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A combination of extensive field surveys and realistic experiments involving an amphibian disease system reveals that biodiversity reduces pathogen transmission due to a predictable link between species richness and the ability of communities to support infection. How biodiversity combats disease Several lines of evidence have suggested that biodiversity loss in an ecosystem can affect pathogen transmission and host disease, with the pathogens gaining the upper hand. Pathogens tend to infect multiple host species, which can vary in their ability to maintain and transmit infections, and for biodiversity to protect against disease risk, resistant hosts would need to be the last to be added to an ecosystem and the first to be lost. Species-poor communities would then dominated by highly susceptible hosts. This study combines wetland field surveys with experimental mesocosms to show that, in an amphibian community experiencing infection with the parasitic flatworm Ribeiroia ondatrae , the negative correlation between disease and diversity is the result of this 'dilution effect'. These findings highlight the value of biodiversity as a cost-effective approach to minimizing the spread of infectious disease. Accelerating rates of species extinctions and disease emergence underscore the importance of understanding how changes in biodiversity affect disease outcomes 1 , 2 , 3 . Over the past decade, a growing number of studies have reported negative correlations between host biodiversity and disease risk 4 , 5 , 6 , 7 , 8 , prompting suggestions that biodiversity conservation could promote human and wildlife health 9 , 10 . Yet the generality of the diversity–disease linkage remains conjectural 11 , 12 , 13 , in part because empirical evidence of a relationship between host competence (the ability to maintain and transmit infections) and the order in which communities assemble has proven elusive. Here we integrate high-resolution field data with multi-scale experiments to show that host diversity inhibits transmission of the virulent pathogen Ribeiroia ondatrae and reduces amphibian disease as a result of consistent linkages among species richness, host composition and community competence. Surveys of 345 wetlands indicated that community composition changed nonrandomly with species richness, such that highly competent hosts dominated in species-poor assemblages whereas more resistant species became progressively more common in diverse assemblages. As a result, amphibian species richness strongly moderated pathogen transmission and disease pathology among 24,215 examined hosts, with a 78.4% decline in realized transmission in richer assemblages. Laboratory and mesocosm manipulations revealed an approximately 50% decrease in pathogen transmission and host pathology across a realistic diversity gradient while controlling for host density, helping to establish mechanisms underlying the diversity–disease relationship and their consequences for host fitness. By revealing a consistent link between species richness and community competence, these findings highlight the influence of biodiversity on infection risk and emphasize the benefit of a community-based approach to understanding infectious diseases.

Increases in the intensity and frequency of wildfires highlight the need to understand how fire disturbance affects ecological interactions. Though the effects of wildfire on free-living aquatic communities are relatively well-studied, how host–parasite interactions respond to fire disturbance is largely unexplored. Using a Before-After-Control-Impact design, we surveyed 10 stream sites (5 burned and 5 unburned) in the Willamette River Basin, Oregon and quantified snail host infection status and trematode parasite community structure 1 year before and two years after historic wildfires. Despite the severity of the wildfires, snail host populations did not show significant shifts in density or size distributions. We detected nine taxa of trematode parasites and overall probability of infection remained consistent over the three-year study period. However, at the taxon-specific level, we found evidence that infection probability by one trematode decreased and another increased after fire. In a larger dataset focusing on the first year after fire (9 burned, 8 unburned sites), we found evidence for subtle differences in trematode community structure, including higher Shannon diversity and evenness at the burned sites. Taken together, host–parasite interactions were remarkably stable for most taxa; for trematodes that did show responses, changes in abundance or behavior of definitive hosts may underlie observed patterns. These results have implications for using parasites as bioindicators of environmental change and suggest that aquatic snail-trematode interactions may be relatively resistant to wildfire disturbance in some ecosystems.

Understanding the drivers of species occurrence is a fundamental goal in basic and applied ecology. Occupancy models have emerged as a popular approach for inferring species occurrence because they account for problems associated with imperfect detection in field surveys. Current models, however, are limited because they assume covariates are independent (i.e., indirect effects do not occur). Here, we combined structural equation and occupancy models to investigate complex influences on species occurrence while accounting for imperfect detection. These two methods are inherently compatible because they both provide means to make inference on latent or unobserved quantities based on observed data. Our models evaluated the direct and indirect roles of cattle grazing, water chemistry, vegetation, nonnative fishes, and pond permanence on the occurrence of six pond‐breeding amphibians, two of which are threatened: the California tiger salamander (Ambystoma californiense) and the California red‐legged frog (Rana draytonii). While cattle had strong effects on pond vegetation and water chemistry, their overall effects on amphibian occurrence were small compared to the consistently negative effects of nonnative fish. Fish strongly reduced occurrence probabilities for four of five native amphibians, including both species of conservation concern. These results could help to identify drivers of amphibian declines and to prioritize strategies for amphibian conservation. More generally, this approach facilitates a more mechanistic representation of ideas about the causes of species distributions in space and time. As shown here, occupancy modeling and structural equation modeling are readily combined, and bring rich sets of techniques that may provide unique theoretical and applied insights into basic ecological questions.

