, 8, 830-840. DOI: 10.1002/ece3.3557
Many exotic species are inadvertently introduced into nonnative ranges across the globe, and chances are they do not make the journey alone. Introduced species bring their own stowaways – parasites, fastened onto or tucked inside their bodies, hitch-hiking to a new frontier. Parasites often go undetected due to their small size and cryptic nature, so revealing their impact in foreign ranges provides an unexplored and compelling research area for scientists focused on invasion ecology.
Consider the parasites of pythons. Exotic Burmese pythons have been introduced in southern Florida through the pet trade, and have since established breeding populations in Everglades National Park and surrounding swamplands. The direct effects of pythons, mainly their voracious predation on native mammals (1,2) have historically received more attention than indirect effects, such as interactions between python parasites and native reptiles.
A team led by researchers from Alabama’s Auburn University have made an important first step in understanding how parasites figure into the high-profile Burmese python invasion. Dr. Melissa Miller and colleagues documented that Burmese pythons have introduced Asian parasites into their North American range and these parasites now infect native Florida snakes. But this is just one strand in a tangled web of interactions between multiple hosts and parasites in the south Florida ecosystem.
Spilling Over and Back
“Spillover” occurs when a parasite adapted to one host jumps ship and transmits to a new host species. In the context of invasion, spillover can work in two directions: introduced hosts can spread their exotic parasites to native species or, in the opposite pattern, native hosts can share their parasites with invaders. In a further plot twist, parasites that spillover can grow and reproduce in the novel host population, and then “spillback” to infect their original hosts with renewed force (3). From a native host’s perspective this is somewhat like a bullet ricocheting back at the shooter – at first glance, spreading parasites to invaders has the promise of suppressing invasion, but it also poses the risk of amplifying infections back to native populations.
In south Florida’s snake community, we know the stage is set for natives to experience parasite spillback. Miller and colleagues documented parasite spillover in both directions: exotic python parasites now infect native snakes, and parasites endemic to native snakes infect the invasive python.
Searching for parasitic arthropods called pentastomes, which mainly infect the host’s respiratory tract, researchers dissected over 1000 invasive and native snakes. These included Burmese pythons and members of 26 native species collected from areas that either overlap with the invasive pythons’ or have remained uninvaded. To avoid killing native snakes for the study, they walked roadsides at night, harvesting snakes that had been killed by vehicles. Parasites dissected from the snakes were identified by inspecting their physical traits and sequencing their DNA.
Researchers discovered a pentastome parasite of Asian origin, Raillietiella orientalis, occurring in Burmese pythons and in the native Florida snakes that share its invaded territory. None of these parasites were discovered in native snakes from regions outside the pythons’ range—a clear implication that pythons are responsible for the spillover of the exotic parasite.
Bad news for native snakes?
But Burmese pythons are not only the instigators—they are also on the receiving side of parasite spillover. A second pentastome that is native to the Americas, Porocephalus crotali, was detected in pythons and native snakes alike. The spillover of the American pentastome into pythons came as a surprise; P. crotali had only been observed in American vipers, so snakes from other families were not expected to be suitable hosts for this parasite. By successfully infecting invasive pythons, P. crotali has tapped into an expansive reservoir of hosts in which it could multiply—a prerequisite for parasite spillback into native American vipers.
Documenting the presence of parasites is simpler than finding conclusive evidence for their effects on host populations, which is the next task these researchers have set for themselves. Measuring the proportion of hosts infected, the intensity of those infections, and the impacts of parasitism on invasive and native snakes can clarify whether parasites will dampen or magnify the severe impact Burmese pythons have on North American wildlife.
1. Dorcas, M.E.; Willson, J.D.; Reed R.N.; Snow, R.W.; Rochford, M.R.; Miller, M.A.; Mehsaka, W.E. Jr.; Andreadis, P.T.; Mazzotti, F.J.; Romagosa, C.M.; Hart, K.M. (2012). Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park, Proceedings of the National Academy of Sciences, 109, 2418-2422. DOI:10.1073/pnas.1115226109
2. McCleery, R.A.; Sovie, A.; Reed, R.N.; Cunningham, M.W.; Hunter M.E.; Hart, K.M. (2015). Marsh rabbit mortalities tie pythons to the precipitous decline of mammals in the Everglades, Proceedings of the Royal Society B: Biological Sciences, 282:1805, 20150120. DOI:10.1098/rspb.2015.0120
3. Chalkowski, K.; Lepczyk, C.A.; Zohdy, S. (2018). Parasite ecology of invasive species: Conceptual framework and new hypotheses, Trends in Parasitology, 34:8, 655-663. DOI:10.1016/j.pt.2018.05.008