Differential impact of a shared nematode parasite on two gamebird hosts: implications for apparent competition

If the deleterious effects of non-specific parasites are greater on vulnera ble host species than on reservoir host species then exclusion of the vulne rable host through apparent competition is more likely. Evidence suggests t hat such a mechanism occurs in interactions between the ring-necked pheasan t (Phasianus colchicus), the grey partridge (Perdix perdix), and their shar ed caecal nematode Heterakis gallinarum. Modelling of the system predicts t hat the reduced parasite impact on the pheasant compared to the partridge r esults in the force of infection transmitted from pheasants to partridges b eing sufficient to cause partridge exclusion. Since the parasite impacts ar e currently estimated from correlational work, controlled infections were c onducted to experimentally compare the impact of H. gallinarum on the two h osts and verify cause and effect. While challenged partridges showed reduce d mass gain, decreased food consumption, and impaired caecal activity, in c omparison to controls, the only detectable effect of parasite challenge on the pheasant was impaired caecal activity. The impact of H. gallinarum on c hallenged partridges conforms with previous correlational data, supporting the prediction that parasite-mediated apparent competition with the ring-ni cked pheasant mag. result in grey. partridge exclusion. However, the observ ed decrease in the caecal activity of challenged pheasants could imply that H. gallinarum may also have an impact on the fecundity and survival of phe asants in the wild, particularly if food is limiting. If this is the case, the associated decrease in the force of infection to which the partridge is exposed mar; be sufficient to change the model prediction from partridge e xclusion to pheasant and partridge coexistence.