Estimating host breadth and cross-species transmission by parasite species
We considered three network-level factors for each parasite species: (1) the number of different host species a parasite infected in a community (host breadth), (2) the number of predicted transmission pairs between different host species (cross-species transmission connections), and (3) a combination of the two (graph density). Calculating each of these required that we first construct networks of the host species infected by a particular parasite species (examples in Fig. 2). To do this, for each parasite species, we constructed undirected networks (i.e., edges/connections do not have a direction) of host species in each site where each node represented the population of a host species in a given community. With only the host species that were present in that site as the nodes, we added edges between host species if they had overlapping epidemics of that same parasite species. For the networks used to calculate host breadth, we added node edges to itself (loops) for each host that was infected by a given parasite (these loops are equivalent to filling a diagonal cell in the adjacency matrix). We then calculated host breadth as the proportion of all hosts in a site (observed across all time points) that were infected with a given parasite species. The networks used to estimate cross-species transmission did not contain loops (that is, node edges from a host species back to itself), because we were only interested in interconnections between species. We then estimated cross-species transmission using the proportion of realized connections between host species out of all possible connections in the network. Finally, because both of these metrics are important for different reasons, we combined the two and calculated graph density as the proportion of realized edges out of all potential edges that could occur in a given network, including loops where a host connects to itself if it was infected. Additionally, since sites varied in their host community composition, we only included a host species in a given network if that species was found in that site; sites where the parasite was not found were dropped from the analysis.