Introduction
Parasites are ubiquitous in nature, and also embedded in complex assemblages of interrelated species. As a result, most host species interact with more than one parasite species and many parasite species interact with multiple potential hosts (Petney and Andrews 1998; Pedersen and Fenton 2007; Rynkiewicz et al. 2015). While we know that the complex web of interactions between species within communities can influence patterns of disease (Johnson et al. 2015a), there are substantial challenges to disentangling the patterns of transmission in these complex communities – leaving an important gap in our understanding of the basic functioning of ecological communities and how changes in species in diversity might influence infectious diseases in wildlife and humans (Cable et al. 2017; Stewart Merrill et al. 2022).
To date, most studies focusing on the complexity of natural communities have focused on the impact of changes in the number of host species on patterns of disease (Johnson et al. 2015b). In particular, studies of the dilution effect have sought to understand how changes in host community diversity might influence the level of infection, especially in one focal host (Keesing et al. 2010; Civitello et al. 2015). Central to the potential for dilution is that host species differ in their ability to host a parasite – to become infected, for parasite replication within a host, and/or in the ability to pass infections on to additional hosts (LoGiudice et al. 2003; Stewart Merrill et al. 2022). However, studies of the dilution effect still tend to focus on one parasite in a community, and we still do not understand whether certain hosts are likely to be important for transmission (or dilution) more broadly across parasites, though there is evidence that certain aspects of a host’s life history might lead to certain hosts generally being highly competent (Martin et al. 2016).
At the same time, we wish to know whether it is likely that the path one parasite takes through an ecological community might be followed by another parasite. Cross-species transmission plays a crucial role in public health, as most human infectious diseases originate from zoonotic sources (Woolhouse and Gowtage-Sequeria 2005; Lloyd-Smith et al. 2009). If one parasite spills over via one particular set of host species, will another parasite most likely follow that same path, or do different parasites tend to have different networks of transmission?
Unfortunately, studying these realistic, complex communities and quantifying their ecological interactions using traditional approaches is challenging due to the sheer number of species involved and the complexity of their interactions (Runghen et al. 2021). Moreover, even in cases where it would be ethical to experimentally manipulate transmission, it often is not logistically feasible (e.g., due to challenges cultivating host species in controlled settings). These challenges are exemplified by a well-studied host-parasite system,Daphnia and their parasites. While this system is unusually well characterized and studied, being used as a model system for questions of disease ecology and evolution (Ebert 2005; McLean and Duffy 2020; Wale and Duffy 2021; Ebert 2022), we still lack a general understanding of basic questions such as whether particular host species are generally more prominent parts of parasite transmission, or whether different parasites tend to be transmitted via different networks of hosts.
In the present study, we took advantage of the large dataset capabilities of network approaches to (1) generate hypotheses about which hosts and parasites might warrant further study in experiments or models (Pilosof et al. 2014), and (2) produce useful metrics approximating the relative “importance” of a species and the number of connections among species (Gómez et al. 2013; Dallas et al. 2017). This allowed us to address three main questions regarding this study system. First, which parasite species in our study system show the most cross-species transmission and infect the widest breadth of host species? Second, are individual host species associated with cross-species transmission of each parasite species or are parasites more commonly host generalists, infecting many species? And third, are patterns of potential cross species transmission similar across parasite species within a community? The insights garnered from these analyses point towards topics for future studies aimed at understanding complex multihost-multiparasite communities in nature.