F. J. Frank van Veen

The ecological and evolutionary implications of merging different types of networks.

Interactions among species drive the ecological and evolutionary processes in ecological communities. These interactions are effectively key components of biodiversity. Studies that use a network approach to study the structure and dynamics of communities of interacting species have revealed many patterns and associated processes. Historically these studies were restricted to trophic interactions, although network approaches are now used to study a wide range of interactions, including for example the reproductive mutualisms. However, each interaction type remains studied largely in isolation from others. Merging the various interaction types within a single integrative framework is necessary if we want to further our understanding of the ecological and evolutionary dynamics of communities. Dividing the networks up is a methodological convenience as in the field the networks occur together in space and time and will be linked by shared species. Herein, we outline a conceptual framework for studying networks composed of more than one type of interaction, highlighting key questions and research areas that would benefit from their study.

Direct and indirect effects of resource quality on food web structure.

The diversity and complexity of food webs (the networks of feeding relationships within an ecological community) are considered to be important factors determining ecosystem function and stability. However, the biological processes driving these factors are poorly understood. Resource quality affects species interactions by limiting energy transfer to consumers and their predators, affecting life history and morphological traits. We show that differences in plant traits affect the structure of an entire food web through a series of direct and indirect effects. Three trophic levels of consumers were influenced by plant quality, as shown by quantitative herbivore–parasitoid–secondary parasitoid food webs. We conclude, on the basis of our data, that changes in the food web are dependent on both trait- and density-mediated interactions among species.

Ecological Networks in a Changing Climate

Summary Attempts to gauge the biological impacts of climate change have typically focussed on the lower levels of organization (individuals to populations), rather than considering more complex multi-species systems, such as entire ecological networks (food webs, mutualistic and host–parasitoid networks). We evaluate the possibility that a few principal drivers underpin network-level responses to climate change, and that these drivers can be studied to develop a more coherent theoretical framework than is currently provided by phenomenological approaches. For instance, warming will elevate individual ectotherm metabolic rates, and direct and indirect effects of changes in atmospheric conditions are expected to alter the stoichiometry of interactions between primary consumers and basal resources; these effects are general and pervasive, and will permeate through the entire networks that they affect. In addition, changes in the density and viscosity of aqueous media could alter interactions among very small organisms and disrupt the pycnoclines that currently compartmentalize many aquatic networks in time and space. We identify a range of approaches and potential model systems that are particularly well suited to network-level studies within the context of climate change. We also highlight potentially fruitful areas of research with a view to improving our predictive power regarding climate change impacts on networks. We focus throughout on mechanistic approaches rooted in first principles that demonstrate potential for application across a wide range of taxa and systems.

Ecosystem engineering and predation: the multi-trophic impact of two ant species

1. Ants are ubiquitous ecosystem engineers and generalist predators and are able to affect ecological communities via both pathways. They are likely to influence any other terrestrial arthropod group either directly or indirectly caused by their high abundance and territoriality. 2. We studied the impact of two ant species common in Central Europe, Myrmica rubra and Lasius niger, on an arthropod community. Colony presence and density of these two ant species were manipulated in a field experiment from the start of ant activity in spring to late summer. 3. The experiment revealed a positive influence of the presence of one ant colony on densities of decomposers, herbivores and parasitoids. However, in the case of herbivores and parasitoids, this effect was reversed in the presence of two colonies. 4. Generally, effects of the two ant species were similar with the exception of their effect on Braconidae parasitoid densities that responded positively to one colony of M. rubra but not of L. niger. 5. Spider density was not affected by ant colony manipulation, but species richness of spiders responded positively to ant presence. This effect was independent of ant colony density, but where two colonies were present, spider richness was significantly greater in plots with two M. rubra colonies than in plots with one colony of each ant species. 6. To test whether the positive ecosystem engineering effects were purely caused by modified properties of the soil, we added in an additional experiment (i) the soil from ant nests (without ants) or (ii) unmodified soil or (iii) ant nests (including ants) to experimental plots. Ant nest soil on its own did not have a significant impact on densities of decomposers, herbivores or predators, which were significantly, and positively, affected by the addition of an intact nest. 7. The results suggest an important role of both ant species in the grassland food web, strongly affecting the densities of decomposers, herbivores and higher trophic levels. We discuss how the relative impact via bottom-up and top-down effects of ants depends on nest density, with a relatively greater top-down predatory impact at higher densities.

