Parasites dominate food web links

PREVALENCE OF THE BOPYRID ISOPOD PROBOPYRUS PANDALICOLA IN DAGGERBLADE GRASS SHRIMP FROM SALT MARSH CREEKS AND PANNES OF CAPE COD, MASSACHUSETTS

In salt marsh ecosystems, daggerblade grass shrimp, Palaemon (Palaemonetes) pugio, play a crucial role in food webs and serve as the definitive host for the bopyrid isopod Probopyrus pandalicola. These ectoparasites infest the branchial chambers of grass shrimp, which can lead to decreased energy availability and sterilization of infected hosts. Although bopyrid isopod infestation of daggerblade grass shrimp has been frequently reported in literature from coastal marshes of the southeastern United States, the prevalence of this parasite has not been recently documented in daggerblade grass shrimp from marshes of the northeastern United States. The goal of this project was to quantify the prevalence of Pr. pandalicola infestations in Pa. pugio across Cape Cod, Massachusetts. We evaluated bopyrid isopod prevalence from shrimp collected from 5 different salt marsh habitats along Cape Cod in August 2021. Bopyrid isopod infestations were found in shrimp at 4 of 5 salt marshes, with prevalence ranging from 0.04 to 14.1%. Seasonal resampling of one of the salt marshes revealed the highest average infestation prevalence in spring (<17.1%) and an isolated high of 30.3% prevalence in a single salt panne. A series of linear and multivariate models showed that panne area, shrimp abundance, and distance to shoreline were related to Pr. pandalicola shrimp infestations in salt pannes in summer. This study describes the prevalence of the bopyrid isopod infesting daggerblade grass shrimp in salt marshes in New England, with implications for how parasitized shrimp influence salt marsh food webs in which they are found.

Parasite-mediated predation determines infection in a complex predator-prey-parasite system

The interplay of host-parasite and predator-prey interactions is critical in ecological dynamics because both predators and parasites can regulate communities. But what is the prevalence of infected prey and predators when a parasite is transmitted through trophic interactions considering stochastic demographic changes? Here, we modelled and analysed a complex predator-prey-parasite system, where parasites are transmitted from prey to predators. We varied parasite virulence and infection probabilities to investigate how those evolutionary factors determine species' coexistence and populations' composition. Our results show that parasite species go extinct when the infection probabilities of either host are small and that success in infecting the final host is more critical for the survival of the parasite. While our stochastic simulations are consistent with deterministic predictions, stochasticity plays an important role in the border regions between coexistence and extinction. As expected, the proportion of infected individuals increases with the infection probabilities. Interestingly, the relative abundances of infected and uninfected individuals can have opposite orders in the intermediate and final host populations. This counterintuitive observation shows that the interplay of direct and indirect parasite effects is a common driver of the prevalence of infection in a complex system.

Parasites alter food-web topology of a subarctic lake food web and its pelagic and benthic compartments

We compared three sets of highly resolved food webs with and without parasites for a subarctic lake system corresponding to its pelagic and benthic compartments and the whole-lake food web. Key topological food-web metrics were calculated for each set of compartments to explore the role parasites play in food-web topology in these highly contrasting webs. After controlling for effects from differences in web size, we observed similar responses to the addition of parasites in both the pelagic and benthic compartments demonstrated by increases in trophic levels, linkage density, connectance, generality, and vulnerability despite the contrasting composition of free-living and parasitic species between the two compartments. Similar effects on food-web topology can be expected with the inclusion of parasites, regardless of the physical characteristics and taxonomic community compositions of contrasting environments. Additionally, similar increases in key topological metrics were found in the whole-lake food web that combines the pelagic and benthic webs, effects that are comparable to parasite food-web analyses from other systems. These changes in topological metrics are a result of the unique properties of parasites as infectious agents and the links they participate in. Trematodes were key contributors to these results, as these parasites have distinct characteristics in aquatic systems that introduce new link types and increase the food web's generality and vulnerability disproportionate to other parasites. Our analysis highlights the importance of incorporating parasites, especially trophically transmitted parasites, into food webs as they significantly alter key topological metrics and are thus essential for understanding an ecosystem's structure and functioning.

Increased virulence due to multiple infection in Daphnia leads to limited growth in 1 of 2 co-infecting microsporidian parasites

Recent outbreaks of various infectious diseases have highlighted the ever-present need to understand the drivers of the outbreak and spread of disease. Although much of the research investigating diseases focuses on single infections, natural systems are dominated by multiple infections. These infections may occur simultaneously, but are often acquired sequentially, which may alter the outcome of infection. Using waterfleas (Daphnia magna) as a model organism, we examined the outcome of sequential and simultaneous multiple infections with 2 microsporidian parasites (Ordospora colligata and Hamiltosporidium tvaerminnensis) in a fully factorial design with 9 treatments and 30 replicates. We found no differences between simultaneous and sequential infections. However, H. tvaerminnensis fitness was impeded by multiple infection due to increased host mortality, which gave H. tvaerminnensis less time to grow. Host fecundity was also reduced across all treatments, but animals infected with O. colligata at a younger age produced the fewest offspring. As H. tvaerminnensis is both horizontally and vertically transmitted, this reduction in offspring may have further reduced H. tvaerminnensis fitness in co-infected treatments. Our findings suggest that in natural populations where both species co-occur, H. tvaerminnensis may evolve to higher levels of virulence following frequent co-infection by O. colligata.

Differential blood cells associated with parasitism in the wild puffer fish Lagocephalus laevigatus (Tetraodontiformes) of the Campeche Coast, southern Mexico

In aquaculture conditions, severe parasitic infections cause negative impacts on fish health and economic losses. The parasite load has been associated with anemia, which reduces the number of erythrocytes in fish. Therefore, the evaluation of hematological parameters as a feasible tool for diagnosing and monitoring fish health allows us to determine the indirect effect of parasites on the health status of fish. Our aim was to evaluate changes in the blood cell parameters of Lagocephalus laevigatus associated with parasitism. A total of 99 puffer fish were collected from the coast of Seybaplaya, Campeche. Each fish had 20 µl of peripheral blood drawn, and blood smears were performed in triplicate. The smears were stained with Giemsa stain, and a quantitative analysis of blood cells (erythrocytes, leukocytes, and monocytes) was obtained with an optical microscope at 100 ×. The parasites recovered from each fish were fixed and identified, and the infection parameters were calculated. Through generalized additive model analysis (GAMLSS), we observed that the infection intensity of puffer fish influenced changes in hematological parameters, principally in erythrocytes, neutrophils, thrombocytes, the total fish length, and the condition factor of the fish. In conclusion, this is the first study that provides baseline data on the hematological parameter variations in uninfected and infected L. laevigatus, the tropical wild puffer fish, as well as the possible effects on fish health. It is necessary to establish reference hematological patterns in wild populations for diagnosis and timely management with emphasis on aquaculture fish.

Climate change effects on terrestrial parasitic nematodes: Where are the knowledge gaps?

