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1. Linking community assembly and structure across scales in a wild mouse parasite community

   https://doi.org/10.1002/ece3.5785  

   Rynkiewicz, Evelyn C. ; Fenton, Andy ; Pedersen, Amy B. ( December 2019 , Ecology and Evolution)

   Understanding what processes drive community structure is fundamental to ecology. Many wild animals are simultaneously infected by multiple parasite species, so host–parasite communities can be valuable tools for investigating connections between community structures at multiple scales, as each host can be considered a replicate parasite community. Like free‐living communities, within‐host–parasite communities are hierarchical; ecological interactions between hosts and parasites can occur at multiple scales (e.g., host community, host population, parasite community within the host), therefore, both extrinsic and intrinsic processes can determine parasite community structure. We combine analyses of community structure and assembly at both the host population and individual scales using extensive datasets on wild wood mice (Apodemus sylvaticus) and their parasite community. An analysis of parasite community nestedness at the host population scale provided predictions about the order of infection at the individual scale, which were then tested using parasite community assembly data from individual hosts from the same populations. Nestedness analyses revealed parasite communities were significantly more structured than random. However, observed nestedness did not differ from null models in which parasite species abundance was kept constant. We did not find consistency between observed community structure at the host population scale and within‐host order of infection. Multi‐state Markov models of parasite community assembly showed that a host's likelihood of infection with one parasite did not consistently follow previous infection by a different parasite species, suggesting there is not a deterministic order of infection among the species we investigated in wild wood mice. Our results demonstrate that patterns at one scale (i.e., host population) do not reliably predict processes at another scale (i.e., individual host), and that neutral or stochastic processes may be driving the patterns of nestedness observed in these communities. We suggest that experimental approaches that manipulate parasite communities are needed to better link processes at multiple ecological scales.

    

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2. Bovine tuberculosis disturbs parasite functional trait composition in African buffalo

   https://doi.org/10.1073/pnas.1903674116  

   Beechler, Brianna R. ; Boersma, Kate S. ; Buss, Peter E. ; Coon, Courtney A. ; Gorsich, Erin E. ; Henrichs, Brian S. ; Siepielski, Adam M. ; Spaan, Johannie M. ; Spaan, Robert S. ; Ezenwa, Vanessa O. ; et al ( July 2019 , Proceedings of the National Academy of Sciences)

   Novel parasites can have wide-ranging impacts, not only on host populations, but also on the resident parasite community. Historically, impacts of novel parasites have been assessed by examining pairwise interactions between parasite species. However, parasite communities are complex networks of interacting species. Here we used multivariate taxonomic and trait-based approaches to determine how parasite community composition changed when African buffalo ( Syncerus caffer ) acquired an emerging disease, bovine tuberculosis (BTB). Both taxonomic and functional parasite richness increased significantly in animals that acquired BTB than in those that did not. Thus, the presence of BTB seems to catalyze extraordinary shifts in community composition. There were no differences in overall parasite taxonomic composition between infected and uninfected individuals, however. The trait-based analysis revealed an increase in direct-transmitted, quickly replicating parasites following BTB infection. This study demonstrates that trait-based approaches provide insight into parasite community dynamics in the context of emerging infections. 

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3. Dose relationships can exacerbate, mute, or reverse the impact of heterospecific host density on infection prevalence

   https://doi.org/10.1002/ecy.3422  

   Clay, Patrick A. ; Cortez, Michael H. ; Duffy, Meghan A. ( July 2021 , Ecology)

   The likelihood an individual becomes infected depends on the community in which it is embedded. For environmentally transmitted parasites, host community composition can alter host density, the density of parasites that hosts encounter in the environment, and the dose to which hosts are subsequently exposed. While some multi‐host theory incorporates some of these factors (e.g., competition among hosts), it does not currently consider the nonlinear relationships between parasite exposure dose and per‐propagule infectivity (dose–infectivity relationships), between exposure dose and infected host mortality (dose–mortality relationships), and between exposure dose and parasite propagule excretion (dose–excretion relationships). This makes it difficult to predict the impact of host species on one another’s likelihood of infection. To understand the implications of these nonlinear dose relationships for multi‐host communities, we first performed a meta‐analysis on published dose–infectivity experiments to quantify the proportion of accelerating, linear, or decelerating dose–infectivity relationships; we found that most experiments demonstrated decelerating dose–infectivity relationships. We then explored how dose–infectivity, dose–mortality, and dose–excretion relationships might alter the impact of heterospecific host density on infectious propagule density, infection prevalence, and density of a focal host using two‐host, one‐parasite models. We found that dose relationships either decreased the magnitude of the impact of heterospecific host density on propagule density and infection prevalence via negative feedback loops (decelerating dose–infectivity relationships, positive dose–mortality relationships, and negative dose–excretion relationships), or increased the magnitude of the impact of heterospecific host density on infection prevalence via positive feedback loops (accelerating dose–infectivity relationships and positive dose–excretion relationships). Further, positive dose–mortality relationships resulted in hosts that traditionally decrease disease (e.g., low competence, strong competitors) increasing infection prevalence, and vice versa. Finally, we found that dose relationships can create positive feedback loops that facilitate friendly competition (i.e., increased heterospecific density has a positive effect on focal host density because the reduction in disease outweighs the negative effects of interspecific competition). This suggests that without taking dose relationships into account, we may incorrectly predict the effect of heterospecific host interactions, and thus host community composition, on environmentally transmitted parasites.

