Human activities have accelerated the introduction of exotic species, leading to diverse and often harmful impacts on native communities. One way invasive plants can influence aquatic ecosystems is by altering water chemistry via leaf litter inputs. Shifts in abiotic conditions may have cascading consequences on ecological relationships between native species (i.e. host-parasite interactions), yet few studies have considered these effects. We created aquatic communities using leaf litter from two pairs of invasive and native plants (autumn olive versus black huckleberry, and purple loosestrife versus swamp loosestrife). Using American toads and Echinostome parasites, we examined how solutions produced by native vs. invasive leaf litter altered host-parasite interactions. We first reared toad tadpoles in outdoor communities containing one of the four leaf litters. To isolate the effect of leaf litter on host susceptibility to parasites, we exposed a subset of tadpoles from the outdoor communities to parasites in clean water in the lab. To quantify net effects of leaf litter on host-parasite interactions, we exposed another subset of tadpoles from the outdoor communities to parasites in leaf litter solutions in the lab. Finally, to mimic more realistic spatial environments, we introduced parasites to tadpoles in outdoor communities and measured parasite encystment.
Results/Conclusions
In the laboratory setting, when tadpoles were exposed to parasites in clean water, tadpoles reared in invasive litters were 58% - 71% more susceptible to parasites than tadpoles reared in native litters. However, when tadpoles were exposed to parasites in leaf litter solutions, tadpoles reared in autumn olive were 65% less susceptible than those reared in black huckleberry solutions, while tadpoles reared in purple loosestrife and swamp loosestrife were similarly susceptible to parasites. Finally, in outdoor communities, we found no effect of leaf litter treatment on disease outcome. We found that tadpoles reared in invasive leaf litters were more developed than those reared in native leaf litters, and development was negatively correlated with encystment. Taken together, it appears that invasive leaf litter may accelerate amphibian development, and subsequently reduce their susceptibility to parasites. However, there seemed to also be strong impacts of leaf litter on parasite infectivity, mitigating impacts on host susceptibility in more realistic settings. We show that the effects of invasive species on native species interaction are complex, underscoring the importance of considering the consequences of invasions in multiple environmental contexts.