Drivers of symbiont diversity in freshwater snails: a comparative analysis of resource availability, community heterogeneity, and colonization opportunities

Abstract

Decades of community ecology research have highlighted the importance of resource availability, habitat heterogeneity, and colonization opportunities in driving biodiversity. Less clear, however, is whether a similar suite of factors explains the diversity of symbionts. Here, we used a hierarchical dataset involving 12,712 freshwater snail hosts representing five species to test the relative importance of potential factors in driving symbiont richness. Specifically, we used model selection to assess the explanatory power of variables related to host species identity, resource availability (average body size, host density), ecological heterogeneity (richness of hosts and other taxa), and colonization opportunities (wetland size and amount of neighboring wetland area) on symbiont richness in 146 snail host populations in California, USA. We encountered a total of 24 taxa of symbionts, including both obligatory parasites such as digenetic trematodes as well as more commensal, mutualistic, or opportunistic groups such as aquatic insect larvae, annelids, and leeches. After validating richness estimates per host population using species accumulative curves, we detected positive effects on symbiont richness from host body size, total richness of the aquatic community, and colonization opportunities. Neither snail density nor the richness of snail species accounted for significant variation in symbiont diversity. Host species identity also affected symbiont richness, with higher gamma and average alpha diversity among more common host species and with higher local abundances. These findings highlight the importance of multiple, concurrent factors in driving symbiont richness that extend beyond epidemiological measures of host abundance or host diversity alone.

Parasite metacommunities: Evaluating the roles of host community composition and environmental gradients in structuring symbiont communities within amphibians

“…(mix of P. acuta and P. gyrina ), of which H. trivolvis and Physa spp. function as first intermediate hosts for many of the trematode species (McCaffrey & Johnson 2017). Amphibian richness at each site ranged from 1 to all 6 species with a mean of 3.1, while snail richness ranged from 0 to 5 taxa, with a mean of 2.1.…”

Tracking the assembly of nested parasite communities: Using β‐diversity to understand variation in parasite richness and composition over time and scale

“…All ponds were small habitats containing both species of focal amphibian host and the snail species Helisoma trivovlis and Physa spp. ( Physa acuta or Physa gyrina ), which are intermediate hosts for a diversity of trematode species found in amphibians (McCaffrey & Johnson, 2017). Each pond was visited four to five times across the summer, during which we sampled 10–20 individuals of each host species, representing juvenile life stages from post‐hatching to peri‐metamorphosis (Appendix S1: Figure S2).…”

Parasite richness and abundance within aquatic macroinvertebrates: testing the roles of host‐ and habitat‐level factors

“…It is worth noting, however, that even the odonates supported fewer parasite taxa than detected in other vertebrate and invertebrate hosts surveyed from these and other similar pond ecosystems. For instance, studies with amphibian, fish, and snail hosts from pond communities have detected anywhere from 12 to 23 macroparasite taxa, compared to the five taxa from odonates in the current study (Muzzall et al 2001, Pulis et al 2011, Johnson et al 2016, McCaffrey and Johnson 2017, Calhoun et al 2018;).…”

“…Teasing apart the effect of each level and their interactions will help better understand and predict changes in both parasite communities and parasite populations. For example, McCaffrey and Johnson (2017) demonstrated that freshwater snail symbiont diversity is driven by both host-level effects, such as snail size and abundance, as well as habitat-level effects that reflect both transmission and colonization opportunities. At the habitat-level, snail symbiont richness correlated positively with wetland size, the proximity of nearby ponds, and diversity of the free-living community, likely reflecting opportunities for colonization and the presence of additional hosts necessary for parasite transmission.…”