Host community composition and defensive symbionts determine trematode parasite abundance in host communities

Jancarla M. Ocampo,J. Drake,Skylar R. Hopkins,Jeremy M. Wojdak,Lisa K. Belden

doi:10.1002/ecs2.1278

Abstract

AbstractHost species vary in their propensity to become infected by and transmit parasites, and this variation in host competency can influence parasite transmission within host communities. Host competency is often attributed to morphological, physiological, and behavioral defenses of hosts, but hosts commonly have an additional, lesser studied form of protection: defensive symbionts. For instance, snails are facultatively defended by ectosymbiotic oligochaete worms (Chaetogaster limnaei) that consume free‐living trematode parasites, bacteria protect amphibians from the fungus that causes chytridiomycosis, and ants protect plants from herbivores. In addition to reducing infection on their hosts, defensive symbionts may influence parasite transmission to other hosts by redirecting parasites toward other hosts and/or removing parasites from the system. We explored these possibilities by examining the relative roles of community composition and the presence of defensive symbionts (C. limnaei) in determining trematode infection intensity among second intermediate host communities composed of snails (Helisoma trivolvis) and tadpoles (Rana catesbeiana). Parasites were dramatically more successful at infecting snails than tadpoles, which led to more total parasites in host communities where snails were present. In addition, defensive symbionts substantially reduced snail infection intensity and thus reduced the total number of parasites in communities containing symbiont‐defended snail hosts. Neither host community composition nor the presence of defensive symbionts on snails influenced individual tadpole infection in our experiments. Therefore, in our experiments, second intermediate host community structure did not influence individual host tadpole infection risk, but did influence total parasite transmission.

Highlights

From a parasite’s perspective, not all host species are created equal
We focused on the portion of the life cycle where first intermediate host snails (Physa gyrina and Helisoma trivolvis) release short-lived, free-living larvae called cercariae into the aquatic environment, and these cercariae search for and infect second intermediate host snails (H. trivolvis in our study) and tadpoles (Rana catesbeiana—bullfrogs in our study)
To better understand the role of defensive symbionts in parasite transmission in multihost communities, we examined the relative roles of community composition and the presence of defensive symbionts in determining infection intensity among individual tadpole hosts and total parasite success within host communities

Summary

Introduction

Host species vary in competency (Keesing et al 2010, Gervasi et al 2015, Johnson et al 2015), which is a measure of the host’s propensity to become infected by, maintain, and transmit a given parasite. Johnson et al 2013, Lacroix et al 2014) and on identifying characteristics of host species that explain why some hosts are more competent than others (e.g., Johnson et al 2012b, Ostfeld et al 2014). Morphological, physiological, and behavioral defenses of hosts—such as investment in immune responses and fastidious grooming behaviors—may explain much of the variation in host competency (e.g., Keesing et al 2009, Johnson et al 2012b)

Methods

Results

Conclusion