Native fly parasites are the principal cause of nestling mortality in endangered Tasmanian pardalotes

Abstract

Abstract Established host–parasite interactions at an evolutionary equilibrium are not predicted to result in host population decline. However, parasites may become a major threat to host species weakened by other factors such as habitat degradation or loss of genetic diversity in small populations. We investigate an unusually virulent Tasmanian ectoparasite, Passeromyia longicornis , in its long‐term hosts, the endangered forty‐spotted pardalote ( Pardalotus quadragintus ) and striated pardalote ( Pardalotus striatus ) in southeastern Tasmania, Australia. We conducted a parasite elimination experiment to determine the net effect of parasites on forty‐spotted pardalote nestling mortality, and monitored nestling parasite load and mortality in forty‐spotted and striated pardalote nestlings during two breeding seasons (Aug–Jan, 2013–2015). Passeromyia longicornis larvae killed 81% of all forty‐spotted pardalotes nestlings. Across 2 years, forty‐spotted pardalotes fledged fewer nestlings (18%) than sympatric striated pardalotes (26%), and this difference was generated by a combination of higher parasite load and virulence in forty‐spotted pardalote nests. These results show that native ectoparasites can cause substantial and potentially unsustainable mortality in their hosts.