STABLE COEXISTENCE IN INSECT COMMUNITIES DUE TO DENSITY- AND TRAIT-MEDIATED INDIRECT EFFECTS

Density-mediated and trait-mediated indirect interactions between species may have important roles in structuring ecological communities. Here we dissect their contributions to community stability in a model herbivore–natural enemy interaction consisting of two aphid species (Acyrthosiphon pisum and Megoura viciae) and a specialist parasitoid (Aphidius ervi) that attacks only one of the aphids (A. pisum). In replicated cage experiments, we found that the two aphid species alone were unable to coexist, with A. pisum competitively excluding M. viciae. We also found that the simple host–parasitoid interaction between A. pisum and the parasitoid was unstable. However, the three-species community persisted for at least 50 weeks. We constructed a series of models to explain the stability of the full community and conclude that it is due to a combination of density-mediated and trait-mediated indirect interactions. Parasitoid attack on the susceptible host reduces the interspecific competition experienced by the...

The loss of indirect interactions leads to cascading extinctions of carnivores

Species extinctions are biased towards higher trophic levels, and primary extinctions are often followed by unexpected secondary extinctions. Currently, predictions on the vulnerability of ecological communities to extinction cascades are based on models that focus on bottom-up effects, which cannot capture the effects of extinctions at higher trophic levels. We show, in experimental insect communities, that harvesting of single carnivorous parasitoid species led to a significant increase in extinction rate of other parasitoid species, separated by four trophic links. Harvesting resulted in the release of prey from top-down control, leading to increased interspecific competition at the herbivore trophic level. This resulted in increased extinction rates of non-harvested parasitoid species when their host had become rare relative to other herbivores. The results demonstrate a mechanism for horizontal extinction cascades, and illustrate that altering the relationship between a predator and its prey can cause wide-ranging ripple effects through ecosystems, including unexpected extinctions.

Evolutionary History and Ecological Processes Shape a Local Multilevel Antagonistic Network

Uncovering the processes that shape the architecture of interaction networks is a major challenge in ecology. Studies have consistently revealed that more closely related taxa tend to show greater overlap in interaction partners, fuelling the idea that interactions are phylogenetically conserved. However, local ecological processes such as exploitative or apparent competition (indirect interactions) might instead cause a decrease in overlap in interacting partners. Because of the taxonomic and geographic coarseness of existing studies, the structuring effect of such processes has been overlooked. Here, we assess the relative importance of phylogeny and ecological processes in a local, highly resolved, four-level antagonistic network. Across all network levels we consistently find that phylogenetic relatedness among resource species is correlated with consumer overlap but that phylogenetic relatedness among consumer species is not or negatively correlated with resource overlap. This pervasive pattern indicates that the antagonistic network has been shaped by both phylogeny on resource range and by exploitative competition limiting resource overlap among closely related consumer species. Intriguingly, the strength of phylogenetic signal varies in a consistent way across the network levels. We discuss the generality of our findings and their implications in a changing world.

