The opposing roles of lethal and nonlethal effects of parasites on host resource consumption

Although parasites can kill their hosts, they also commonly cause nonlethal effects on their hosts, such as altered behaviors or feeding rates. Both the lethal and nonlethal effects of parasites can influence host resource consumption. However, few studies have explicitly examined the joint lethal and nonlethal effects of parasites to understand the net impacts of parasitism on host resource consumption. To do this, we adapted equations used in the indirect effects literature to quantify how parasites jointly influence basal resource consumption through nonlethal effects (altered host feeding rate) and lethal effects (increased host mortality). To parametrize these equations and to examine the potential temperature sensitivity of parasite influences, we conducted a fully factorial lab experiment (crossing trematode infection status and a range of temperatures) to quantify feeding rates and survivorship curves of snail hosts. We found that infected snails had significantly higher mortality and ate nearly twice as much as uninfected snails and had significantly higher mortality, resulting in negative lethal effects and positive nonlethal effects of trematodes on host resource consumption. The net effects of parasites on resource consumption were overall positive in this system, but did vary with temperature and experimental duration, highlighting the context dependency of outcomes for the host and ecosystem. Our work demonstrates the importance of jointly investigating lethal and nonlethal effects of parasites and provides a novel framework for doing so.

Automated summary

Parasites can have both lethal and nonlethal effects on their hosts, influencing resource consumption. However, little research has investigated the joint impacts of these effects. In this study, we used equations from the indirect effects literature to quantify the combined influence of altered feeding rate and increased mortality due to parasitic infection. Our experimental results showed that infected snails had higher mortality rates and consumed more resources, resulting in positive nonlethal effects and negative lethal effects on resource consumption. These effects varied with temperature and experimental duration, emphasizing the context-dependent nature of parasite-host interactions.