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- PublicationRestriction temporaireEnvironmental gradients and the evolution of tri‐trophic interactionsLong‐standing theory predicts herbivores and predators should drive selection for increased plant defences, such as the specific production of volatile organic compounds for attracting predators near the site of damage. Along elevation gradients, a general pattern is that herbivores and predators are abundant at low elevation and progressively diminish at higher elevations. To determine whether plant adaptation along such a gradient influences top‐down control of herbivores, we manipulated soil predatory nematodes, root herbivore pressure and plant ecotypes in a reciprocal transplant experiment. Plant survival was significantly higher for low‐elevation plants, but only when in the presence of predatory nematodes. Using olfactometer bioassays, we showed correlated differential nematode attraction and plant ecotype‐specific variation in volatile production. This study not only provides an assessment of how elevation gradients modulate the strength of trophic cascades, but also demonstrates how habitat specialisation drives variation in the expression of indirect plant defences.
- PublicationAccès libreChicks of the great spotted cuckoo may turn brood parasitism into mutualism by producing a foul-smelling secretion that repels predatorsThe great spotted cuckoo (Clamator glandarius) is an important brood parasite of carrion crows (Corvus corone corone) in northern Spain. We recently found that, unlike what is commonly known for cuckoo-host interactions, the great spotted cuckoo has no negative impact on average crow fitness in this region. The explanation for this surprising effect is a repulsive secretion that the cuckoo chicks produce when they are harassed and that may protect the brood against predation. Here, we provide details on the chemical composition of the cuckoo secretion, as well as conclusive evidence that the dominating volatile chemicals in the secretion are highly repellent to model species representative of common predators of the crows. These results support the notion that, in this particular system, the production of a repulsive secretion by the cuckoo chicks has turned a normally parasitic interaction into a mutualistic one.
- PublicationAccès libreDo induced responses mediate the ecological interactions between the specialist herbivores and phytopathogens of an alpine plant?Plants are not passive victims of the myriad attackers that rely on them for nutrition. They have a suite of physical and chemical defences, and are even able to take advantage of the enemies of their enemies. These strategies are often only deployed upon attack, so may lead to indirect interactions between herbivores and phytopathogens. In this study we test for induced responses in wild populations of an alpine plant (Adenostyles alliariae) that possesses constitutive chemical defence (pyrrolizidine alkaloids) and specialist natural enemies (two species of leaf beetle, Oreina elongata and Oreina cacaliae, and the phytopathogenic rust Uromyces cacaliae). Plants were induced in the field using chemical elicitors of the jasmonic acid (JA) and salicylic acid (SA) pathways and monitored for one month under natural conditions. There was evidence for induced resistance, with lower probability and later incidence of attack by beetles in JA-induced plants and of rust infection in SA-induced plants. We also demonstrate ecological cross-effects, with reduced fungal attack following JA-induction, and a cost of SA-induction arising from increased beetle attack. As a result, there is the potential for negative indirect effects of the beetles on the rust, while in the field the positive indirect effect of the rust on the beetles appears to be over-ridden by direct effects on plant nutritional quality. Such interactions resulting from induced susceptibility and resistance must be considered if we are to exploit plant defences for crop protection using hormone elicitors or constitutive expression. More generally, the fact that induced defences are even found in species that possess constitutively-expressed chemical defence suggests that they may be ubiquitous in higher plants.
- PublicationAccès libreCoping with an antagonist: the impact of a phytopathogenic fungus on the development and behaviour of two species of alpine leaf beetleHerbivorous insects and phytopathogenic fungi often share their host plants. This creates a network of direct and indirect interactions, with far-reaching consequences for the ecology and evolution of all three parties. In the Alps, the leaf beetles Oreina elongata and Oreina cacaliae (Coleoptera: Chrysomelidae), and the rust fungus Uromyces cacaliae (Uredinales: Pucciniaceae) are found on the same host plant, Adenostyles alliariae (Asterales: Asteraceae). We compare the impact of rust infection on these two closely-related beetle species, one of which, O. cacaliae, is a specialist on A. alliariae, while the other, O. elongata, moves repeatedly between Adenostyles and an alternative host, Cirsium spinosissimum. Larval performance, feeding preference, oviposition choice and dispersal behaviour were studied in field and laboratory experiments. When reared on rust-infected leaves, larvae of both beetle species had lower growth rates, lower maximum weights and longer development times. Larvae and adults discriminated among diets in feeding trials, showing a preference for discs cut from healthy leaves over those bearing a patch of sporulating rust, those from elsewhere on an infected leaf, and those from an upper leaf on an infected plant. Females of the two species differed in behaviour: in O. cacaliae they favoured healthy leaves for larviposition, while in O. elongata they showed no significant preference during oviposition. In the field, larvae and adults of both species dispersed more rapidly when placed on infected host plants. The results demonstrate that rust infection reduces the quality of the plant as a host for both Oreina species, and they combine the ability to detect systemic infection with the evolution of evasive behaviours. For these beetles, competition with a rust clearly increases the difficulty of survival in the harsh conditions of alpine environments, and may have a profound impact on the evolution of their life history traits and host plant use.
