Temperate freshwater ecosystems are currently being salinised through anthropogenic activities. These increases in freshwater salinity can impact individuals, populations, and species interactions. We studied the effects of salinity on freshwater host–parasite interactions at multiple scales using the zooplankton host, Daphnia dentifera, and a fungal parasite (Metschnikowia bicuspidata). We conducted one experiment at the individual‐level to quantify the effects of salinity on infection prevalence and another to understand the combined population‐level effects of salinity and parasitism. In our first experiment, we found that the effects of salinity on infection prevalence varied by host genotype; increased salinity reduced infection prevalence in one genotype but had no effect on infection prevalence in another. In our second experiment, infection prevalence was lower when NaCl was added to the microcosms compared to the control (no added salt) treatments. We also found a significant parasite × salinity interaction on D. dentifera density in our second experiment, where the parasite only reduced host densities in our control treatment, probably due to the reduced infection prevalence as salinity increased. This study demonstrates that salinity can influence infection prevalence in freshwater hosts and that host population density may respond to the combined effects of salinisation and parasitism in a non‐additive manner.

It is well-established that both resources and infectious disease can influence species invasions, but little is known regarding interactive effects of these two factors. We performed a series of experiments to understand how resources and parasites can jointly affect the ability of a freshwater invasive zooplankton to establish in a population of a native zooplankton. In a life history trial, we found that both species increased offspring production to the same degree as algal resources increased, suggesting that changes in resources would have similar effects on both species. In a microcosm experiment simulating an invasion, we found that the invasive species reached its highest densities when there was a combination of both high resources and the presence of a shared parasite, but not for each of these conditions alone (i.e., a significant resource x parasite interaction). This result can be explained by changes in native host population density; high resource levels initially led to an increase in the density of the native host, which caused larger epidemics when the parasite was present. This high infection prevalence caused a subsequent reduction in native host density, increasing available resources and allowing the invasive species to establish relatively dense populations. Thus, in this system, native communities with a combination of high resource levels and parasitism may be the most vulnerable to invasions. More generally, our results suggest that parasitism and resource availability can have interactive, non-additive effects on the outcome of invasions.