Host–parasite interactions are embedded within complex communities composed of multiple host species and a cryptic assemblage of other parasites. To date, however, surprisingly few studies have explored the joint effects of host and parasite richness on disease risk, despite growing interest in the diversity–disease relationship. Here, we combined field surveys and mechanistic experiments to test how transmission of the virulent trematode Ribeiroia ondatrae was affected by the diversity of both amphibian hosts and coinfecting parasites. Within natural wetlands, host and parasite species richness correlated positively, consistent with theoretical predictions. Among sites that supported Ribeiroia , however, host and parasite richness interacted to negatively affect Ribeiroia transmission between its snail and amphibian hosts, particularly in species-poor assemblages. In laboratory and outdoor experiments designed to decouple the relative contributions of host and parasite diversity, increases in host richness decreased Ribeiroia infection by 11–65%. Host richness also tended to decrease total infections by other parasite species (four of six instances), such that more diverse host assemblages exhibited ∼40% fewer infections overall. Importantly, parasite richness further reduced both per capita and total Ribeiroia infection by 15–20%, possibly owing to intrahost competition among coinfecting species. These findings provide evidence that parasitic and free-living diversity jointly regulate disease risk, help to resolve apparent contradictions in the diversity–disease relationship, and emphasize the challenges of integrating research on coinfection and host heterogeneity to develop a community ecology-based approach to infectious diseases.

1. Ecologists often measure the biomass and productivity of organisms to understand the importance of populations and communities in the flow of energy through ecosystems. Despite the central role of such studies in the advancement of freshwater ecology, there has been little effort to incorporate parasites into studies of freshwater energy flow. This omission is particularly important considering the roles that parasites sometimes play in shaping community structure and ecosystem processes. 2. Using quantitative surveys and dissections of over 1600 aquatic invertebrate and amphibian hosts, we calculated the ecosystem-level biomass and productivity of trematode parasites alongside the biomass of free-living aquatic organisms in three freshwater ponds in California, USA. 3. Snails and amphibian larvae, which are both important intermediate trematode hosts, dominated the dry biomass of free-living organisms across ponds (snails = 3.2 g m -2 ; amphibians = 3.1 g m -2 ). An average of 33.5% of mature snails were infected with one of six trematode taxa, amounting to a density of 13 infected snails m -2 of pond substrate. Between 18% and 33% of the combined host and parasite biomass within each infected snail consisted of larval trematode tissue, which collectively accounted for 87% of the total trematode biomass within the three ponds. Mid-summer trematode dry biomass averaged 0.10 g m -2 , which was equal to or greater than that of the most abundant insect orders (coleoptera = 0.10 g m -2 , odonata = 0.08 g m -2 , hemiptera = 0.07 g m -2 and ephemeroptera = 0.03 g m -2 ). 4. On average, each trematode taxon produced between 14 and 1660 free-swimming larvae (cercariae) infected snail -1 24 h -1 in mid-summer. Given that infected snails release cercariae for 3–4 months a year, the pond trematode communities produced an average of 153 mg m -2 yr -1 of dry cercarial biomass (range = 70–220 mg m -2 yr -1 ). 5. Our results suggest that a significant amount of energy moves through trematode parasites in freshwater pond ecosystems, and that their contributions to ecosystem energetics may exceed those of many free-living taxa known to play key roles in structuring aquatic communities.

In lotic ecosystems, the River Continuum Concept (RCC) provides a framework for understanding changes in environmental factors and free‐living communities, yet how parasite populations shift along river continua remains less clear. We quantified infections by a pathogenic trematode parasite (Nanophyetus salmincola) in >14,000 host snails across 130 stream reaches spanning 165 km in the Willamette River Basin in western Oregon, USA. Environmental factors—including flow volume, temperature, benthic algae, canopy cover, woody debris, and land cover—changed predictably with stream order, consistent with the RCC. From first‐ to eighth‐order reaches, infection prevalence decreased by ˜42‐fold and infected snail density, a measure of disease risk to fish hosts, decreased ˜3‐fold. Infected snail density, but not prevalence, was positively associated with snail biomass density, and individual infection probability increased strongly with host size. Shifts in snail population characteristics across stream orders, however, did not explain the observed changes in N. salmincola populations, suggesting that environmental variables and corresponding changes in non‐snail hosts explain the downstream decrease in infections. Our findings show predictable spatial variation in disease risk to vertebrate hosts from N. salmincola and indicate the RCC can help explain shifts in parasite populations in lotic ecosystems.