- PublicationAccès libreFormal comment to Soler et al.: Great spotted cuckoo nestlings have no antipredatory effect on magpie or carrion crow host nests in southern SpainReplicating research is crucial to assess the generality of findings. Yet, in ecology, the complexity of data collection and experimentation often precludes the possibility of going beyond single–population studies. The study by Soler et al. is therefore most welcome, as it provides new insights on the possible role of great spotted cuckoo in protecting the nest of its corvid hosts. In a previous article, we suggested a mechanism based on the malodorous secretion of great spotted cuckoo chicks to explain why the presence of the parasite in the nests of carrion crows in northern Spain increased the probability of nest success (i.e. fledging at least one host chick) as compared to non-parasitized nests. Soler et al. found no evidence supporting an anti-depredatory function of cuckoo chicks in their studied populations and proposed an alternative mechanism that may explain our experimental results. Here we would like to address a) the differences between the results of the two studies and b) the proposed interpretation of our translocation experiment. We will also respond to the concerns raised by Soler et al. on some of the analyses presented in our paper.
- PublicationAccès libreInsights into the encapsulation process of photovoltaic modules: GCMS analysis on the curing step of poly(ethylene-co-vinyl acetate) (EVA) encapsulantAppropriate encapsulation schemes are essential in protecting the active components of the photovoltaic (PV) module against weathering and to ensure long term reliability. For crystalline cells, poly(ethylene-co-vinyl acetate) (EVA) is the most commonly used PV encapsulant. Additives like peroxides and silanes are formulated in EVA encapsulants to obtain the desired properties, e.g. the desired gel content value and sufficient adhesion after the encapsulation process etc. The identification and control of volatile organic compounds (VOCs) released by the polymeric encapsulant during PV module encapsulation is important for understanding and optimizing processes in order to enhance the encapsulation quality of the manufactured modules. The authors demonstrate how gas chromatography and mass spectrometry (GC-MS) techniques can be used to help understand the curing process, mainly by identifying the VOCs emanating from EVA under the effect of temperature and pressure. The results provide chemical insights into the EVA encapsulation process, which are valuable for further optimisation of the PV module manufacturing process and evaluation of its environmental impact. 26 Refs.
- PublicationAccès libreCanopy gaps promote selective stem-cutting by small mammals of two dominant tree species in an African lowland forest: the importance of seedling chemistrySmall mammals can impede tree regeneration by injuring seedlings and saplings in several ways. One fatal way is by severing their stems, but apparently this type of predation is not well-studied in tropical rain forest. Here, we report on the incidence of stem-cutting' to new, wild seedlings of two locally dominant, canopy tree species monitored in 40 paired forest understorey and gap-habitat areas in Korup, Cameroon following a 2007 masting event. In gap areas, which are required for the upward growth and sapling recruitment of both species, 137 seedlings of the long-lived, light-demanding, fast-growing large tropical tree (Microberlinia bisulcata) were highly susceptible to stem-cutting (83% of deaths) it killed 39% of all seedlings over a c. 2-y period. In stark contrast, seedlings of the more shade-tolerant, slower-growing tree species (Tetraberlinia bifoliolata) were hardly attacked (4.3%). In the understorey, however, stem-cutting was virtually absent. Across the gap areas, the incidence of stem-cutting of M. bisulcata seedlings showed significant spatial variation that could not be explained significantly by either canopy openness or Janzen-Connell type effects (proximity and basal area of conspecific adult trees). To examine physical and chemical traits that might explain the species difference to being cut, bark and wood tissues were collected from a separate sample of seedlings in gaps (i.e. not monitored for stem-cutting). These analyses suggested that, compared with T. bifoliolata, the lower stem density, higher Mg and K and fatty acid concentrations in bark, and fewer phenolic and terpene compounds in M. bisulcata seedlings made them more palatable and attractive to small-mammal predators, likely rodents. We conclude that selective stem-cutting is a potent countervailing force to the current local canopy dominance of the grove-forming M. bisulcata by limiting the recruitment and abundance of its saplings. Given the ubiquity of gaps and ground-dwelling rodents in pantropical forests, it would be surprising if this form of lethal browsing was restricted to Korup.
