It is becoming increasingly appreciated that the structure and functioning of ecological food webs are controlled by the nature and level of plant chemicals. It is hypothesized that intraspecific variation in plant chemical resistance, in which individuals of a host-plant population exhibit genetic differences in their chemical contents (called 'plant chemotypes'), may be an important determinant of variation in food web structure and functioning. We evaluated this hypothesis using field assessments and plant chemical assays in the tansy plant Tanacetum vulgare L. (Asteraceae). We examined food webs in which chemotypes of tansy plants are the resource for two specialized aphids, their predators and mutualistic ants. The density of the ant-tended aphid Metopeurum fuscoviride was significantly higher on particular chemotypes (borneol) than others. Clear chemotype preferences between predators were also detected. Aphid specialist seven-spotted ladybird beetles (Coccinella septempunctata) were more often found on camphor plants, while significantly higher numbers of the polyphagous nursery web spider (Pisaura mirabilis) were observed on borneol plants. The analysis of plant chemotype effects on the arthropod community clearly demonstrates a range of possible outcomes between plant-aphid-predator networks. The findings help to offer a deeper insight into how one important factor--plant chemical content--influences which species coexist within a food web on a particular host plant and the nature of their trophic linkages.

Field assessments and genetic analyses on tansy aphid, Metopeurum fuscoviride Stroyan were made aiming to investigate whether a specialist aphid species and its associated predator density differ between chemotypes of the plant host, tansy, and whether the essential oil composition of the plant host determines the genetic differences observed between colonies of M. fuscoviride inhabiting these different chemotypes. Tansy plants were defined after volatile extractions as Camphor Pure and Borneol Pure, respectively, with hybrid chemotypes defined as Camphor Hybrid, Borneol Hybrid, and Thujone Hybrid, respectively. Ladybird beetles, Coccinella septempunctata (L.), clearly dominated Pure Camphor plants, whereas significantly higher numbers of the nursery web spider, Pisaura mirabilis (Clerck), were detected on Borneol Pure and Borneol Hybrid plants. Genetic analyses using five polymorphic microsatellite markers revealed that higher genetic similarity existed between M. fuscoviride aphids from Borneol Pure and Borneol Hybrid plants and between Camphor Pure and Camphor Hybrid plants, respectively, whereas a lower level of genetic similarity was seen between these aphids from Thujone Hybrid plants. Accordingly, it is possible that host plant‐adapted species can create much context‐dependency in the nature and strength of predation effects, which may in turn alter costs and benefits of host plant chemical differentiations, or even the prey genotype variations. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114, 709–719.

Aims We tested whether chemical content of individual Tansy plants influences aboveground arthropod and belowground soil microbial community composition. Methods We use Tansy chemical defence composition as focus for our analysis, given that changes in chemical defence are a dominant mechanism expected to modify aboveground and belowground community composition and nutrient cycling. Detailed assessment of arthropod food webs were made on tansy chemotypes. Next generation soil DNA analyses were used to assess soil bacterial community. Results Tansy plants defended by Camphor and Borneol had higher total soil bacterial diversity than control plants (soils under non-Tansy plants) and Tansy chemotypes containing high concentrations of Thujon. Furthermore, the frequency of bacterial genera with a significant role in litter mineralization processes decreased with time in soil associated with Camphor and Borneol chemotypes, indicative of complete microbial decomposition and mineralization of plant material. No such changes occurred in soils associated with Thujon chemotypes. Thujon plants had fewer aphids, ladybirds, spiders, and Orius sp. than other chemotypes. Conclusions Our experiments revealed that Thujon chemotypes had different associated aboveground arthropod and belowground soil microbial communities than Camphor or Borneol chemotypes. The common qualitative response between aboveground and belowground communities to Tansy plant defense is consistent with our hypothesis and extensive literature on plant defense and aboveground and belowground feedbacks.

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