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1. Co‐parasitism in the face of predation: Effects of natural enemies on a neotropical mockingbird

   https://doi.org/10.1111/1365-2656.13991  

   Herman, Jordan M. ; Fiorini, Vanina D. ; Crudele, Ignacio ; Reboreda, Juan C. ; Pladas, Shawn A. ; Watson, André P. ; Bush, Sarah E. ; Clayton, Dale H. ( August 2023 , Journal of Animal Ecology)

   Co‐parasitism is ubiquitous and has important consequences for the ecology and evolution of wild host populations. Studies of parasite co‐infections remain limited in scope, with few experimental tests of the fitness consequences of multiple parasites, especially in natural populations.

   We measured the separate and combined effects ofPhilornis seguyinest flies and shiny cowbirdsMolothrus bonariensison the fitness of a shared host, the chalk‐browed mockingbird (Mimus saturninus) in Argentina.

   Using a two‐factor experimental approach, we manipulated the presence of nest flies and cowbirds in mockingbird nests and assessed their effects on mockingbird haemoglobin levels, begging and provisioning rates, body size, and fledging success. We also monitored rates of nest predation in relation to parasitism by flies and cowbirds.

   Nest flies reduced the haemoglobin concentration, body size, and fledging success of mockingbirds, likely because mockingbirds did not compensate for parasitism by begging more or feeding their nestlings more. Cowbirds also reduced the fledging success of mockingbirds, even though they had no detectable effect on haemoglobin or body size. Nests with cowbirds, which beg more than mockingbirds, attracted more nest predators. There was no significant interaction between the effects of flies and cowbirds on any component of mockingbird fitness. The combined effects of nest flies and cowbirds were strictly additive.

   In summary, we show that nest flies and cowbirds both reduce host fitness, but do not have interactive effects in co‐parasitized nests. Our results further suggest that predators exacerbate the effects of nest flies and cowbirds on their hosts. Our study shows that the fitness consequences of co‐parasitism are complex, especially in the context of community‐level interactions.

    

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2. It's a worm‐eat‐worm world: Consumption of parasite free‐living stages protects hosts and benefits predators

   https://doi.org/10.1111/1365-2656.13591  

   Hobart, Brendan K. ; Moss, Wynne E. ; McDevitt‐Galles, Travis ; Stewart Merrill, Tara E. ; Johnson, Pieter T. J. ( September 2021 , Journal of Animal Ecology)

   Predation on parasites is a common interaction with multiple, concurrent outcomes. Free‐living stages of parasites can comprise a large portion of some predators' diets and may be important resources for population growth. Predation can also reduce the density of infectious agents in an ecosystem, with resultant decreases in infection rates. While predator–parasite interactions likely vary with parasite transmission strategy, few studies have examined how variation in transmission mode influences contact rates with predators and the associated changes in consumption risk.

   To understand how transmission mode mediates predator–parasite interactions, we examined associations between an oligochaete predatorChaetogaster limnaeithat lives commensally on freshwater snails and nine trematode taxa that infect snails.Chaetogasteris hypothesized to consume active (i.e. mobile), free‐living stages of trematodes that infect snails (miracidia), but not the passive infectious stages (eggs); it could thus differentially affect transmission and infection prevalence of parasites, including those with medical or veterinary importance. Alternatively, when infection does occur,Chaetogastercan consume and respond numerically to free‐living trematode stages released from infected snails (cercariae). These two processes lead to contrasting predictions about whetherChaetogasterand trematode infection of snails correlate negatively (‘protective predation’) or positively (‘predator augmentation’).

   Here, we tested how parasite transmission mode affectedChaetogaster–trematode relationships using data from 20,759 snails collected across 4 years from natural ponds in California. Based on generalized linear mixed modelling, snails with moreChaetogasterwere less likely to be infected by trematodes that rely on active transmission. Conversely, infections by trematodes with passive infectious stages were positively associated with per‐snailChaetogasterabundance.

   Our results suggest that trematode transmission mode mediates the net outcome of predation on parasites. For trematodes with active infectious stages, predatoryChaetogasterlimited the risk of snail infection and its subsequent pathology (i.e. castration). For taxa with passive infectious stages, no such protective effect was observed. Rather, infected snails were associated with higherChaetogasterabundance, likely owing to the resource subsidy provided by cercariae. These findings highlight the ecological and epidemiological importance of predation on free‐living stages while underscoring the influence of parasite life history in shaping such interactions.

    

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3. Host exposure to a common pollutant can influence diversity–disease relationships

   https://doi.org/10.1111/1365-2656.13988  

   Buss, Nicholas ; Hua, Jessica ( August 2023 , Journal of Animal Ecology)

   Hosts and parasites are embedded in communities where species richness and composition can influence disease outcomes (diversity–disease relationships). The direction and magnitude of diversity–disease relationships are influenced by variation in competence (ability to support and transmit infections) of hosts in a community. However, host susceptibility to parasites, which mediates host competence, is not static and is influenced by environmental factors, including pollutants. Despite the role that pollutants can play in augmenting host susceptibility, how pollutants influence diversity–disease dynamics is not well understood.

