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.