Hidden Host Mortality from an Introduced Parasitoid: Conventional and Molecular Evaluation of Non-Target Risk

Hidden trophic interactions are important in understanding food web ecology and evaluating the ecological risks and benefits associated with the introduction of exotic natural enemies in classical biological control programs. Although non-target risk is typically evaluated based on evidence of successful parasitism, parasitoid-induced host mortality not resulting in visible evidence of parasitism (i.e., nonreproductive effects) is often overlooked. The adventive establishment of Trissolcus japonicus, an exotic parasitoid of the introduced stink bug Halyomorpha halys, provides an opportunity to investigate the total impact of this parasitoid on target and non-target hosts in the field. We developed a new methodology to measure nonreproductive effects in this system, involving a species-specific diagnostic PCR assay for T. japonicus. We applied this methodology to field-deployed eggs of four pentatomid species, coupled with traditional rearing techniques. Nonreproductive effects were responsible for the mortality of an additional 5.6% of H. halys eggs due to T. japonicus, and were even more substantial in some of the non-target species (5.4–43.2%). The observed hidden mortality of native non-target species from an introduced parasitoid could change predictions about direct and indirect ecological interactions and the efficacy of biological control of the target pest.

Stable isotopes of fatty acids: current and future perspectives for advancing trophic ecology

To understand consumer dietary requirements and resource use across ecosystems, researchers have employed a variety of methods, including bulk stable isotope and fatty acid composition analyses. Compound-specific stable isotope analysis (CSIA) of fatty acids combines both of these tools into an even more powerful method with the capacity to broaden our understanding of food web ecology and nutritional dynamics. Here, we provide an overview of the potential that CSIA studies hold and their constraints. We first review the use of fatty acid CSIA in ecology at the natural abundance level as well as enriched physiological tracers, and highlight the unique insights that CSIA of fatty acids can provide. Next, we evaluate methodological best practices when generating and interpreting CSIA data. We then introduce three cutting-edge methods: hydrogen CSIA of fatty acids, and fatty acid isotopomer and isotopologue analyses, which are not yet widely used in ecological studies, but hold the potential to address some of the limitations of current techniques. Finally, we address future priorities in the field of CSIA including: generating more data across a wider range of taxa; lowering costs and increasing laboratory availability; working across disciplinary and methodological boundaries; and combining approaches to answer macroevolutionary questions. This article is part of the theme issue ‘The next horizons for lipids as ‘trophic biomarkers’: evidence and significance of consumer modification of dietary fatty acids’.

Into the soil labyrinth: soil physical structure as a driver of trophic interactions and soil biodiversity

<p>The high diversity of densely packed organisms occurring in small volumes of soils has long been intriguing and we still poorly understand what drives such diversity. Exploring the role of small scale physical structure of the soil, constituting the habitat of these organisms offers unprecedented clues for explaining how organisms interact, notably through trophic interactions and how, in turn, these interactions drive this extraordinary diversity. We review here how restrictions on soil organisms’ ability to sense (e.g. volatiles) and access food resources/prey imposed by the soil physical structure and aqueous habitats within are important drivers for trophic interactions, and consequently, of soil biodiversity. Examples from micro- to macrofauna are presented, focusing on organisms unable to create their own pore space, such as bacteria, fungi, protists, nematodes and microarthropods. Finally, we discuss interdisciplinary challenges to develop research merging soil physics and soil food web ecology.</p>

Stable isotopes as tracers in aquatic ecosystems

The addition of stable isotopes (SI) of 13C and 15N has been used to study several aquatic processes, thus avoiding environmental disturbance by the observer. This approach, employed for the last three decades, has contributed to expanding our knowledge of food-web ecology and nutrient dynamics in aquatic systems. Currently, SI addition is considered a powerful complementary tool for studying several ecological and biogeochemical processes at the whole-aquatic-ecosystem scale, which could not be addressed otherwise. However, their contributions have not been considered jointly nor have they been evaluated with a view to assessing the reliability and scope of their results from an ecosystem perspective. We intend to bridge this gap by providing a comprehensive review (78 scientific publications reporting in situ 13C/15N additions at the whole-aquatic-ecosystem scale) addressing the main results arising from their use as tracers. Specifically, we focus on: (i) reasons for SI additions at the whole-ecosystem scale to study ecological processes, (ii) the paradigms resulting from its use and the insights achieved, (iii) uncertainties and drawbacks arising from these SI addition experiments, and (iv) the potential of this approach for tackling new paradigms. SI tracer addition at the ecosystem scale has provided new functional insights into numerous ecological processes in aquatic sciences (importance of subsidies in lakes; heterotrophy dominance in benthic food webs in lakes, wetlands and estuaries; the decrease in N removal efficiency in most aquatic ecosystems due to anthropogenic alteration; the recognition of hyporheic zones and floodplains as hot spots for stream denitrification; and high rates of internal N recycling in tidal freshwater marshes). However, certain constraints such as the high cost of isotopes, the maintenance of the new isotopic steady state, and avoidance of biomass changes in any compartment or pool during tracer addition bear witness to the difficulties of applying this approach to all fields of aquatic ecology and ecosystems. The future development of this approach, rather than expanding to larger and complex aquatic ecosystems, should include other stable isotopes such as phosphorus (P18O4).