   Using an amphibian–trematode model, we tested how NaCl influences diversity–disease dynamics. We predicted that NaCl exposure can alter relative susceptibility of host species to trematodes, leading to cascading effects on the diversity–disease relationship. To test these predictions, we exposed hosts to benign or NaCl environments and generated communities that differed in number and composition of host species. We exposed these communities to trematodes and measured disease outcomes at the community (total infections across all hosts within a community) and species levels (average number of infections per host species within a community).

   Host species differed in their relative susceptibility to trematodes when exposed to NaCl. Consequently, at the community level (total infections across all hosts within a community), we only detected diversity–disease relationships (dilution effects) in communities where hosts were exposed to NaCl. At the species level, disease outcomes (average number of infections/species) and whether multi‐species communities supported lower number of infections relative to single‐species communities depended on community composition. Notably, however, as with overall community infection, diversity–disease relationships only emerged when hosts were exposed to NaCl.

   Synthesis.Pollutants are ubiquitous in nature and can influence disease dynamics across a number of host–parasite systems. Here, we show that NaCl exposure can alter the relative susceptibility of host species to parasites, influencing the relationship between biodiversity and disease at both community and species levels. Collectively, our study contributes to the limited knowledge surrounding environmental mediators of host susceptibility and their influence on diversity–disease dynamics.

    

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4. A Diverse Flea (Siphonaptera) Assemblage from the Small Mammals of Central New Mexico

   https://doi.org/10.32873/unl.dc.manter31  

   Peterson, D. E. ; Dunnum, J. L. ; Liphardt, S. W. ; Boegler, K. A. ; Eckerlin, R. P. ; Hoberg, E. P. ; Perez-Arriaga, M. O. ; Dereig, K. ; Garnand, D. ; Campbell, M. L. ; et al ( April 2023 , MANTER: Journal of Parasite Biodiversity)

   The geographical ranges of many mammals and their associated parasites are dynamic. Comprehensive documentation of these communities over time provides a foundation for interpreting how changing environmental conditions, driven by accelerating climate change, other anthropogenic disturbances, and natural events, may influence host-parasite interactions. Fleas (Order Siphonaptera) are obligate, hematophagous parasites of birds and mammals with medical interest because of their role in transmitting pathogens. From 2016 to 2019, we sampled the small mammal and associated flea communities in El Malpais National Conservation Area (El Malpais) in Cibola County, New Mexico. Among 898 mammalian specimens, 925 fleas representing 29 species were collected from 18 host species. Pleochaetis exilis was the most abundant flea species, composing 27% of the total fleas collected, whereas Aetheca wagneri was the most prevalent flea species, parasitizing 8% of the community sampled. Across a total of 284 hosts recorded with fleas, A. wagneri, Malaraeus eremicus, and Peromyscopsylla hesperomys adelpha parasitized the most host species (n = 6 each). Onychomys leucogaster (Wied-Neuwied, 1841), the northern grasshopper mouse, a rodent highly implicated in plague dynamics, was host for the highest number of flea species (n = 15), followed by Peromyscus truei (Shufeldt, 1885) (n = 10). Our aims are to (a) describe the flea-mammal assemblage of a central New Mexico site, creating a baseline for diversity against which changing patterns of association can be assessed over time; (b) identify previously unrecognized host associations; and (c) examine infestation parameters, including the relationships of flea prevalence and mean abundance to host sex, host abundance, and seasonality. As such, our study exemplifies the Documentation and Assessment phases of the DAMA protocol (Document, Assess, Monitor, Act), a central component of exploring distribution and diversity of complex pathogen-host communities across space and time that are essential to a proactive understanding of emerging disease. 

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5. Trematode parasites exceed aquatic insect biomass in Oregon stream food webs

   https://doi.org/10.1111/1365-2656.13409  

   Preston, Daniel L. ; Layden, Tamara J. ; Segui, Leah M. ; Falke, Landon P. ; Brant, Sara V. ; Novak, Mark ; Hoye, ed., Bethany ( January 2021 , Journal of Animal Ecology)

   Although parasites are increasingly recognized for their ecosystem roles, it is often assumed that free‐living organisms dominate animal biomass in most ecosystems and therefore provide the primary pathways for energy transfer.

   To examine the contributions of parasites to ecosystem energetics in freshwater streams, we quantified the standing biomass of trematodes and free‐living organisms at nine sites in three streams in western Oregon, USA. We then compared the rates of biomass flow from snailsJuga pliciferainto trematode parasites relative to aquatic vertebrate predators (sculpin, cutthroat trout and Pacific giant salamanders).

   The trematode parasite community had the fifth highest dry biomass density among stream organisms (0.40 g/m²) and exceeded the combined biomass of aquatic insects. Only host snails (3.88 g/m²), sculpin (1.11 g/m²), trout (0.73 g/m²) and crayfish (0.43 g/m²) had a greater biomass. The parasite ‘extended phenotype’, consisting of trematode plus castrated host biomass, exceeded the individual biomass of every taxonomic group other than snails. The substantial parasite biomass stemmed from the high snail density and infection prevalence, and the large proportional mass of infected hosts that consisted of trematode tissue (M = 31% per snail).

   Estimates of yearly biomass transfer from snails into trematodes were slightly higher than the combined estimate of snail biomass transfer into the three vertebrate predators. Pacific giant salamanders accounted for 90% of the snail biomass consumed by predators.

   These results demonstrate that trematode parasites play underappreciated roles in the ecosystem energetics of some freshwater streams.

    

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