- PublicationRestriction temporaireThe combined use of an attractive and repellent sex pheromonal component by a gregarious parasitoidGregarious parasitoids usually clump their cocoons together and the adults emerge in a synchronized fashion. This makes it easy for them to find mating partners and most copulations indeed take place at the natal patch. Yet, males should leave such sites when females are no longer receptive. As yet, this decision-making process and the possible involvement of pheromones were poorly understood. Here we report on a remarkable use of attractive and repellent pheromones of the well-studied gregarious parasitoid species Cotesia glomerata (L.) (Hymenoptera: Braconidae). Virgin C. glomerata females were found to release an attractive as well as a repellent compound, which in combination arrest males on the natal patch, but after mating the females stop the production of the attractant and the males are repelled. The repellent compound was identified as heptanal, which was also released by males, probably reducing male-male competition on the natal patch. We also confirmed that the sex ratio of the emerging wasps can vary considerably among patches, depending on the relative quality of hosts and the number of females that parasitize a host. The newly revealed use of attractive and repellent pheromone compounds by C. glomerata possibly helps maximize mating success under these variable conditions.
- PublicationAccès libreCounter-intuitive developmental plasticity induced by host qualityAdaptation to different hosts plays a central role in the evolution of specialization and speciation in phytophagous insects and parasites, and our ability to experimentally rank hosts by their quality is critical to research to understand these processes. Here we provide a counter-intuitive example in which growth is faster on poor quality hosts. The leaf beetles Oreina elongata and Oreina cacaliae share their host plant with the rust Uromyces cacaliae. Larvae reared on infected Adenostyles alliariae show reduced growth rate, reduced maximum weight and longer development time. However, they normally respond adaptively to the rust's mid-season arrival. When switched during development from healthy to infected leaves, larvae accelerate growth and reduce development time, but pupate at lower body weight. In this novel plant–insect–fungus interaction, infection forms the cue to trade off life-history traits in order to complete development within the brief alpine summer. It represents a novel mode of developmental plasticity, which is likely to be found in other host–parasite systems whenever host quality deteriorates due to multiple infection or ageing. This phenotypic plasticity would modify competition after co-infection and the mutual selection imposed by hosts and parasites, and creates a paradoxical negative correlation between growth rate and environmental quality.
- PublicationAccès libreHost plant location by chemotaxis in an aquatic beetleInteractions between plants and aquatic insects are poorly documented, especially for turbid freshwater ecosystems. Many Swiss lakes offer such habitats, several of which are inhabited by the leaf beetle Macroplea appendiculata (Panzer 1794). This donaciid beetle is the only coleopteran species known to complete its life cycle entirely under water, where it lives primarily on perfoliate pondweed (Potamogeton perfoliatus L.), with Eurasian watermilfoil (Myriophyllum spicatum L.) as an alternative host plant. Direct observations during diving trips, aquatic olfactometer bioassays, and stir bar sorptive extractions (SBSE) coupled with GC–MS analysis were used to understand how these beetles locate their patchily distributed host plants and congeners in a harsh, often swirling environment. In olfactometer assays we observed that the aquatic beetle is strongly attracted to water extracts of pondweed, whereas neither mature males nor females beetles seem to produce attractive cues. The chemical analyses revealed that perfoliate pondweed releases one dominating compound, eucalyptol. Olfactometer assays confirmed that this is a potent attractant for the beetle. We also observed attraction to phytol, which is released by the main, as well as the alternative host plant. These finding are somewhat surprising as eucalyptol has never been reported for aquatic plants and phytol is poorly soluble in water. In addition, both are frequently described as insect repellents in terrestrial ecosystems. We suggest that these terpenoids normally have a defensive function against herbivores and pathogens, but that the highly specialized leaf beetle has evolved to exploit its host’s defence chemistry for optimal foraging.