Climate change is expected to affect parasitic nematodes and hence possibly parasite-host dynamics and may have far-reaching consequences for animal health, livestock production, and ecosystem functioning. However, there has been no recent overview of current knowledge to identify how studies could contribute to a better understanding of terrestrial parasitic nematodes under changing climates. Here we screened almost 1,400 papers to review 57 experimental studies on the effects of temperature and moisture on hatching, development, survival, and behaviour of the free-living stages of terrestrial parasitic nematodes with a direct life cycle in birds and terrestrial mammals. Two major knowledge gaps are apparent. First, research should study the temperature dependency curves for hatching, development, and survival under various moisture treatments to test the interactive effect of temperature and moisture. Second, we specifically advocate for more studies that investigate how temperature, and its interaction with moisture, affect both vertical and horizontal movement of parasitic nematodes to understand infection risks. Overall, we advocate for more field experiments that test environmental effects on life-history traits and behaviour of parasitic nematodes in their free-living stages under natural and realistic circumstances. We also encourage studies to expand the range of used hosts and parasitic nematodes because 66% of results described in the available studies use sheep and cattle as hosts and 32% involve just three nematode species. This new comprehension brings attention to understudied abiotic impacts on terrestrial parasitic nematodes and will have broader implications for livestock management, wildlife conservation, and ecosystem functioning in a rapidly warming climate.

Fish endoparasite metacommunity in environments with different degrees of conservation in the western Brazilian Amazon

Parasite communities correspond to the definition of metacommunity, as species interact and disperse within hosts. The present study evaluated parasite metacommunities in a tropical floodplain. The study was conducted in the Western Amazon around the municipalities of Cruzeiro do Sul, state of Acre, and Guajará, state of Amazonas, Brazil. Six sampling sites were selected and grouped into conserved and degraded environments. Fish were caught between periods of drought and flood, using passive and active sampling methods; in the laboratory, they were measured weighed, and necropsied. Parasites found were fixed, evaluated, and identified. Physical and chemical variables and environmental conservation characteristics were measured in all sites. Diversity index, ANOVA, Tukey, local contribution to beta diversity (LCBD), species contribution to beta diversity by individual species (SCBD), and variance partitioning were summarized. The α species diversity increased in conserved environments and varied between seasonal periods, mainly in detritivorous and omnivorous hosts. Local contributions to beta diversity showed significantly higher values in conserved environments for the endoparasite fauna of piscivorous and omnivorous hosts, indicating that these environments presented unique parasite infracommunities and revealing the conservation status of these environments. Variations in infracommunities were explained mainly by niche-based processes, including environmental conditions, degree of conservation, and host characteristics. Thus, these data will serve as a tool to understand the way parasite communities are structured, which is important information for the management and conservation of aquatic environments.

Blood meal identification reveals extremely broad host range and host-bias in a temporary ectoparasite of coral reef fishes

Appreciation for the role of cryptofauna in ecological systems has increased dramatically over the past decade. The impacts blood-feeding arthropods, such as ticks and mosquitos, have on terrestrial communities are the subject of hundreds of papers annually. However, blood-feeding arthropods have been largely ignored in marine environments. Gnathiid isopods, often referred to as "ticks of the sea", are temporary external parasites of fishes. They are found in all marine environments and have many consequential impacts on host fitness. Because they are highly mobile and only associated with their hosts while obtaining a blood meal, their broader trophic connections are difficult to discern. Conventional methods rely heavily on detecting gnathiids on wild-caught fishes. However, this approach typically yields few gnathiids and does not account for hosts that avoid capture. To overcome this limitation, we sequenced blood meals of free-living gnathiids collected in light traps to assess the host range and community-dependent exploitation of Caribbean gnathiid isopods. Using fish-specific COI (cox1) primers, sequencing individual blood meals from 1060 gnathiids resulted in the identification of 70 host fish species from 27 families. Comparisons of fish assemblages to blood meal identification frequencies at four collection sites indicated that fishes within the families Haemulidae (grunts) and Lutjanidae (snappers) were exploited more frequently than expected based on their biomass, and Labrid parrotfishes were exploited less frequently than expected. The broad host range along with the biased exploitation of diel-migratory species has important implications for the role gnathiid isopods play in Caribbean coral reef communities.

Global perspective of environmental distribution and diversity of Perkinsea (Alveolata) explored by a meta-analysis of eDNA surveys

Perkinsea constitutes a lineage within the Alveolata eukaryotic superphylum, mainly composed of parasitic organisms. Some described species represent significant ecological and economic threats due to their invasive ability and pathogenicity, which can lead to mortality events. However, the genetic diversity of these described species is just the tip of the iceberg. Environmental surveys targeting this lineage are still scarce and mainly limited to the Northern Hemisphere. Here, we aim to conduct an in depth exploration of the Perkinsea group, uncovering the diversity across a variety of environments, including those beyond freshwater and marine ecosystems. We seek to identify and describe putative novel organisms based on their genetic signatures. In this study, we conducted an extensive analysis of a metabarcoding dataset, focusing on the V4 region of the 18S rRNA gene (the EukBank dataset), to investigate the diversity, distribution and environmental preferences of the Perkinsea. Our results reveal a remarkable diversity within the Perkinsea, with 1568 Amplicon Sequence Variants (ASVs) identified across thousands of environmental samples. Surprisingly, we showed a substantial diversity of Perkinsea within soil samples (269 ASVs), challenging the previous assumption that this group is confined to marine and freshwater environments. In addition, we revealed that a notable proportion of Perkinsea ASVs (428 ASVs) could correspond to putative new organisms, encompassing the well-established taxonomic group Perkinsidae. Finally, our study shed light on previously unveiled taxonomic groups, including the Xcellidae, and revealed their environmental distribution. These findings demonstrate that Perkinsea exhibits far greater diversity than previously detected and surprisingly extends beyond marine and freshwater environments. The meta-analysis conducted in this study has unveiled the existence of previously unknown clusters within the Perkinsea lineage, solely identified based on their genetic signatures. Considering the ecological and economic importance of described Perkinsea species, these results suggest that Perkinsea may play a significant, yet previously unrecognized, role across a wide range of environments, spanning from soil environments to the abyssal zone of the open ocean with important implications for ecosystem functioning.

How might bacteriophages shape biological invasions?

Invasions by eukaryotes dependent on environmentally acquired bacterial mutualists are often limited by the ability of bacterial partners to survive and establish free-living populations. Focusing on the model legume-rhizobium mutualism, we apply invasion biology hypotheses to explain how bacteriophages can impact the competitiveness of introduced bacterial mutualists. Predicting how phage-bacteria interactions affect invading eukaryotic hosts requires knowing the eco-evolutionary constraints of introduced and native microbial communities, as well as their differences in abundance and diversity. By synthesizing research from invasion biology, as well as bacterial, viral, and community ecology, we create a conceptual framework for understanding and predicting how phages can affect biological invasions through their effects on bacterial mutualists.

The parasitic nematode Strongyloides ratti exists predominantly as populations of long-lived asexual lineages

Nematodes are important parasites of people and animals, and in natural ecosystems they are a major ecological force. Strongyloides ratti is a common parasitic nematode of wild rats and we have investigated its population genetics using single-worm, whole-genome sequencing. We find that S. ratti populations in the UK consist of mixtures of mainly asexual lineages that are widely dispersed across a host population. These parasite lineages are likely very old and may have originated in Asia from where rats originated. Genes that underly the parasitic phase of the parasite's life cycle are hyperdiverse compared with the rest of the genome, and this may allow the parasites to maximise their fitness in a diverse host population. These patterns of parasitic nematode population genetics have not been found before and may also apply to Strongyloides spp. that infect people, which will affect how we should approach their control.