Application of Nitrogen and Carbon Stable Isotopes (δ15N and δ13C) to Quantify Food Chain Length and Trophic Structure

Increasingly, stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) are used to quantify trophic structure, though relatively few studies have tested accuracy of isotopic structural measures. For laboratory-raised and wild-collected plant-invertebrate food chains spanning four trophic levels we estimated nitrogen range (NR) using δ15N, and carbon range (CR) using δ13C, which are used to quantify food chain length and breadth of trophic resources respectively. Across a range of known food chain lengths we examined how NR and CR changed within and between food chains. Our isotopic estimates of structure are robust because they were calculated using resampling procedures that propagate variance in sample means through to quantified uncertainty in final estimates. To identify origins of uncertainty in estimates of NR and CR, we additionally examined variation in discrimination (which is change in δ15N or δ13C from source to consumer) between trophic levels and among food chains. δ15N discrimination showed significant enrichment, while variation in enrichment was species and system specific, ranged broadly (1.4‰ to 3.3‰), and importantly, propagated variation to subsequent estimates of NR. However, NR proved robust to such variation and distinguished food chain length well, though some overlap between longer food chains infers a need for awareness of such limitations. δ13C discrimination was inconsistent; generally no change or small significant enrichment was observed. Consequently, estimates of CR changed little with increasing food chain length, showing the potential utility of δ13C as a tracer of energy pathways. This study serves as a robust test of isotopic quantification of food chain structure, and given global estimates of aquatic food chains approximate four trophic levels while many food chains include invertebrates, our use of four trophic level plant-invertebrate food chains makes our findings relevant for a majority of ecological systems.

Body Size, Life History and the Structure of Host–Parasitoid Networks

Abstract Recent studies of the allometric scaling of metabolism, resource handling and space use have provided a mechanistic understanding of how interactions within ecological networks are arranged. Especially, the ‘allometric diet breadth model’ (ADBM), which considers the association between consumer size, resource availability and handling costs, has shown that food webs are predictably shaped according to the body-size relationships of the organisms within them. However, size-based models of network structure are more applicable to predator–prey webs than to insect host–parasitoid networks because the relationship between body size and host use appears to be less straightforward in host–parasitoid interactions. Herein, we describe the structuring of host–parasitoid networks using frameworks that are based not only upon parasitoid body-size considerations but also upon the life-history characteristics that are commonly used to describe variation among hymenopteran parasitoids: the degree of ovigeny, idio/koinobosis and endo/ectoparasitism. We compare these frameworks with those suggested by the ADBM and elucidate upon why it has been unable to successfully predict host–parasitoid network structure. For instance, body-size constraints upon foraging capability are a stronger determinant of whether an interaction is possible in predator–prey webs than they are in host–parasitoid networks because the ultimate determinant of host suitability is its phylogeny. Further, the degree to which the taxonomic host range of a parasitoid is constrained by phylogeny is largely determined by parasitoid life history, for example, whether the larva develops as an endo- or ectobiont. In addition, we describe how parasitoid life history influences host-choice decisions, which are expected to be tailored towards the optimal allocation of scarce resources, through the determination of how species are limited in their reproductive success. To conclude, we describe some fruitful avenues for future research and highlight the importance of considering how temporal or spatial variation in the characteristics of parasitoids or their hosts affects how networks are structured.

Indirect Effects, Apparent Competition and Biological Control

In biological control in its simplest form only direct interactions between the control agent and the pest and potential non-targets are considered. Ecologists are however amassing an ever increasing body of evidence for the importance of indirect effects in ecological communities. Indirect effects are the effects of one species on another mediated by at least one intermediate species. An example is so-called apparent competition which is the negative indirect effect that prey species have on each other when they share natural enemies. This effect is thought to play a particularly significant role in phytophagous insect communities where the scope for resource competition is limited. We show that there is experimental evidence for apparent competition amongst phytophagous insects. We describe a community of aphids and their parasitoids, predators and pathogens that we have been studying for over 10 years. We discuss how this species-rich community in a relatively natural environment may be structured by indirect effects. Returning to biological control we discuss how these ideas from community ecology can be applied to enhance pest control and to assess the ecological risks of the introduction of control agents. Introducing or encouraging species that share natural enemies with a target pest may lead to increased pest control through an apparent competition effect by boosting the natural enemy population. We conclude that although occasional attempts are made, such techniques are currently still much underutilised. Equally, we show how indirect effects may cause or increase the impact of introduced control agents on native flora and fauna but that these possible effects are rarely taken into account.