Bottom-up and trait-mediated effects of resource quality on amphibian parasitism

Top-down and bottom-up effects and relationships with local environmental factors in the water frog-helminth systems in Latvia

Semi-aquatic European water frogs (Pelophylax spp.) harbour rich helminth infra-communities, whose effects on host population size in nature are poorly known. To study top-down and bottom-up effects, we conducted calling male water frog counts and parasitological investigations of helminths in waterbodies from different regions of Latvia, supplemented by descriptions of waterbody features and surrounding land use data. We performed a series of generalized linear model and zero-inflated negative binomial regressions to determine the best predictors for frog relative population size and helminth infra-communities. The highest-ranked (by Akaike information criterion correction, AICc) model explaining the water frog population size contained only waterbody variables, followed by the model containing only land use within 500 m, while the model containing helminth predictors had the lowest rank. Regarding helminth infection responses, the relative importance of the water frog population size varied from being non-significant (abundances of larval plagiorchiids and nematodes) to having a similar weight to waterbody features (abundances of larval diplostomids). In abundances of adult plagiorchiids and nematodes the best predictor was the host specimen size. Environmental factors had both direct effects from the habitat features (e.g., waterbody characteristics on frogs and diplostomids) and indirect effects through parasite-host interactions (impacts of anthropogenic habitats on frogs and helminths). Our study suggests the presence of synergy between top-down and bottom-up effects in the water frog-helminth system that creates a mutual dependence of frog and helminth population sizes and helps to balance helminth infections at a level that does not cause over-exploitation of the host resource.

Bottom-up effects of fungicides on tadpoles of the European common frog (Rana temporaria)

Biodiversity is under pressure worldwide, with amphibians being particularly threatened. Stressors related to human activity, such as chemicals, are contributing to this decline. It remains, however, unclear whether chemicals exhibiting a fungicidal activity could indirectly affect tadpoles that depend on microbially conditioned leaf litter as food source. The indirect effect of fungicides (sum concentration of a fungicide mixture composed of azoxystrobin, carbendazim, cyprodinil, quinoxyfen, and tebuconazole: 100 µg/L) on tadpoles was assessed relative to leaf litter colonized by microbes in absence of fungicides (control) and a worst-case scenario, that is leached leaf litter without microbial colonization. The quality of leaf litter as food for tadpoles of the European common frog (Rana temporaria) was characterized through neutral lipid fatty acid profiles and microbial sum parameters and verified by sublethal responses in tadpoles (i.e., feeding rate, feces production, growth, and fatty acid composition). Fungicides changed the nutritious quality of leaf litter likely through alterations in leaves' neutral lipid fatty acid profiles (i.e., changes in some physiologically important highly unsaturated fatty acids reached more than 200%) in combination with a potential adsorption onto leaves during conditioning. These changes were reflected by differences in the development of tadpoles ultimately resulting in an earlier start of metamorphosis. Our data provide a first indication that fungicides potentially affect tadpole development indirectly through bottom-up effects. This pathway is so far not addressed in fungicide environmental risk assessment and merits further attention.

Temporal and spatial variations of macrofouling organisms on ecological floating beds in Yundang Lagoon, China

Spatial-temporal variations of macrofouling organisms that attach to ecological floating beds (EFBs) in the Yundang Lagoon were investigated to identify factors that influence the appearance of macrofouling organisms. Results show that the composition, abundance, biomass, and dominance of macrofouling organisms on EFBs exhibited significant seasonal variation. Pearson correlation analysis indicates that the abundance and biomass of the bivalve Mytilopsis sallei showed negative correlation with root biomass (p < 0.05) and particulate matter (p < 0.05). Environmental (temperature and salinity, p < 0.05) and biological (bottom-up control) factors are the main drivers of population turnover. There were significant species differences of macrofouling organisms within the different parts of the lagoon, which were attributed to environmental characteristics such as hydrodynamics, dissolved oxygen, and the degree of eutrophication. Results of this study provide a basis for controlling macrofouling organisms, while improving the stability of EFBs and the efficiency of ecological restoration.

