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Consumers can influence the competitive outcomes of prey species in various ways. Modern coexistence theory predicts that consumers can promote prey coexistence by preferably targeting a competitively superior one, thereby reducing fitness differences. However, previous studies yielded mixed conclusions. In this study, we tested the hypothesis that a parasitic annual plant, Cuscuta campestris, facilitates the coexistence of two common annual plants. We performed field surveys and parasitism experiments to parameterize a plant competition dynamics model. The model suggested a competition–defence tradeoff: the legume Lespedeza striata was a better competitor than the grass Setaria faberi, while it was more susceptible to the parasite. Moreover, an empirical host–parasite dynamics model, extended from the plant competition model, predicted their coexistence within broad, biologically reasonable ranges of parameters. This work provides field evidence of the coexisting–promoting role of a parasitic plant, as caused by stabilising feedback between host and parasite densities. [ABSTRACT FROM AUTHOR]

Background and Aims Plants have evolved various defences against herbivores, including direct chemical and structural defences and co-opted biological defences by predatory insects. However, the effects of abiotic habitat conditions on the quantitative expression of defence traits of spiny species have not been elucidated. Methods Here, we investigated whether a spiny deciduous tree, Aralia elata (Miq.) Seem. changes its defence expression across light and nutrient gradients. We measured allocation to spines and C-based secondary metabolites (condensed tannins and total phenols) on A. elata plants growing across light and nutrient gradients in situ in natural landscapes in Japan. Second, we examined the effects of light and soil nutrient condition on allocation to shoot organs, spines and chemical defences of juveniles of two genotypes of the species, respectively spiny (mainland population) and non-spiny (island population), grown in a glasshouse. Key Results In the field investigation, absolute spine mass, spine mass fraction, total phenols and condensed tannins all responded positively to canopy openness. Total phenol content was also negatively related to soil N. In the glasshouse, spiny genotype individuals had less total biomass, had lower stem allocation and were shorter than non-spiny genotype individuals. In spiny genotype trees, both spine mass fraction and total phenols decreased under low light conditions. Nutrient additions had negative effects on spine mass fraction and total phenols, but no effect on absolute spine mass. Conclusions These results suggest that development of spines is costly for A. elata and receives greater allocation when carbohydrate supply is more plentiful. Thus, light is a more important determinant of spine allocation than soil nutrients for A. elata. [ABSTRACT FROM AUTHOR]

Recent studies suggest that changes in leaf traits due to interactions between plants affect resource utilisation by herbivores, as well as herbivore distribution. However, this has not yet been confirmed experimentally. Here, we investigated the effects of phenotypic plasticity in leaf traits of Rumex obtusifolius (host plant) in response to intraspecific and interspecific interactions on the distribution of two leaf beetles, Gastrophysa atrocyanea (specialist herbivore) and Galerucella grisescens (generalist herbivore).We investigated the local population density of R. obtusifolius plants and the presence of leaf beetles on the plants at five study sites. Leaf chemicals (condensed tannins and total phenolics) were compared between aggregated and solitary R. obtusifolius plants. To clarify the effects of the interaction environment of R. obtusifolius plants on their leaf traits and on resource utilisation by the leaf beetles, we compared leaf chemicals and preferences of adult leaf beetles among treatments where R. obtusifolius experienced intraspecific interaction, interspecific interaction or no interaction in cultivation experiments. Finally, we evaluated the independent and combined effects of patch size and intraspecific interaction on leaf beetle distribution in mesocosm experiments.In the field, the presence of the specialist leaf beetle G. atrocyanea was positively correlated with the local population density (rosette overlap ratio) of R. obtusifolius plants; however, there was no correlation in the case of the generalist leaf beetle G. grisescens. In the cultivation experiments, plants in the intraspecific interaction treatment increased their leaf concentrations of condensed tannins and total phenolics, and G. atrocyanea consumed more of these leaves than leaves in other treatments. Similar results were observed in the field. In the mesocosm experiments, larger numbers of G. atrocyanea were distributed on R. obtusifolius plants exposed to below‐ground intraspecific interaction than on plants not exposed to intraspecific interaction.Our results provide experimental evidence that leaf‐trait changes in response to intraspecific interaction between host plants influence specialist herbivore distribution. This highlights the need to integrate plant–plant interactions into our understanding of plant–animal interactions. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]