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Animals often engage in mutualistic associations with microorganisms that protect them from predation, parasitism or pathogen infection. Studies of these interactions in insects have mostly focussed on the direct effects of symbiont infection on natural enemies without studying community- wide effects. Here, we explore the effect of a defensive symbiont on population dynamics and species extinctions in an experimental community composed of three aphid species and their associated specialist parasitoids. We found that introducing a bacterial symbiont with a protective (but not a non-protective) phenotype into one aphid species led to it being able to escape from its natural enemy and increase in density. This changed the relative density of the three aphid species which resulted in the extinction of the two other parasitoid species. Our results show that defensive symbionts can cause extinction cascades in experimental communities and so may play a significant role in the stability of consumer-herbivore communities in the field. [ABSTRACT FROM AUTHOR]

Summary Species extinction rates due to human activities are high [ 1–3 ], and initial extinctions can trigger cascades of secondary extinctions, leading to further erosion of biodiversity [ 4 ]. A potential major mechanism for secondary extinction cascades is provided by the long-standing theory that the diversity of consumer species is maintained due to the positive indirect effects that these species have on each other by reducing competition among their respective resource species [ 5–7 ]. This means that the loss of one carnivore species could lead to competitive exclusion at the prey trophic level, leading to extinctions of further carnivore species. Evidence for these effects is difficult to obtain due to many confounding factors in natural systems, but extinction cascades that could be due to this mechanism have been demonstrated in simplified laboratory microcosms [ 8 ]. We established complex insect food webs in replicated field mesocosms and found that the overharvesting of one parasitoid wasp species caused increased extinction rates of other parasitoid species, compared to controls, but only when we manipulated the spatial distribution of herbivore species such that the potential for interspecific competition at this level was high. This provides clear evidence for horizontal extinction cascades at high trophic levels due to the proposed mechanism. Our results demonstrate that the loss of carnivores can have widespread effects on other species at the same trophic level due to indirect population-dynamic effects that are rarely considered in this context. [ABSTRACT FROM AUTHOR]

The function of a predator within a community is greatly based on its trophic niche, that is the number and the strength of feeding links. In generalist predators, which feed on a wide range of prey, the size and position of the trophic niche is likely determined by traits such as hunting mode, the stratum they occur in, their body size and age., We used stable isotope analyses (13C and 15N) to measure the trophic niche size of nine spider species within a forest hedge community and tested for species traits and individual traits that influence stable isotope enrichment, niche size and resource use., The spiders Enoplognatha, Philodromus, Floronia, and Heliophanus had large isotopic niches, which correspond to a more generalistic feeding behaviour. In contrast, Araneus, Metellina and Agelena, as top predators in the system, had rather narrow niches. We found a negative correlation between trophic position and niche size. Differences in trophic position in spiders were explained by body size, hunting modes and stratum, while niche size was influenced by hunting mode. In Philodromus, the size of the trophic niche increased significantly with age., Fitting spiders to functional groups according to their mean body size, hunting mode and their habitat domain resulted in largely separated niches, which indicates that these traits are meaningful for separating functional entities in spiders. Functional groups based on habitat domain (stratum) caught the essential functional differences between the species with species higher up in the vegetation feeding on flying insects and herb and ground species also preying on forest floor decomposers., Interestingly, we found a gradient from large species using a higher habitat domain and having a smaller niche to smaller species foraging closer to the ground and having a larger niche. This shows that even within generalist predators, such as spiders, there is a gradient of specialism that can be predicted by functional traits. [ABSTRACT FROM AUTHOR]

The positive relationship between biodiversity and ecosystem functioning is mainly derived from studies concerning primary producers, whereas a generalization of this relationship for higher trophic levels is more difficult. Furthermore, most evidence of the biodiversity-ecosystem functioning relationship is derived from experiments manipulating only one trophic level and, as a consequence, interactive diversity effects at multiple trophic levels have mostly been ignored. Here, we performed a mesocosm experiment in which we manipulated functional group diversity at two trophic levels (primary and secondary consumers) applying a full-factorial design. More specifically, we asked whether 1) predator functional diversity affects prey mortality rates, 2) prey functional diversity affects prey mortality rates, 3) whether there are interactive effects of simultaneous diversity changes at both trophic levels. For each trophic level we used two functional groups, i.e. organisms belonging to two different habitat domains: at the higher trophic position 1) a ground foraging spider species and 2) a spider species foraging in the vegetation canopy and at the lower trophic position 3) a ground living cricket species and 4) leafhoppers living in the vegetation canopy. Increasing predator functional group diversity increased prey mortality by 53%, and increasing prey functional group diversity increased prey mortality by 24%. Further, prey mortality was highest at the uppermost level of functional group diversity (142% increase in prey mortality compared to single prey and predator functional diversity), most likely due to resource partitioning between the predators. This finding demonstrates that a multi-trophic perspective is necessary, and that previous studies focusing on only one trophic level have most likely underestimated the strength of the relationship between biodiversity and ecosystem functioning. [ABSTRACT FROM AUTHOR]