Prey selection along a predators' body size gradient evidences the role of different trait-based mechanisms in food web organization

An increase in prey richness, prey size and predator trophic position with predator body size has been consistently reported as prime features of food web organization. These trends have been explained by non-exclusive mechanisms. First, the increase in energy demand with body size determines that larger predators must reduce prey selectivity for achieving the required number of resources, being consumption relationships independent of prey traits. Second, when consumption is restricted by gape limitation, small predators are constrained to select among small prey. However, this selection weakens over large predators, which progressively consume more and larger prey. Finally, the optimal foraging mechanism predicts that larger predators optimize their diet by selecting only large prey with high energy reward. Each one of these mechanisms can individually explain the increase in prey richness, prey size and predator trophic position with predator body size but their relative importance or the direct evidence for their combined role was seldom considered. Here we use the community assembly by trait selection (CATS) theory for evaluating the support for each one of these mechanisms based on the prey selection patterns that they predict. We analyzed how prey body size and trophic guild determine prey selection by predators of increasing body size in a killifish guild from a temporary pond system. Results support the combination of the three mechanisms to explain the structural trends in our food web, although their strength is contingent on prey trophic group. Overall, high energy prey are preferred by larger predators, and small predators select small prey of all trophic status. However, large predators prefer large primary producers and avoid large carnivorous prey, probably because of the inherent risk of consuming other carnivorous. Our study provides a mechanistic understanding of how predator traits determine the selection of prey traits affecting food web assembly.

The Reemergence of Phycopathology: When Algal Biology Meets Ecology and Biosecurity

Viruses, bacteria, and eukaryotic symbionts interact with algae in a variety of ways to cause disease complexes, often shaping marine and freshwater ecosystems. The advent of phyconomy (a.k.a. seaweed agronomy) represents a need for a greater understanding of algal disease interactions, where underestimated cryptic diversity and lack of phycopathological basis are prospective constraints for algal domestication. Here, we highlight the limited yet increasing knowledge of algal pathogen biodiversity and the ecological interaction with their algal hosts. Finally, we discuss how ecology and cultivation experience contribute to and reinforce aquaculture practice, with the potential to reshape biosecurity policies of seaweed cultivation worldwide.

The fast-food effect: costs of being a generalist in a human-dominated landscape

Agricultural expansion in Southeast Asia has converted most natural landscapes into mosaics of forest interspersed with plantations, dominated by the presence of generalist species that benefit from resource predictability. Dietary shifts, however, can result in metabolic alterations and the exposure of new parasites that can impact animal fitness and population survival. Our study focuses on the Asian water monitor lizard (Varanus salvator), one of the largest predators in the Asian wetlands, as a model species to understand the health consequences of living in a human-dominated landscape in Sabah, Malaysian Borneo. We evaluated the effects of dietary diversity on the metabolism of monitor lizards and the impact on the composition of their parasite communities in an oil palm-dominated landscape. Our results showed that (1) rodent-dominated diets were associated with high levels of lipids, proteins and electrolytes, akin to a fast-food-based diet of little representativeness of the full nutritional requirements, but highly available, and (2) lizards feeding on diverse diets hosted more diverse parasite communities, however, at overall lower parasite prevalence. Furthermore, we observed that the effect of diet on lipid concentration differed depending on the size of individual home ranges, suggesting that sedentarism plays an important role in the accumulation of cholesterol and triglycerides. Parasite communities were also affected by a homogeneous dietary behaviour, as well as by habitat type. Dietary diversity had a negative effect on both parasite richness and prevalence in plantations, but not in forested areas. Our study indicates that human-dominated landscapes can pose a negative effect on generalist species and hints to the unforeseen health consequences for more vulnerable taxa using the same landscapes. Thus, it highlights the potential role of such a widely distributed generalist as model species to monitor physiological effects in the ecosystem in an oil palm-dominated landscape.

Tapping into natural history collections to assess latitudinal gradients of parasite diversity

Parasites are key components of the biosphere not only due to their huge diversity, but also because they exert important influences on ecological processes. Nevertheless, we lack an understanding of the biogeographical patterns of parasite diversity. Here, we tap into the potential of biodiversity collections for understanding parasite biogeography. We assess species richness of supracommunities of helminth parasites infecting mammal assemblages in the Nearctic, and describe its relation to latitude, climate, host diversity, and land area. We compiled data from parasitology collections and assessed parasite diversity in Nearctic ecoregions for the entire parasite supracommunity of mammals in each ecoregion, as well as separately from carnivores and rodents to explore the effect of host taxonomic resolution on observed patterns. For carnivores, we found evidence of a negative latitudinal gradient, while parasites of rodents displayed no clear pattern. We found that parasite diversity was positively correlated with mean annual temperature and negatively correlated with seasonal precipitation. Parasite richness shows a diversity peak at intermediate host richness values and in carnivores correlates with temperature and seasonal precipitation. Rodent parasite diversity did not correlate with explored factors. Other researchers are encouraged to use parasitology collections to continue exploring patterns of parasite biogeography and macroecology.

A Rhizocephalan Parasite Induces Pervasive Effects on Its Shrimp Host

AbstractRhizocephalan barnacles are parasites of crustaceans that are known for dramatic effects on hosts, including parasitic castration, feminization, molt inhibition, and the facilitation of epibiosis. Most research on rhizocephalans has focused on carcinized hosts, with relatively little research directed to shrimp hosts that may experience distinct consequences of infection. Here, we describe a high-prevalence rhizocephalan-shrimp system in which multiple host changes are associated with infection: the dock shrimp Pandalus danae infected by the rhizocephalan Sylon hippolytes. In field-collected P. danae, infection by Sylon was associated with development of female sex characters at a smaller size and greater probability of epibiosis. Standardized video observations showed that infected P. danae performed grooming activities at higher rates than uninfected shrimp, suggesting that inhibited molting rather than direct behavioral modification is a likely mechanism for higher epibiosis rates. There was no difference in the composition of grooming behavior types or in general activity between infected and uninfected shrimp. Fatty acid compositions differed with infection, but total lipid concentrations did not, suggesting that parasite-driven shifts in host resource allocation were compensated or redirected from unmeasured tissues. Our results show that Sylon alters its host's role by provisioning an epibiotic substrate and also that it influences host physiology, resulting in feminization and fatty acid shifts. This study lays the groundwork for expanding rhizocephalan-shrimp research and encourages recognition of oft-ignored roles of parasitism in ecological communities.

