Host Density and Competency Determine the Effects of Host Diversity on Trematode Parasite Infection

Abstract

Variation in host species composition can dramatically alter parasite transmission in natural communities. Whether diverse host communities dilute or amplify parasite transmission is thought to depend critically on species traits, particularly on how hosts affect each other’s densities, and their relative competency as hosts. Here we studied a community of potential hosts and/or decoys (i.e. non-competent hosts) for two trematode parasite species, Echinostoma trivolvis and Ribeiroia ondatrae, which commonly infect wildlife across North America. We manipulated the density of a focal host (green frog tadpoles, Rana clamitans), in concert with manipulating the diversity of alternative species, to simulate communities where alternative species either (1) replace the focal host species so that the total number of individuals remains constant (substitution) or (2) add to total host density (addition). For E. trivolvis, we found that total parasite transmission remained roughly equal (or perhaps decreased slightly) when alternative species replaced focal host individuals, but parasite transmission was higher when alternative species were added to a community without replacing focal host individuals. Given the alternative species were roughly equal in competency, these results are consistent with current theory. Remarkably, both total tadpole and per-capita tadpole infection intensity by E. trivolvis increased with increasing intraspecific host density. For R. ondatrae, alternative species did not function as effective decoys or hosts for parasite infective stages, and the diversity and density treatments did not produce clear changes in parasite transmission, although high tank to tank variation in R. ondatrae infection could have obscured patterns.

Competition and host size mediate larval anuran interactions with trematode parasites

“…Nevertheless, other recent studies have similarly reported no effect (Raffel et al, 2010;Marino & Werner, 2013) or even a positive effect (Johnson et al, 2013;Wojdak et al, 2014) of density on larval amphibian trematode infection at the mesocosm scale. Our sample sizes for dissection were limited and necessarily did not include animals that died during the experiments, as dead tadpoles are typically not visible in the large mesocosms and rapidly degrade.…”

Success, failure and ambiguity of the dilution effect among competitors

“…; Wojdak et al . ). However, these designs still obscure the dynamical feedbacks and interactions described above.…”

“…; Wojdak et al . ), we allow our host species to dynamically interact and mechanistically influence each other's densities via these traits. Then, we explore a range of realistic outcomes in our community module (three case studies) by measuring intraspecific variation in the traits of our focal host ( sensu Bolnick et al .…”

“…Dobson 2004;Keesing et al 2006;Johnson & Thieltges 2010;Roche et al 2012) andexperiments (e.g. Johnson et al 2008;Becker et al 2014;Venesky et al 2014;Wojdak et al 2014), we allow our host species to dynamically interact and mechanistically influence each other's densities via these traits. Then, we explore a range of realistic outcomes in our community module (three case studies) by measuring intraspecific variation in the traits of our focal host (sensu Bolnick et al 2011).…”

Linking host traits, interactions with competitors and disease: Mechanistic foundations for disease dilution

“…Intuitively, host regulation might matter more when predation (Rohr et al., ) or competition (Strauss et al., ) depresses focal host densities more strongly. Encounter reduction appears stronger when diluters remove parasites more rapidly and strongly resist infection (Venesky, Liu, Sauer, & Rohr, ; but see Wojdak, Edman, Wyderko, Zemmer, & Belden, ). Yet, intraspecific variation in susceptibility among focal hosts may counter either dilution mechanism by fuelling uncontrollably large or inconsequentially small epidemics (Strauss et al., ).…”