Lakes are sentinels of environmental change. In cold climates, lake ice phenology—the timing and duration of ice cover during winter—is a key control on ecosystem function. Ice phenology is likely driven by a complex interplay between physical characteristics and climatic conditions. Under climate change, lakes are generally freezing later, melting out earlier, and experiencing a shorter duration of ice cover; however, few long-term records exist for large, high-elevation lakes which may be particularly vulnerable to climate impacts. Here, we quantified ice phenology over the last century (1927–2022) for North America’s largest high-elevation lake—Yellowstone Lake—and compared it to seven similar lakes in northern Europe. We show that contrary to expectation, the ice phenology of Yellowstone Lake has been uniquely resistant to climate change. Indeed, despite warming temperatures in the region, no change in the timing nor duration of ice cover has occurred at Yellowstone Lake due to buffering by increased snowfall. However, with projections of continued warming and shifting precipitation regimes in the high Rocky Mountains, it is unclear how long this buffering will last.

While often studied in isolation, host-parasite interactions are typically embedded within complex communities. Other community members, including predators and alternative hosts, can therefore alter parasite transmission (e.g., the dilution effect), yet few studies have experimentally evaluated more than one such mechanism. Here, we used data from natural wetlands to design experiments investigating how alternative hosts and predators of parasites mediate trematode ( Ribeiroia ondatrae ) infection in a focal amphibian host ( Pseudacris regilla ). In short-term predation bioassays involving mollusks, zooplankton, fish, larval insects, or newts, four of seven tested species removed 62-93% of infectious stages. In transmission experiments, damselfly nymphs (predators) and newt larvae (alternative hosts) reduced infection in P. regilla tadpoles by ∼50%, whereas mosquitofish (potential predators and alternative hosts) did not significantly influence transmission. Additional bioassays indicated that predators consumed parasites even in the presence of alternative prey. In natural wetlands, newts had similar infection intensities as P. regilla , suggesting that they commonly function as alternative hosts despite their unpalatability to downstream hosts, whereas mosquitofish had substantially lower infection intensities and are unlikely to function as hosts. These results underscore the importance of studying host-parasite interactions in complex communities and of broadly linking research on predation, biodiversity loss, and infectious diseases.

While the recent inclusion of parasites into food-web studies has highlighted the role of parasites as consumers, there is accumulating evidence that parasites can also serve as prey for predators. Here we investigated empirical patterns of predation on parasites and their relationships with parasite transmission in eight topological food webs representing marine and freshwater ecosystems. Within each food web, we examined links in the typical predator–prey sub web as well as the predator–parasite sub web, i.e. the quadrant of the food web indicating which predators eat parasites. Most predator–parasite links represented 'concomitant predation' (consumption and death of a parasite along with the prey/host; 58–72%), followed by 'trophic transmission' (predator feeds on infected prey and becomes infected; 8–32%) and predation on free-living parasite life-cycle stages (4–30%). Parasite life-cycle stages had, on average, between 4.2 and 14.2 predators. Among the food webs, as predator richness increased, the number of links exploited by trophically transmitted parasites increased at about the same rate as did the number of links where these stages serve as prey. On the whole, our analyses suggest that predation on parasites has important consequences for both predators and parasites, and food web structure. Because our analysis is solely based on topological webs, determining the strength of these interactions is a promising avenue for future research.

Predators can increase the biomass of their prey, particularly when prey life stages differ in competitive ability and predation is stage specific. Akin to predators, parasites influence host population sizes and engage in stage-structured interactions, yet whether parasites can increase host population biomass remains relatively unexplored. Using a stage-structured consumer–resource model and a mesocosm experiment with snails and castrating trematodes, we examined responses of host biomass to changes in infection prevalence under variation in host pathology and resource competition. Equilibrium adult host biomass increased with infection prevalence in the model when parasites castrated hosts and adults were superior competitors to juveniles. Juvenile biomass increased with infection prevalence whether parasites caused mortality or castration, but only when juveniles were superior competitors. In mesocosms, increases in infection by castrating trematodes reduced snail egg production, juvenile abundance, and adult survival. At high competition, juvenile growth and total biomass increased with infection prevalence due to competitive release. At low competition, juvenile biomass decreased with infection due to reduced reproduction. These results highlight how disease-induced biomass overcompensation depends on infection pathology, resource availability, and competitive interactions within and between host life stages. Considering such characteristics may benefit biocontrol efforts using parasites.