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. 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. 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. 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. 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. 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. 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. [ABSTRACT FROM AUTHOR]

1. Mutualistic and antagonistic interactions, although often studied independently, may affect each other, and food web dynamics are likely to be determined by the two processes working in concert. 2. The structure, and hence dynamics, of food webs depends on the relative abundances of generalist and specialist feeding guilds. Secondary parasitoids of aphids can be divided into two feeding guilds: (i) the more specialised endoparasitoids, which attack the primary parasitoid larvae in the still living aphid, and (ii) the generalist ectoparasitoids, which attack the pre-pupa of the primary or secondary parasitoid in the mummified aphid. 3. We studied the effect of an ant-aphid mutualism on the relative abundance of these two functional groups of secondary parasitoids. We hypothesised that generalists will be negatively affected by the presence of ants, thus leading to a greater dominance of specialists. 4. We manipulated the access of ants ( Lasius niger) to aphid colonies in which we placed parasitised aphids. Aphid mummies were collected and reared to determine the levels of endo- and ecto-secondary parasitism. 5. When aphids were attended by L. niger the proportion of secondary parasitism by ectoparasitoids dropped from 26 to 8% of the total number of parasitised aphids, with Pachyneuron aphidis most strongly affected, while endoparasitoids as a group did not respond. However, among these Syrphophagus mamitus profited from ant attendance becoming the dominant secondary parasitoid, while parasitisation rates of Alloxysta and Phaenoglyphis declined. 6. The shift to S. mamitus as dominant secondary parasitoid in ant-attended aphid colonies is likely due to the behavioural plasticity of this species in response to ant aggression, and a release from tertiary parasitism by generalist ectoparasitoids. 7. The reduction of secondary parasitism by generalist ectoparasitoids reduces the potential for apparent competition among primary parasitoids with consequences for the dynamics of the wider food web. [ABSTRACT FROM AUTHOR]

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Predator-prey interactions are ubiquitous and powerful forces that structure ecological communities. 1–3 Habitat complexity has been shown to be particularly important in regulating the strength of predator-prey interactions. 4–6 While it is well established that changes in habitat structure can alter the efficacy of predatory and anti-predatory behaviors, 7–9 little is known about the consequences of engineering activity by prey species who modify the external environment to reduce their own predation risk. Using field surveys and manipulative experiments, we evaluated how habitat modification by Brandt's voles (Lasiopodomys brandtii) influences predation risk from a principal avian predator (shrike; Lanius spp.) in a steppe grassland, located in Inner Mongolia, China. We found that voles actively modify habitat structure by cutting down a large, unpalatable bunchgrass species (Achnatherum splendens) in the presence of shrikes, a behavior that disappeared when these avian predators were excluded experimentally. The damage activities of these voless dramatically decreased the volume of unpalatable grasses, which in turn reduced visitations by shrikes and thus mortality rates. Our study shows that herbivorous prey that act as ecosystem engineers can directly reduce their own predation risk by modifying habitat structure. Given the ubiquity of predation risks faced by consumers, and the likely ability of many consumers to alter the habitat structure in which they live, the interplay between predation risk and ecosystem engineering may be an important but unappreciated mechanism at play in natural communities. [Display omitted] • Voles modified habitat structure by destroying a large, unpalatable bunchgrass • Damage activities of voles decreased the plant volume of the bunchgrasses • Vole damage activities were enhanced at the presence of the predatory birds • Habitat modifications by voles reduced predation risk and increased vole survival Zhong et al. show that by damaging a large, unpalatable bunch grass, Brandt's voles in steppe grasslands in China can reduce the predation risk from avian predators (Lanius spp.) probably due to improved vision. Habitat modifications by voles can reduce the visiting frequency of avian predators and thus increase the survival of voles. [ABSTRACT FROM AUTHOR]