High food quality increases infection of Gammarus pulex (Crustacea: Amphipoda) by the acanthocephalan parasite Pomphorhynchus laevis

Parasitism is an important process in ecosystems, but has been largely neglected in ecosystem research. However, parasites are involved in most trophic links in food webs with, in turn, a major role in community structure and ecosystem processes. Several studies have shown that higher nutrient availability in ecosystems tends to increase the prevalence of parasites. Yet, most of these studies focused on resource availability, whereas studies investigating resource quality remain scarce. In this study, we tested the impact of the quality of host food resources on infection by parasites, as well as on the consequences for the host. Three resources were used to individually feed Gammarus pulex (Crustacea: Amphipoda) experimentally infected or not infected with the acanthocephalan species Pomphorhynchus laevis: microbially conditioned leaf litter without phosphorus input (standard resource); microbially conditioned leaf litter enriched in phosphorus; and microbially conditioned leaf litter without phosphorus input but complemented with additional inputs of benthic diatoms rich in both phosphorus and eicosapentaenoic acid. During the 110 day experiment, infection rate, parasite load, host survival, and parasite-mediated behavioral traits implicated in trophic transmission were measured (refuge use, geotaxis and locomotor activity). The resources of higher quality, regardless of the infection status, reduced gammarid mortality and increased gammarid growth. In addition, higher quality resources increased the proportion of infected gammarids, and led to more cases of multi-infections. While slightly modifying the geotaxis behavior of uninfected gammarids, resource quality did not modulate the impact of parasites on host behavior. Finally, for most parameters, consumption of algal resources had a greater impact than did phosphorus-enriched leaf litter. Therefore, manipulation of resource quality significantly affected host-parasite relationships, which stressed the need for future research to investigate in natura the relationships between resource availability, resource quality and parasite prevalence.

Nitrogen and Phosphorus Retranslocation of Leaves and Stemwood in a Mature Eucalyptus Forest Exposed to 5 Years of Elevated CO2

Elevated CO2 affects C cycling processes which in turn can influence the nitrogen (N) and phosphorus (P) concentrations of plant tissues. Given differences in how N and P are used by plants, we asked if their stoichiometry in leaves and wood was maintained or altered in a long-term elevated CO2 experiment in a mature Eucalyptus forest on a low P soil (EucFACE). We measured N and P concentrations in green leaves at different ages at the top of mature trees across 6 years including 5 years in elevated CO2. N and P concentrations in green and senesced leaves and wood were determined to evaluate both spatial and temporal variation of leaf N and P concentrations, including the N and P retranslocation in leaves and wood. Leaf P concentrations were 32% lower in old mature leaves compared to newly flushed leaves with no effect of elevated CO2 on leaf P. By contrast, elevated CO2 significantly decreased leaf N concentrations in newly flushed leaves but this effect disappeared as leaves matured. As such, newly flushed leaves had 9% lower N:P ratios in elevated CO2 and N:P ratios were not different in mature green leaves (CO2 by Age effect, P = 0.02). Over time, leaf N and P concentrations in the upper canopy slightly declined in both CO2 treatments compared to before the start of the experiment. P retranslocation in leaves was 50%, almost double that of N retranslocation (29%), indicating that this site was P-limited and that P retranslocation was an important mechanism in this ecosystem to retain P in plants. As P-limited trees tend to store relatively more N than P, we found an increased N:P ratio in sapwood in response to elevated CO2 (P < 0.01), implying N accumulation in live wood. The flexible stoichiometric ratios we observed can have important implications for how plants adjust to variable environmental conditions including climate change. Hence, variable nutrient stoichiometry should be accounted for in large-scale Earth Systems models invoking biogeochemical processes.

Bottom-up trait-mediated indirect effects decrease pathogen transmission in a tritrophic system

A plant's induction of secondary defenses helps to decrease herbivore damage by changing resource quality. While these chemical or physical defenses may directly decrease herbivory, they can also have indirect consequences. In a tritrophic system consisting of a plant, an insect herbivore, and an insect pathogen, plant based trait-mediated indirect effects (TMIEs) can alter host-pathogen interactions and, thereby, indirectly affect disease transmission. In a series of field experiments, individual soybean plants (Glycine max) were sprayed with either a jasmonic acid (JA) solution to trigger induction of plant defenses or a similar control compound. Fall armyworm (Spodoptera frugiperda) larvae along with varying amounts of a lethal baculovirus were placed on the plants to measure transmission. Induction of plant defenses decreased viral transmission due to increased population heterogeneity arising from changes in individual susceptibility. The change in susceptibility via TMIEs was driven by a decrease in feeding rates and an increase viral dose needed to infect larvae. While the induction against herbivore attack may decrease herbivory, it can also decrease the efficacy of the herbivore's pathogen potentially to the plant's detriment. While TMIEs have been well-recognized for being driven by top-down forces, bottom-up interactions can dictate community dynamics and, here, epizootic severity.