Moderate Grazing Promotes Arthropod Species Diversity in an Alpine Meadow

Livestock grazing is an important tool used in grassland land management practices. Studies have substantially addressed the effect of grazing on plant species diversity, revealing that moderate grazing increases plant species diversity. However, few studies have dealt with the relationship between grazing and arthropod species diversity, which remains unclear. Here, we hypothesize that moderate grazing promotes arthropod species diversity because arthropods are directly or indirectly dependent on plant diversity. In this study, we conducted a two-year plant and arthropod survey from 2020 to 2021 at four levels of grazing intensity, i.e., nongrazing (as a control), light grazing, moderate grazing, and heavy grazing, of the long-term grazing experiment starting in 2016. The data show that plant species diversity peaked in the moderate grazing treatment, and herbivore species diversity was positively correlated with plant species diversity (and hence peaked in the moderate grazing treatment). Moderate grazing promoted parasitoid species diversity, which was positively correlated with herbivore species diversity. However, predator species diversity did not significantly differ among the four treatments. In addition, saprophage species diversity decreased, whereas coprophages increased with increasing grazing levels, such that species richness (but not species diversity of detritivores statistically) was highest in the moderate grazing treatment. Consequently, the species diversity of arthropods as a whole peaked at the moderate grazing level, a phenomenology that is consistent with the intermediate disturbance hypothesis. Considering that moderate grazing has been found to increase plant species diversity, facilitate soil carbon accumulation, and prevent soil erosion, we suggest that moderate grazing would maximize multi-functional ecosystem services.

Variation in the parasite communities of three co-occurring herbivorous coral reef fishes

Parasites are important, diverse, and abundant components of natural ecosystems and can influence the behaviour and health of their hosts, inter- and intraspecific interactions, and ultimately community structure. Coral reefs are one of the world's most biodiverse ecosystems, yet our understanding of the abundance, diversity, and composition of parasite communities of coral reef fishes is limited. Here, the authors aimed to compare the abundance, richness and composition of parasite communities among three co-occurring herbivorous coral reef fishes (the barred rabbitfish Siganus doliatus, Ward's damsel Pomacentrus wardi and the obscure damsel Pomacentrus adelus) from an inshore reef of the Great Barrier Reef (GBR). In total, 3978 parasites (3869 endoparasites and 109 ectoparasites) from 17 families were recovered from 30 individuals of each of the three fish species (mean = 44 ± 22 s.e. parasites per fish; range = 0-1947 parasites per fish). The parasite communities of P. wardi and P. adelus were characterised by pennellid copepods, derogenid and lecithasterid digeneans and were distinct from those of S. doliatus that were characterised by a higher abundance of atractotrematid and gyliauchenid digeneans. The abundance and family richness of all parasites were greatest in S. doliatus (abundance: 22.1 ± 5.0 parasites per fish; richness: 3.2 ± 0.3 families per fish), intermediate in P. wardi (abundance: 4.8 ± 1.1 parasites per fish; richness: 2.3 ± 0.3 families per fish) and lowest in P. adelus (abundance: 1.4 ± 0.4 parasites per fish; richness: 0.9 ± 0.2 families per fish). Similarly, the abundance of endoparasites was greatest in S. doliatus (19.7 ± 5.1 endoparasites per fish), intermediate in P. wardi (2.6 ± 0.7 endoparasites per fish) and lowest in P. adelus (1.2 ± 0.4 endoparasites per fish). Ectoparasite abundances were also lowest for P. adelus (0.2 ± 0.1 ectoparasites per fish), and S. doliatus and P. wardi had comparable abundances of ectoparasites (1.3 ± 0.3 and 2.1 ± 0.5 parasites per fish, respectively). Similarities between the parasite assemblages of the two pomacentrids may be related to their similar behaviours and/or diets vs. those of the larger-bodied and more mobile rabbitfish. Investigating the causes and consequences of variation in parasite communities across a broader range of fish species will be critical to understand the potential role of parasites in coral reef ecosystems.

Host genetic identity determines parasite community structure across time and space in oyster restoration

Intraspecific variation in host susceptibility to individual parasite species is common, yet how these effects scale to mediate the structure of diverse parasite communities in nature is less well understood. To address this knowledge gap, we tested how host genetic identity affects parasite communities on restored reefs seeded with juvenile oysters from different sources-a regional commercial hatchery or one of two wild progenitor lines. We assessed prevalence and intensity of three micro- and two macroparasite species for 4 years following restoration. Despite the spatial proximity of restored reefs, oyster source identity strongly predicted parasite community prevalence across all years, with sources varying in their relative susceptibility to different parasites. Oyster seed source also predicted reef-level parasite intensities across space and through time. Our results highlight that host intraspecific variation can shape parasite community structure in natural systems, and reinforce the importance of considering source identity and diversity in restoration design.

Consumption of trematode parasite infectious stages: from conceptual synthesis to future research agenda

Given their sheer cumulative biomass and ubiquitous presence, parasites are increasingly recognized as essential components of most food webs. Beyond their influence as consumers of host tissue, many parasites also have free-living infectious stages that may be ingested by non-host organisms, with implications for energy and nutrient transfer, as well as for pathogen transmission and infectious disease dynamics. This has been particularly well-documented for the cercaria free-living stage of digenean trematode parasites within the Phylum Platyhelminthes. Here, we aim to synthesize the current state of knowledge regarding cercariae consumption by examining: (a) approaches for studying cercariae consumption; (b) the range of consumers and trematode prey documented thus far; (c) factors influencing the likelihood of cercariae consumption; (d) consequences of cercariae consumption for individual predators (e.g. their viability as a food source); and (e) implications of cercariae consumption for entire communities and ecosystems (e.g. transmission, nutrient cycling and influences on other prey). We detected 121 unique consumer-by-cercaria combinations that spanned 60 species of consumer and 35 trematode species. Meaningful reductions in transmission were seen for 31 of 36 combinations that considered this; however, separate studies with the same cercaria and consumer sometimes showed different results. Along with addressing knowledge gaps and suggesting future research directions, we highlight how the conceptual and empirical approaches discussed here for consumption of cercariae are relevant for the infectious stages of other parasites and pathogens, illustrating the use of cercariae as a model system to help advance our knowledge regarding the general importance of parasite consumption.

Bioenergetics modelling of growth processes in parasitized Eastern Baltic cod (Gadus morhua L.)

Changes in physiological processes can reveal how individuals respond to environmental stressors. It can be difficult to link physiological responses to changes in vital rates such as growth, reproduction and survival. Here, bioenergetics modelling can aid in understanding non-intuitive outcomes from stressor combinations. Building on an established bioenergetics model, we examine the potential effects of parasite infection on growth rate and body condition. Parasites represent an overlooked biotic factor, despite their known effects on the physiology of the host organism. As a case study, we use the host-parasite system of Eastern Baltic cod (Gadus morhua) infected with the parasitic nematode Contraceacum osculatum. Eastern Baltic cod have during the past decade experienced increasing infection loads with C. osculatum that have been shown to lead to physiological changes. We hypothesized that infection with parasites affects cod growth negatively as previous studies reveal that the infections lead to reduced energy turnover, severe liver disease and reduced nutritional condition. To test this, we implemented new variables into the bioenergetics model representing the physiological changes in infected fish and parameterized these based on previous experimental data. We found that growth rate and body condition decreased with increased infection load. Highly infected cod reach a point of no return where their energy intake cannot maintain a surplus energy balance, which may eventually lead to induced mortality. In conclusion, parasite infections cannot be ignored when assessing drivers of fish stock dynamics.

