Parasite spillover to native hosts from more tolerant, supershedding invasive hosts: implications for management.
Introduced hosts are capable of introducing parasite species and altering the abundance of parasites that are already present in native hosts, but few studies have compared the tolerances of native and invasive hosts to introduced parasites or identified the traits of introduced hosts that make them supershedders of non-native parasites. Here, we compare the effects of a nematode Aplectana hamatospicula that is native to Cuba but appears to be introduced to Florida on the native Floridian treefrog, Hyla femoralis, and on the Cuban treefrog (CTF), Osteopilus septentrionalis. We were particularly interested in CTFs because their introduction to Florida has led to reported declines of native treefrogs. In the laboratory, infection with A. hamatospicula caused a greater loss in body mass of H. femoralis than CTFs despite H. femoralis shedding fewer total worms in their faeces than CTFs. Field collections of CTFs, H. femoralis, and another native Floridian treefrog, H. squirella (Squirrel treefrog) from Tampa, FL also showed that CTFs shed more larval worms in their faeces than both native frogs when controlling for body size. Hence, the non-native CTF is a supershedder of this non-native parasite that is spilling over to less tolerant native treefrogs. Any conservation intervention to reduce the effects of CTFs on native treefrogs would benefit from knowing the traits that contribute to the invasive host being a supershedder of this parasite. Hence, we conducted necropsies on 330 CTFs to determine how host sex and body size affect the abundance of A. hamatospicula, and two other common parasites in this species (acuariid nematodes and trematode metacercariae). There was a significant linear increase in A. hamatospicula and encysted acuariids with CTF body size, but there was no detectable relationship between host body size and the intensity of metacercariae. Female CTFs were bigger, lived longer and, on average, had more A. hamatospicula than male CTFs. Synthesis and applications. These results of the study suggest that there is parasite spillover from the invasive Cuban treefrog (CTF) to native treefrogs in Florida. Additionally, at least some of the adverse effects of CTFs on native treefrogs could be caused by the introduction and amplification of this introduced parasite, and female and larger CTFs seem to be amplifying these infections more than males and smaller CTFs, respectively, suggesting that management could benefit from targeting these individuals.