Whether larval amphibians school does not affect the parasite aggregation rule: testing the effects of host spatial heterogeneity in field and experimental studies

Abstract

Almost all macroparasites show over‐dispersed infections within natural host populations such that most parasites are distributed among a few heavily‐infected individuals. Despite the importance of parasite aggregation for understanding system stability, the potential for population regulation, and super‐spreading events, many questions persist about its underlying drivers. Theoretically, aggregation results from heterogeneity in host exposure, resistance, and tolerance. However, few studies have examined how host spatial arrangement – which likely affects both parasite encounter and density‐dependent interactions – influences infection and dispersion, representing a critical gap in our current knowledge regarding the possible drivers of parasite aggregation. Using field data from over 165 ponds and 8000 hosts, we evaluated how the spatial clustering of amphibian larvae within ponds 1) varied among different amphibian species, and 2), affected the distribution of parasites within the host population using Taylor's power law. A complementary mesocosm experiment used field‐guided manipulations of the spatial arrangement of larval amphibians to create a gradient in host clustering while controlling host density, thereby testing for spatial effects on both infection success and aggregation by three different trematode species. Our field data indicated that larval amphibians exhibited significant spatial clustering that was well captured by Taylor's power law (R2 0.92 to 0.97 for different host species), but the residual variation only weakly correlated with observed patterns of trematode parasite over‐dispersion. Correspondingly, experimental manipulation of host clustering had no effects on parasite infection success or the degree of parasite aggregation among cages or mesocosms. Given the importance of parasite over‐dispersion for host populations and disease dynamics, we advocate for further investigations of host and parasite spatial aggregation, particularly studies that incorporate and/or control for heterogeneity in exposure and susceptibility.

Measuring aggregation in parasite populations

“…Taylor [35] suggested that the slope was not only a true population statistic, but also an 'index of aggregation' that described an intrinsic property of the organisms concerned. Consequently, the slope has been used by a number of authors as the basis for a measure of aggregation [8,10,[36][37][38][39][40][41][42][43]. While the methodology has been used to explore the relationship between the distribution of parasites and biologically meaningful covariates [21], it is far from clear how the slope from Taylor's law relates to other concepts of aggregation.…”

Biological and statistical processes jointly drive population aggregation: using host–parasite interactions to understand Taylor's power law

“…the residual variance in host body size after accounting for mean host body size), log-host density, and spatial heterogeneity in host density within the pond (i.e. the residual variance in amphibians captured per dipnet sweep after accounting for mean density; see [50] and electronic supplementary material for calculations). At the parasite level, we included parasite body mass, rspb.royalsocietypublishing.org Proc.…”

Trematode parasite cercariae as food in model freshwater trophic interactions

“…All treatments received three snails (Stagnicola elodes) collected from a field site in Waterloo, Ontario (N43.476450, W80.554990). These were contained in mesh cylinders (15.2 x 14 cm) that rested at the bottom of each bin to prevent their escape (Figure 2-1), but the mesh size (0.32 cm) was sufficient to easily allow passage by cercariae and has been successfully used in other studies (e.g., Koprivnikar et al, 2018). This ensured that the treatments requiring cercariae (C + D) had a constant influx present in the form of infected snails; treatments not requiring cercariae (A + B) had uninfected snails added to account for any possible effect of snail presence irrespective of parasites.…”