Prey-mimetism in cercariae of Apatemon (Digenea, Strigeidae) in freshwater in northern latitudes

Cercariae, the free-living larval stages of trematodes, have adopted an amazing variety of transmission strategies. One of them is prey-mimetism, i.e. cercariae mimicking prey to attract motile hosts to be eaten. In a period between 2002 and 2019, we examined small planorbid snails, Bathyomphalus contortus, Gyraulus parvus and Planorbis planorbis from lakes in Finland and Iceland and from the Curonian Lagoon in Lithuania. Cercariae with conspicuously enlarged tails and unusual swimming behaviour, likely mimicking invertebrate prey, were detected and studied by the use of morphological and molecular (cox1, ITS1-5.8S-ITS2 and 28S rDNA) methods. Cercariae of two species belonging to the genus Apatemon (Strigeidae) were recognised. We consider Apatemon sp. 5 ex P. planorbis from the Curonian Lagoon identical to Cercaria globocaudata U. Szidat, 1940. Cercariae ex G. parvus from Iceland and ex B. contortus from Finland were conspecific, and we named them Apatemon sp. 6; these cercariae could not be associated with any known species. For the first time, we verified that cercariae of the Bulbocauda group belong to the genus Apatemon. We provide a mini-review on records of furcocercariae of the family Strigeidae with enlarged tails reported in freshwaters of the northern hemisphere and reveal that it is not only Apatemon but also Australapatemon and most likely Strigea which belong to the Bulbocauda group, rendering it a purely ecological assemblage. Understanding which invertebrate swimming behaviour these cercariae are mimicking will enhance our knowledge of the processes behind trematode transmission and will help to assess evolutionary pathways of host-finding strategies in trematodes.

Combining generalized modeling and specific modeling in the analysis of ecological networks

The complexity of real food webs involves uncertainty in data and in underlying ecological processes, and modeling approaches deal with these challenges differently. Generalized modeling provides a linear stability analysis without narrow specification of all processes, and conventional dynamical systems models approximate functional forms to discuss trajectories in phase space. This study compares results and ecological interpretations from both methods in four-species ecological networks at steady state. We find that a specific (dynamical systems) model only provides a subset of stability data from the generalized model, which spans many plausible dynamic scenarios, allowing for conflicting results. Nevertheless, both approaches reveal that fixed points become stable when nutrient flows to predators are fettered and even more when the basal growth rate approaches a maximum. The specific model identifies a distinct ecosystem response to bottom-up forcing, the enrichment of lower trophic levels. Enrichment stabilizes a fixed point when basal species are in a resource-deprived environment but destabilizes it if resources become more abundant. The generalized model provides less specific information since infinitely many paths of enrichment are hypothetical. Nevertheless, generalized modeling of ecological systems is a powerful technique that enables a meta analysis of these uncertain complex systems.

Parasites either reduce or increase host vulnerability to fishing: a case study of a parasitic copepod and its salmonid host

Parasites generally increase host vulnerability to predators via host manipulation for trophic transmission and reduction of host activities. Predators also select prey depending on the parasite infection status. Despite such parasites' roles in prey-predator interactions in wild animals, how parasites affect human hunting probability and resource consumption remains unknown. We examined the effects of the ectoparasitic copepod Salmincola cf. markewitschi on fish vulnerability to angling. We found that infected fish were less vulnerable compared with non-infected fish when the fish body condition was low, which was probably due to reduced foraging activity. On the contrary, infected fish were more vulnerable when the host body condition was high, probably due to the compensation of parasites' negative effects. A Twitter analysis also suggested that people avoided eating fish with parasites and that anglers' satisfaction decreased when captured fish were parasitized. Thus, we should consider how animal hunting is affected by parasites not only for catchability but also for avoiding parasite infection sources in many local regions.

Environmental refuges from disease in host-parasite interactions under global change

The physiological performance of organisms depends on their environmental context, resulting in performance-response curves along environmental gradients. Parasite performance-response curves are generally expected to be broader than those of their hosts due to shorter generation times and hence faster adaptation. However, certain environmental conditions may limit parasite performance more than that of the host, thereby providing an environmental refuge from disease. Thermal disease refuges have been extensively studied in response to climate warming, but other environmental factors may also provide environmental disease refuges which, in turn, respond to global change. Here, we (1) showcase laboratory and natural examples of refuges from parasites along various environmental gradients, and (2) provide hypotheses on how global environmental change may affect these refuges. We strive to synthesize knowledge on potential environmental disease refuges along different environmental gradients including salinity and nutrients, in both natural and food-production systems. Although scaling up from single host-parasite relationships along one environmental gradient to their interaction outcome in the full complexity of natural environments remains difficult, integrating host and parasite performance-response can serve to formulate testable hypotheses about the variability in parasitism outcomes and the occurrence of environmental disease refuges under current and future environmental conditions.

The trematode Podocotyle atomon modulates biochemical responses of Gammarus locusta to thermal stress but not its feeding rate or survival

Although parasitism is one of the most common species interactions in nature, the role of parasites in their hosts' thermal tolerance is often neglected. This study examined the ability of the trematode Podocotyle atomon to modulate the feeding and stress response of Gammarus locusta towards temperature. To accomplish this, infected and uninfected females and males of Gammarus locusta were exposed to temperatures (2, 6, 10, 14, 18, 22, 26, 30 °C) for six days. Shredding (change in food biomass) and defecation rates (as complementary measure to shredding rate) were measured as proxies for feeding activity. Lipid and glycogen concentrations (energy reserves), catalase (oxidative stress indicator), and phenoloxidase (an immunological response in invertebrates) were additionally measured. Gammarid survival was optimal at 10 °C as estimated by the linear model and was unaffected by trematode infection. Both temperature and sex influenced the direction of infection effect on phenoloxidase. Infected females presented lower phenoloxidase activity than uninfected females at 14 and 18 °C, while males remained unaffected by infection. Catalase activity increased at warmer temperatures for infected males and uninfected females. Higher activity of this enzyme at colder temperatures occurred only for infected females. Infection decreased lipid content in gammarids by 14 %. Infected males had significantly less glycogen than uninfected, while infected females showed the opposite trend. The largest infection effects were observed for catalase and phenoloxidase activity. An exacerbation of catalase activity in infected males at warmer temperatures might indicate (in the long-term) unsustainable, overwhelming, and perhaps lethal conditions in a warming sea. A decrease in phenoloxidase activity in infected females at warmer temperatures might indicate a reduction in the potential for fighting opportunistic infections. Results highlight the relevance of parasites and host sex in organismal homeostasis and provide useful insights into the organismal stability of a widespread amphipod in a warming sea.

Habitat fragmentation matters more than habitat loss: The case of host-parasite interactions

While ecologists agree that habitat loss has a substantial negative effect on biodiversity it is still very much a matter of debate whether habitat fragmentation has a lesser effect and whether this effect is positive or negative for biodiversity. Here, we assess the relative influence of tropical forest loss and fragmentation on the prevalence of vector-borne blood parasites of the genera Plasmodium and Haemoproteus in six forest bird species. We also determine whether habitat loss and fragmentation are associated with a rise or fall in prevalence. We sample more than 4000 individual birds from 58 forest sites in Guadeloupe and Martinique. Considering 34 host-parasite combinations independently and a fine characterization of the amount and spatial configuration of habitat, we use partial least square regressions to disentangle the relative effects of forest loss, forest fragmentation, landscape heterogeneity, and local weather conditions on spatial variability of parasite prevalence. Then we test for the magnitude and the sign of the effect of each environmental descriptor. Strikingly, we show that forest fragmentation explains twice as much of the variance in prevalence as habitat loss or landscape heterogeneity. In addition, habitat fragmentation leads to an overall rise in prevalence in Guadeloupe, but its effect is variable in Martinique. Both habitat loss and landscape heterogeneity exhibit taxon-specific effects. Our results suggest that habitat loss and fragmentation may have contrasting effects between tropical and temperate regions and that inter-specific interactions may not respond in the same way as more commonly used biodiversity metrics such as abundance and diversity.

