Effect of Ascaridia compar infection on rock partridge population dynamics: empirical and theoretical investigations

Helminth parasites have the potential to significantly affect the dynamics of their hosts. As a consequence, they can dramatically threaten the persistence of endangered species, such as rock partridge (Alectoris graeca saxatilis) in the Province of Trento (northern Italy). The aim of this work was to understand the effect of helminth parasites on rock partridge fitness, informing on their potential effects on host population dynamics. In particular, we investigated the hypothesis that infections from Ascaridia compar induce the rock partridge population cycles observed in Trentino. In order to support this hypothesis, we compared the predictions obtained from a host-parasite interaction model including variable parasite aggregation with multi-annual empirical data of A. compar infection in natural host populations. We estimated host demographic parameters from rock partridge census data in Trentino, and the parasitological parameters from a series of experimental infections in a rock partridge captive population. The host-parasite model predicts higher A. compar abundance in rock partridge populations exhibiting cyclic dynamics compared to non cyclic ones. In addition, for cyclic host populations, the model predicts an increase in mean parasite burden with the length of cycle period. Model predictions are well-supported by field data: significant differences in parasite infection between cyclic and non cyclic populations and among cyclic populations with different oscillation periods were observed. On the basis of these results, we conclude that helminth parasites can not be ruled out as drivers of rock partridge population dynamics in Trentino and must be considered when planning conservation strategies of this threatened species.