Parasite life-cycle studies: a plea to resurrect an old parasitological tradition

Many helminth taxa have complex life cycles, involving different life stages infecting different host species in a particular order to complete a single generation. Although the broad outlines of these cycles are known for any higher taxon, the details (morphology and biology of juvenile stages, specific identity of intermediate hosts) are generally unknown for particular species. In this review, we first provide quantitative evidence that although new helminth species are described annually at an increasing rate, the parallel effort to elucidate life cycles has become disproportionately smaller over time. We then review the use of morphological matching, experimental infections and genetic matching as approaches to elucidate helminth life cycles. Next we discuss the various research areas or disciplines that could benefit from a solid knowledge of particular life cycles, including integrative taxonomy, the study of parasite evolution, food-web ecology, and the management and control of parasitic diseases. Finally, we end by proposing changes to the requirements for new species descriptions and further large-scale attempts to genetically match adult and juvenile helminth stages in regional faunas, as part of a plea to parasitologists to bring parasite life-cycle studies back into mainstream research.

Effects of juvenile fish predation (Cyprinus carpio L.) on the composition and diversity of free-living freshwater nematode assemblages

Free-living nematodes are well recognised as an abundant and ubiquitous component of meiobenthic communities, where they serve as a link between microbial production and higher trophic levels. However, the effect of fish predation on nematode assemblages is almost unknown. In this study, the predation effects of the benthivorous juvenile carp (Cyprinus carpio) on nematode abundance, biomass, diversity and species composition in the littoral zone of a natural freshwater pond were examined over 310 days using field enclosures and exclosures. Fish predation altered the abundance and biomass of nematodes, and especially of the dominant species Tobrilus gracilis, Eumonhystera filiformis and Monhystera paludicola/stagnalis. Species richness and species composition, but not the diversity and feeding type of nematode assemblages, were affected by fish predation. Our study provides insights into the food-web ecology of lakes and the first evidence of freshwater fish predation effects on nematode assemblages in a natural habitat.

Microbes are trophic analogs of animals

In most ecosystems, microbes are the dominant consumers, commandeering much of the heterotrophic biomass circulating through food webs. Characterizing functional diversity within the microbiome, therefore, is critical to understanding ecosystem functioning, particularly in an era of global biodiversity loss. Using isotopic fingerprinting, we investigated the trophic positions of a broad diversity of heterotrophic organisms. Specifically, we examined the naturally occurring stable isotopes of nitrogen (15N:14N) within amino acids extracted from proteobacteria, actinomycetes, ascomycetes, and basidiomycetes, as well as from vertebrate and invertebrate macrofauna (crustaceans, fish, insects, and mammals). Here, we report that patterns of intertrophic 15N-discrimination were remarkably similar among bacteria, fungi, and animals, which permitted unambiguous measurement of consumer trophic position, independent of phylogeny or ecosystem type. The observed similarities among bacterial, fungal, and animal consumers suggest that within a trophic hierarchy, microbiota are equivalent to, and can be interdigitated with, macrobiota. To further test the universality of this finding, we examined Neotropical fungus gardens, communities in which bacteria, fungi, and animals are entwined in an ancient, quadripartite symbiosis. We reveal that this symbiosis is a discrete four-level food chain, wherein bacteria function as the apex carnivores, animals and fungi are meso-consumers, and the sole herbivores are fungi. Together, our findings demonstrate that bacteria, fungi, and animals can be integrated within a food chain, effectively uniting the macro- and microbiome in food web ecology and facilitating greater inclusion of the microbiome in studies of functional diversity.