Sensitivity of bipartite network analyses to incomplete sampling and taxonomic uncertainty

Bipartite network analysis is a powerful tool to study the processes structuring interactions in ecological communities. In applying the method, it is assumed that the sampled interactions provide an accurate representation of the actual community. However, acquiring a representative sample may be difficult as not all species are equally abundant or easily identifiable. Two potential sampling issues can compromise the conclusions of bipartite network analyses: failure to capture the full range of interactions (sampling completeness) and use of a taxonomic level higher than species to evaluate the network (taxonomic resolution). We asked how commonly used descriptors of bipartite antagonistic communities (modularity, nestedness, connectance, and specialization [H₂ ']) are affected by reduced host sampling completeness, parasite taxonomic resolution, and their crossed effect, as they are likely to co-occur. We used a quantitative niche model to generate weighted bipartite networks that resembled natural host-parasite communities. The descriptors were more sensitive to uncertainty in parasite taxonomic resolution than to host sampling completeness. When only 10% of parasite taxonomic resolution was retained, modularity and specialization decreased by ~76% and ~12%, respectively, and nestedness and connectance increased by ~114% and ~345% respectively. The loss of taxonomic resolution led to a wide range of possible communities, which made it difficult to predict its effects on a given network. With regards to host sampling completeness, standardized nestedness, connectance, and specialization were robust, whereas modularity was sensitive (~30% decrease). The combination of both sampling issues had an additive effect on modularity. In communities with low effort for both sampling issues (50%-10% of sampling completeness and taxonomic resolution), estimators of modularity, and nestedness could not be distinguished from those of random assemblages. Thus, the categorical description of communities with low sampling effort (e.g., if a community is modular or not) should be done with caution. We recommend evaluating both sampling completeness and taxonomic certainty when conducting bipartite network analyses. Care should also be exercised when using nonrobust descriptors (the four descriptors for parasite taxonomic resolution; modularity for host sampling completeness) when sampling issues are likely to affect a dataset.

A new technique to study nutrient flow in host-parasite systems by carbon stable isotope analysis of amino acids and glucose

Stable isotope analysis of individual compounds is emerging as a powerful tool to study nutrient origin and conversion in host-parasite systems. We measured the carbon isotope composition of amino acids and glucose in the cestode Schistocephalus solidus and in liver and muscle tissues of its second intermediate host, the three-spined stickleback (Gasterosteus aculeatus), over the course of 90 days in a controlled infection experiment. Similar linear regressions of δ¹³C values over time and low trophic fractionation of essential amino acids indicate that the parasite assimilates nutrients from sources closely connected to the liver metabolism of its host. Biosynthesis of glucose in the parasite might occur from the glucogenic precursors alanine, asparagine and glutamine and with an isotope fractionation of - 2 to - 3 ‰ from enzymatic reactions, while trophic fractionation of glycine, serine and threonine could be interpreted as extensive nutrient conversion to fuel parasitic growth through one-carbon metabolism. Trophic fractionation of amino acids between sticklebacks and their diets was slightly increased in infected compared to uninfected individuals, which could be caused by increased (immune-) metabolic activities due to parasitic infection. Our results show that compound-specific stable isotope analysis has unique opportunities to study host and parasite physiology.

The consumption of viruses returns energy to food chains

Viruses impact host cells and have indirect effects on ecosystem processes. Plankton such as ciliates can reduce the abundance of virions in water, but whether virus consumption translates into demographic consequences for the grazers is unknown. Here, we show that small protists not only can consume viruses they also can grow and divide given only viruses to eat. Moreover, the ciliate Halteria sp. foraging on chloroviruses displays dynamics and interaction parameters that are similar to other microbial trophic interactions. These results suggest that the effect of viruses on ecosystems extends beyond (and in contrast to) the viral shunt by redirecting energy up food chains.

Resetting our expectations for parasites and their effects on species interactions: a meta-analysis

Despite the ubiquitous nature of parasitism, how parasitism alters the outcome of host-species interactions such as competition, mutualism and predation remains unknown. Using a phylogenetically informed meta-analysis of 154 studies, we examined how the mean and variance in the outcomes of species interactions differed between parasitized and non-parasitized hosts. Overall, parasitism did not significantly affect the mean or variance of host-species interaction outcomes, nor did the shared evolutionary histories of hosts and parasites have an effect. Instead, there was considerable variation in outcomes, ranging from strongly detrimental to strongly beneficial for infected hosts. Trophically-transmitted parasites increased the negative effects of predation, parasites increased and decreased the negative effects of interspecific competition for parasitized and non-parasitized heterospecifics, respectively, and parasites had particularly strong negative effects on host species interactions in freshwater and marine habitats, yet were beneficial in terrestrial environments. Our results illuminate the diverse ways in which parasites modify critical linkages in ecological networks, implying that whether the cumulative effects of parasitism are considered detrimental depends not only on the interactions between hosts and their parasites but also on the many other interactions that hosts experience.

Seasonal patterns of infestation by monogenean parasites of fish and their relationship with water parameters in two rivers with different disturbance gradients in southeastern Brazil

Here, we evaluate the relationships between the infestation rates of five monogenean parasites species with the dry and wet seasons, with the organic and inorganic parameters of the water of two rivers: the Jacaré-Pepira and Jacaré-Guaçú, and with the condition factors of its fish hosts: Serrasalmus maculatus and Astronotus crassipinnis, in the state of São Paulo, southeastern Brazil. Fish were collected between January and December 2017. Anacanthorus serrasalmi, Amphithecium speirocamarotum and Gussevia asota had higher abundance rates (Student's t test, p ≤ 0.05) in the wet season. Gussevia asota had its abundance negatively correlated to nitrate in the Jacaré-Pepira River and with total nitrogen and potassium in the Jacaré-Guaçú River. Regarding the fish hosts condition factors, was observed a positive correlation with the abundances of G. asota in the Jacaré-Guaçú River, and with A. serrasalmi in the Jacaré-Pepira River. In general, wet season favored an increasing in the infestation rates of the monogeneans parasites in their host species, mainly in the river considered as the most polluted, the Jacaré-Guaçú River. Of the five parasites species analyzed in this study, only Gussevia astronoti and Rhinoxenus piranhus had no interaction with seasonality, river water variables, or fish host condition factors. On the other hand, G. asota had interactions both with water parameters (nitrate and total nitrogen) and with the hosts condition factors, which reflected in the abundance and intensity rates, showing itself as a species sensitive to changes in the environment and, therefore, that can be considered as a bioindicator organism.

