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

Abstract

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.

The role of small‐scale environmental gradients on trematode infection

“…These mortality rates may result in skewed estimations of infection risk, if infected snails are more likely to die in the field. Evidence from the lab suggests that infected snails are more likely to die, so this may also hold true in the field (Resetarits et al, 2023). This mortality resulted in small numbers of surviving snails in some of the ponds, which probably increased stochasticity and limited some of our analyses.…”

“…Snails that shed cercariae were marked as infected and those that did not shed and laid eggs (indicating intact gonads) were marked as uninfected. This method was used rather than simply dissecting the snails because infected and uninfected snails were used for a subsequent experiment (Resetarits et al, 2023). Cercariae were identified to morphotype under a light microscope using relevant literature (Schell, 1985).…”