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.