Ecology and effects of metazoan parasites of fish in transitional waters

Given the abundance, heterogeneity and ubiquity of parasitic organisms, understanding how they influence biodiversity, evolution, health and ecosystem functionality is crucial, especially currently when anthropogenic pressures are altering host–parasite balances. This review describes the features, roles and impacts of metazoan parasites of fish occurring in transitional waters (TW). These aquatic ecosystems are highly productive and widespread around the globe and represent most favourable theatres for parasitism given the availability of hosts (invertebrates, fishes and birds) and an increased probability of parasite transmission, especially of those having complex life cycles. Fascinating examples of how parasitism can influence different hierarchical levels of biological systems, from host individuals and populations to entire aquatic communities, through effects on food webs come from this kind of ecosystem. Edible fish of commercial value found in TW can harbour some parasite species, significantly reducing host health, marketability and food safety, with possible economic and public health consequences. Many TW are historically exploited by humans as sources of relevant ecosystem services, including fisheries and aquaculture, and they are highly vulnerable ecosystems. Alteration of TW can be revealed through the study of parasite communities, contributing, as bioindicators, for assessing environmental changes, health and restoration. Fish parasites can provide much information about TW, but this potential appears to be not fully exploited. More studies are necessary to quantify the ecological, economic and medical impacts fish parasites can have on these important ecosystems.

Daily dynamics of contrasting spring algal blooms in Santa Monica Bay (central Southern California Bight)

Protistan algae (phytoplankton) dominate coastal upwelling ecosystems where they form massive blooms that support the world's most important fisheries and constitute an important sink for atmospheric CO₂ . Bloom initiation is well understood, but the biotic and abiotic forces that shape short-term dynamics in community composition are still poorly characterized. Here, high-frequency (daily) changes in relative abundance dynamics of the metabolically active protistan community were followed via expressed 18S V4 rRNA genes (RNA) throughout two algal blooms during the spring of 2018 and 2019 in Santa Monica Bay (central Southern California Bight). A diatom bloom formed after wind-driven, nutrient upwelling events in both years, but different taxa dominated each year. Whereas diatoms bloomed following elevated nutrients and declined after depletion each year, a massive dinoflagellate bloom manifested under relatively low inorganic nitrogen conditions following diatom bloom senescence in 2019 but not 2018. Network analysis revealed associations between diatoms and cercozoan putative parasitic taxa and syndinean parasites during 2019 that may have influenced the demise of the diatoms, and the transition to a dinoflagellate-dominated bloom.

Taxonomic and functional dynamics during chytrid epidemics in an aquatic ecosystem

Fungal parasitism is common in plankton communities and plays a crucial role in the ecosystem by balancing nutrient cycling in the food web. Previous studies of aquatic ecosystems revealed that zoosporic chytrid epidemics represent an important driving factor in phytoplankton seasonal successions. In this study, host-parasite dynamics in Lake Pavin (France) were investigated during the spring diatom bloom while following chytrid epidemics using next generation sequencing (NGS). Metabarcoding analyses were applied to study changes in the eukaryotic microbial community throughout diatom bloom-chytrid epidemics. Relative read abundances of metabarcoding data revealed potential "beneficiaries" and "victims" during the studied period. Subsequently, metatranscriptomic analyses on samples before and during the chytrid epidemic unveiled the active part of the community and functional/metabolic dynamics in association with the progress of chytrid infection. Diatom functions involving lipases, transporters, histones, vacuolar systems, the proteasome, proteases and DNA/RNA polymerases were more abundant during the diatom bloom. Chytrid functions related to a parasitic lifestyle including invasion, colonization and stress tolerance were up-regulated during the chytrid epidemic. In addition, functions related to the degradation/metabolism of proteins, lipids and chitin were in higher proportion in the community during the epidemic event. Results of NGS and bioinformatics analyses offered a panorama of dynamic biodiversity and biological functioning of the community.

Insect herbivory immediately before the eclipse of the gymnosperms: The Dawangzhangzi plant assemblage of Northeastern China

The Early Cretaceous terrestrial revolution involved global shifts from gymnosperm- to angiosperm-dominated floras. However, responses of insect herbivores to these changes remain unexamined. We evaluated 2 176 highly sampled plant specimens representing 62 species/morphotypes from the 126 Ma Dawangzhangzi plant assemblage of Northeastern China. Our study consisted of horsetails, ferns, ginkgoaleans, czekanowskialeans, conifers, and an angiosperm. Their herbivory was evaluated by the functional feeding groups of hole feeding, margin feeding, and surface feeding (ectophytic feeders); piercer and suckers, and ovipositing insects (ectoendophytic feeders); mining, galling, and borings (endophytic feeders); and pathogens, collectively constituting 65 damage types (DTs). The plant assemblage was assessed for herbivory richness by DT richness, component community structure, and DT specialization on plant hosts; for herbivory intensity, it was evaluated for DT frequency, herbivorized surface area, and feeding event occurrences. Using feeding event occurrences, the data supported seven species/morphotypes as most intensely herbivorized: Liaoningocladus boii (76.6%), Czekanowskia sp. 1 (8.4%), Czekanowskia rigida (4.10%), Lindleycladus lanceolatus (3.5%), Ginkgoites sp. 2 (2.0%), Podozamites sp. 1 (1.1%), and Solenites sp. 1 (0.9%). The most herbivorized taxa were pinaleans (conifers), then czekanowskialeans, and lastly ginkgoaleans; the monodominant component community was the conifer Liaoningocladus boii. DT host specialization levels were low. The plant assemblage had an overall low 0.86% of foliage removed by herbivores, explained by physical and chemical antiherbivore defenses, and parasitoid attack. Although Paleozoic, gymnosperm-dominated assemblages had greater herbivory, component community structure of the three most herbivorized taxa are more similar to modern bracken fern and willow than modern gymnosperm taxa.

Where traditional extinction estimates fall flat: using novel cophylogenetic methods to estimate extinction risk in platyhelminths

Today parasites comprise a huge proportion of living biodiversity and play a major role in shaping community structure. Given their ecological significance, parasite extinctions could result in massive cascading effects across ecosystems. It is therefore crucial that we have a way of estimating their extinction risk. Attempts to do this have often relied on information about host extinction risk, without explicitly incorporating information about the parasites. However, assuming an identical risk may be misleading. Here, we apply a novel metric to estimate the cophylogenetic extinction rate, Ec, of parasites with their hosts. This metric incorporates information about the evolutionary history of parasites and hosts that can be estimated using event-based cophylogenetic methods. To explore this metric, we investigated the use of different cophylogenetic methods to inform the Ec rate, based on the analysis of polystome parasites and their anuran hosts. We show using both parsimony- and model-based approaches that different methods can have a large effect on extinction risk estimation. Further, we demonstrate that model-based approaches offer greater potential to provide insights into cophylogenetic history and extinction risk.

Root hemiparasitic plants are associated with more even communities across North America

Root hemiparasitic plants both compete with and extract resources from host plants. By reducing the abundance of dominant plants and releasing subordinates from competitive exclusion, they can have an outsized impact on plant communities. Most research on the ecological role of hemiparasites is manipulative and focuses on a small number of hemiparasitic taxa. Here, we ask whether patterns in natural plant communities match the expectation that hemiparasites affect the structure of plant communities. Our data were collected on 129 national park units spanning the continental United States. The most common hemiparasite genera were Pedicularis, Castilleja, Krameria, and Comandra. We used null models and linear mixed models to determine whether hemiparasites were associated with changes in community richness and evenness. Hemiparasite presence did not affect community metrics. Hemiparasite abundance was positively associated with increasing evenness of herbaceous species, but not with species richness. The associations that we observed on a continental scale are consistent with evidence that the impacts of root hemiparasitic plants on evenness can be substantial and abundance dependent but that effects on richness are less pronounced. Hemiparasites mediate competitive exclusion in communities to facilitate species coexistence and merit consideration of inclusion in ecological theories of coexistence.

