Divergence in host–parasite interactions during the cane toad's invasion of Australia

Coevolutionary host–parasite “arms races” can be disrupted by new evolutionary forces imposed by biological invasions, affecting both host and parasite densities, as well as their traits. The spread of cane toads (Rhinella marina) and their parasitic lungworm (Rhabdias pseudosphaerocephala) across tropical Australia provides an ideal opportunity to study a perturbed host–parasite system. We conducted a cross-infection experiment using common-garden-reared toads and lungworms from three regions (comprising long-established, intermediate, and recently invaded sites across tropical Australia) to quantify traits of the parasite (infectivity and virulence) and the host (resistance and tolerance). Specifically, we assessed whether patterns of host–parasite co-adaptation were better explained by spatial distances versus time since the separation of host and parasite populations. Infection success was highest when toads from long-established populations were exposed to lungworms from close to the invasion front and lowest when frontal toads were exposed to parasites from older populations. This suggests that both parasite infectivity and host resistance have increased over the course of invasion. In contrast, most virulence/tolerance traits have not consistently increased or decreased during invasion. Higher parasite burdens were associated with increased feeding performance in toads and faster growth in lungworms, counter-intuitive results that suggest host manipulation by the parasite.