    

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4. Novel parasite invasion leads to rapid demographic compensation and recovery in an experimental population of guppies

   https://doi.org/10.1073/pnas.2006227117  

   Rogowski, Emma L. B. ; Van Alst, Andy D. ; Travis, Joseph ; Reznick, David N. ; Coulson, Tim ; Bassar, Ronald D. ( August 2020 , Proceedings of the National Academy of Sciences)

   The global movement of pathogens is altering populations and communities through a variety of direct and indirect ecological pathways. The direct effect of a pathogen on a host is reduced survival, which can lead to decreased population densities. However, theory also suggests that increased mortality can lead to no change or even increases in the density of the host. This paradoxical result can occur in a regulated population when the pathogen’s negative effect on survival is countered by increased reproduction at the lower density. Here, we analyze data from a long-term capture–mark–recapture experiment of Trinidadian guppies (Poecilia reticulata) that were recently infected with a nematode parasite (Camallanus cotti). By comparing the newly infected population with a control population that was not infected, we show that decreases in the density of the infected guppy population were transient. The guppy population compensated for the decreased survival by a density-dependent increase in recruitment of new individuals into the population, without any change in the underlying recruitment function. Increased recruitment was related to an increase in the somatic growth of uninfected fish. Twenty months into the new invasion, the population had fully recovered to preinvasion densities even though the prevalence of infection of fish in the population remained high (72%). These results show that density-mediated indirect effects of novel parasites can be positive, not negative, which makes it difficult to extrapolate to how pathogens will affect species interactions in communities. We discuss possible hypotheses for the rapid recovery.

    

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5. Infectivity of the parasite Metschnikowia bicuspidata is decreased by time spent as a transmission spore, but exposure to phycotoxins in the water column has no effect

   https://doi.org/10.1111/fwb.14082  

   Sánchez, Kristel F. ; Zhong, Baili ; Agudelo, Jorge A. ; Duffy, Meghan A. ( April 2023 , Freshwater Biology)

   Transmission from one host to another is a crucial component of parasite fitness. For some aquatic parasites, transmission occurs via a free‐living stage that spends time in the water, awaiting an encounter with a new host. These parasite transmission stages can be impacted by biotic and abiotic factors that influence the parasite's ability to successfully infect or grow in a new host.

   Here we tested whether time spent in the water column and/or exposure to common cyanobacterial toxins impacted parasite transmission stages. More specifically, we tested whether the infectivity, within host growth, and virulence of the fungal parasiteMetschnikowia bicuspidatachanged as a result of time spent in the water or from exposure to cyanotoxins in the water column. We exposed parasite transmission spores to different levels of one of two ecologically important cyanotoxins, microcystin‐LR and anatoxin‐a, and factorially manipulated the amount of time spores were incubated in water. We removed the toxins and used those same spores to infect one genotype of the common lake zooplanktonDaphnia dentifera.

   We found that cyanotoxins did not impact parasite fitness (infection prevalence and spore yield per infected host) or virulence (host lifetime reproduction and survivorship) at the tested concentrations (10 and 30 μg/L). However, we found that spending longer as a transmission spore decreased a spore's chances for successful infection: spores that were only incubated for 24 hr infected approximately 75% of exposed hosts, whereas spores incubated for 10 days infected less than 50% of exposed hosts.

   We also found a negative relationship between the final spore yield from infected hosts and the proportion of hosts that became infected. In treatments where spores spent longer in the water column prior to encountering a host, infection prevalence was lower (indicating lower per spore infectivity), but each infected host yielded more spores at the end of infection. We hypothesise that this pattern may result from intraspecific parasite competition within the host.

   Overall, these results suggest that transmission spores of this parasite are not strongly influenced by cyanotoxins in the water column, but that other aspects of spending time in the water strongly influence parasite fitness.

    

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