Food webs for three burn severities after wildfire in the Eldorado National Forest, California

Wildfire dynamics are changing around the world and understanding their effects on ecological communities and landscapes is urgent and important. We report detailed food webs for unburned, low-to-moderate and high severity burned habitats three years post-fire in the Eldorado National Forest, California. The cumulative cross-habitat food web contains 3,084 ontogenetic stages (nodes) or plant parts comprising 849 species (including 107 primary producers, 634 invertebrates, 94 vertebrates). There were 178,655 trophic interactions between these nodes. We provide information on taxonomy, body size, biomass density and trophic interactions under each of the three burn conditions. We detail 19 sampling methods deployed across 27 sites (nine in each burn condition) used to estimate the richness, body size, abundance and biomass density estimates in the node lists. We provide the R code and raw data to estimate summarized node densities and assign trophic links.

International shipping as a potent vector for spreading marine parasites

Aim

The global shipping fleet, the primary means of transporting goods among countries, also serves as a major dispersal mechanism for marine invasive species. To date, researchers have primarily focussed on the role of ships in transferring marine macrofauna, often overlooking transfers of associated parasites, which can have larger impacts on naïve host individuals and populations. Here, we re-examine three previously published metabarcode datasets targeting zooplankton and protists in ships' ballast water to assess the diversity of parasites across life stages arriving to three major US ports.

Location

Port of Hampton Roads in the Chesapeake Bay, Virginia; Ports of Texas City, Houston and Bayport in Galveston Bay, Texas; and Port of Valdez in Prince William Sound, Alaska.

Methods

We selected all known parasitic taxa, using sequences generated from the small subunit gene (SSU) from ribosomal RNA (rRNA) amplified from (1) zooplankton collected from plankton tows (35 and 80 μm datasets) and (2) eukaryotes collected from samples of ships' ballast water (3 μm dataset).

Results

In all three datasets, we found a broad range of parasitic taxa, including many protistan and metazoan parasites, that infect a wide range of hosts, from teleost fish to dinoflagellates. Parasite richness was highest in the 3 μm dataset and relatively uniform across arrival regions. Several parasite taxa were found in high relative abundance (based on number of sequences recovered) either in ships entering a single or across multiple regions.

Main Conclusions

The ubiquity, diversity and relative abundance of parasites detected demonstrate ships are a potent vector for spreading marine parasites across the world's oceans, potentially contributing to reported increases in outbreaks of marine diseases. Future research is urgently needed to evaluate the fate of parasites upon arrival and the efficacy of ballast water treatment systems to reduce future transfers and colonization.

Complex networks of parasites and pollinators: moving towards a healthy balance

Parasites are viewed as a major threat to wild pollinator health. While this may be true for epidemics driven by parasite spillover from managed or invasive species, the picture is more complex for endemic parasites. Wild pollinator species host and share a species-rich, generalist parasite community. In contrast to the negative health impacts that these parasites impose on individual hosts, at a community level they may act to reduce competition from common and abundant pollinator species. By providing rare species with space in which to exist, this will act to support and maintain a diverse and thus healthier pollinator community. At this level, and perhaps paraxodically, parasites may be good for pollinators. This stands in clear contrast to the obvious negative impacts of epidemic and spillover parasites on wild pollinator communities. Research into floral resources that control parasites could be best employed to help design landscapes that provide pollinators with the opportunity to moderate their parasite community, rather than attempting to eliminate specific parasites from wild pollinator communities. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Infected food web and ecological stability

Parasites are widespread in nature. Nevertheless, they have only recently been incorporated into food web studies and community ecology. Earlier studies revealed the large effects of parasites on food web network structures, suggesting that parasites affect food web dynamics and their stability. However, our understanding of the role of parasites in food web dynamics is limited to a few theoretical studies, which only assume parasite-induced mortality or virulence as a typical characteristic of parasites, without any large difference in terms of predation effects. Here, I present a food web model with parasites in which parasites change the mortality and interaction strengths of hosts by affecting host activity. The infected food web shows that virulence and infection rate have virtually no effect on food web stability without any difference in interaction strengths between susceptible and infected individuals. However, if predation rates are weakened through a restriction of the activity of infected individuals, virulence and infection rate can greatly influence stability: diseases with lower virulence and higher transmission rate tend to increase stability. The stabilization is stronger in cascade than random food webs. The present results suggest that parasites can greatly influence food web stability if parasite-induced diseases prevent host foraging activity. Parasite-induced infectious disease, by weaking species interactions, may play a key role in maintaining food webs.

Context-dependent parasite infection affects trophic niche in populations of sympatric stickleback species

How parasites alter host feeding ecology remains elusive in natural populations. A powerful approach to investigate the link between infection and feeding ecology is quantifying unique and shared responses to parasite infection in related host species within a common environment. Here, 9 pairs of sympatric populations of the three-spined and nine-spined stickleback fishes were sampled across a range of freshwater and brackish habitats to investigate how parasites alter host feeding ecology: (i) biotic and abiotic determinants of parasite community composition, and (ii) to what extent parasite infection correlates with trophic niche specialization of the 2 species, using stable isotope analyses (δ15N and δ13C). It was determined that parasite community composition and host parasite load varied among sites and species and were correlated with dissolved oxygen. It was also observed that the digenean Cyathocotyle sp.'s abundance, a common directly infecting parasite with a complex life cycle, correlated with host δ¹³C in a fish species-specific manner. In 6 sites, correlations were found between parasite abundance and their hosts' feeding ecology. These effects were location-specific and occasionally host species or host size-specific. Overall, the results suggest a relationship between parasite infection and host trophic niche which may be an important and largely overlooked ecological factor. The population specificity and variation in parasite communities also suggest this effect is multifarious and context-dependent.

Large-herbivore nemabiomes: patterns of parasite diversity and sharing

Amidst global shifts in the distribution and abundance of wildlife and livestock, we have only a rudimentary understanding of ungulate parasite communities and parasite-sharing patterns. We used qPCR and DNA metabarcoding of fecal samples to characterize gastrointestinal nematode (Strongylida) community composition and sharing among 17 sympatric species of wild and domestic large mammalian herbivore in central Kenya. We tested a suite of hypothesis-driven predictions about the role of host traits and phylogenetic relatedness in describing parasite infections. Host species identity explained 27-53% of individual variation in parasite prevalence, richness, community composition and phylogenetic diversity. Host and parasite phylogenies were congruent, host gut morphology predicted parasite community composition and prevalence, and hosts with low evolutionary distinctiveness were centrally positioned in the parasite-sharing network. We found no evidence that host body size, social-group size or feeding height were correlated with parasite composition. Our results highlight the interwoven evolutionary and ecological histories of large herbivores and their gastrointestinal nematodes and suggest that host identity, phylogeny and gut architecture-a phylogenetically conserved trait related to parasite habitat-are the overriding influences on parasite communities. These findings have implications for wildlife management and conservation as wild herbivores are increasingly replaced by livestock.