Quantifying potential cross-species transmission
To generate a plausible measure of potential cross-species transmission, we chose a method that accounts for temporal and spatial overlap of a single parasite species in multiple host species (i.e., to what degree did the same parasite species infect more than one host species on the same day in the same lake?). Our metric, which we term “epidemic overlap”, measures the degree to which epidemics of a single parasite species overlapped in multiple host species within the same site over the same time period. Looking at a single parasite species in a single lake (hereafter: site) in a single year of data at a time, we compared parasite prevalence and infected host density of the parasite species in each pairwise combination of hosts (e.g., shaded region in Fig. 1, comparing S. cienkowskii infected host density in D. pulicaria and D. retrocurva in Bruin Lake during 2014). Only including days when two hosts were infected with the same parasite, we took the geometric mean of infected host density for each pair of host species through time and calculated the area under the overlapping epidemic curve. We used the geometric mean because, unlike the arithmetic mean, it yields a zero for any dates where prevalence in either of the hosts was zero. In this way, epidemic overlap served as a proxy for the potential amount of cross-species transmission. In the overlapping region, infections in one host could plausibly result from transmission stages of parasites produced by another host species. In other words, epidemic overlap quantifies the amount of synchrony of epidemics of the same parasite species in different host species.