Trait-mediated indirect effects, predators, and disease: test of a size-based model

Abstract

Increasing prevalence of wildlife disease accentuates the need to uncover drivers of epidemics. Predators can directly influence disease prevalence via density-mediated effects (e.g., culling infected hosts leading to reduced disease prevalence). However, trait-mediated indirect effects (TMIEs) of predators can also strongly influence disease--but predicting a priori whether TMIEs should increase or decrease disease prevalence can be challenging, especially since a single predator may elicit responses that have opposing effects on disease prevalence. Here, we pair laboratory experiments with a mechanistic, size-based model of TMIEs in a zooplankton host, fungal parasite, multiple predator system. Kairomones can either increase or decrease body size of the host Daphnia, depending on the predator. These changes in size could influence key traits of fungal disease, since infection risk and spore yield increase with body size. For six host genotypes, we measured five traits that determine an index of disease spread (R 0). Although host size and disease traits did not respond to kairomones produced by the invertebrate predator Chaoborus, cues from fish reduced body size and birth rate of uninfected hosts and spore yield from infected hosts. These results support the size model for fish; the birth and spore yield responses should depress disease spread. However, infection risk did not decrease with fish kairomones, thus contradicting predictions of the size model. Exposure to kairomones increased per spore susceptibility of hosts, countering size-driven decreases in exposure to spores. Consequently, synthesizing among the relevant traits, there was no net effect of fish kairomones on the R 0 metric. This result accentuates the need to integrate the TMIE-based response to predators among all key traits involved in disease spread.

Poor resource quality lowers transmission potential by changing foraging behaviour

“…; Bertram et al . ). The increase in parasite reproduction with host size is also consistent with studies of many other invertebrate hosts (Johnson et al .…”

“…body size and initial and final concentrations of food; Sarnelle & Wilson ; Bertram et al . ). Best‐fit estimates of and were obtained by minimizing the sum of the negative log‐likelihood values produced from fitting the transmission and foraging models.…”

“…; Bertram et al . ). Additionally, high‐quality food could enhance transmission potential by promoting host growth and production of fungal spores (Hall et al .…”

Bigger is better: changes in body size explain a maternal effect of food on offspring disease resistance

“…In addition to maternal food, Daphnia body size is determined by a number of factors (e.g., genetics, predator cues, clutch position), and these factors also have the potential to affect susceptibility through their effect on size. Body size effects might explain the variation in infection levels observed amongst Daphnia genotypes (Stjernman & Little, 2011) as well as providing a mechanistic link for phenomena such as the interplay between predator and parasite defense (Bertram, Pinkowski, Hall, Duffy, & Cáceres, 2013). Anything that changes the size-structure of populations, like size-selective predation (Galbraith, 1967;Gibson, 1980;Riessen & Young, 2005), also has the potential to influence disease resistance.…”

Habitat, predators, and hosts regulate disease in Daphnia through direct and indirect pathways

“…In our system, fish and midge predation can regulate the depths at which focal hosts and spore predators migrate and reside (Leibold 1991, Gonzalez andTessier 1997), possibly influencing contact with parasites. One such trait for Daphnia is body size: larger hosts have higher exposure rates and larger spore yields, both of which can increase disease (Hall et al 2007, Duffy et al 2011, Bertram et al 2013, Civitello et al 2015b, Strauss et al 2015. One such trait for Daphnia is body size: larger hosts have higher exposure rates and larger spore yields, both of which can increase disease (Hall et al 2007, Duffy et al 2011, Bertram et al 2013, Civitello et al 2015b, Strauss et al 2015.…”