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The Plant Vigour Hypothesis states that herbivores preferentially feed on the most vigorous plants within a plant population and/or the most vigorous modules within a plant. The goal of this study was to evaluate how shoot size (as an indication of module vigour) affects leaf herbivory in the host plant Calotropis procera, an exotic xerophyte perennial milkweed shrub. We predicted that the proportion of leaf area removed by insect herbivores would be positively related to shoot size. Eight patches were selected containing a varied number of C. procera individuals (5, 8, 29, 31, 55, 79, 116, and 172 individuals/patch) in the Brazilian seasonally dry vegetation (Caatinga), of which five individuals were randomly selected for further analysis. From each individual, three to six shoots were randomly selected, measured and had their leaves collected, for a total of approximately 200 leaves per patch. At the regional scale, the proportion of leaf area removed was positively affected by shoot size. In addition, this pattern was also found for the majority of the studied patches (29, 31, 55, 116, and 172 individuals/patch). Among the insect herbivores associated with C. procera, larvae of Danaus spp. (Lepidoptera: Nymphalidae) were commonly observed feeding on all patches. These herbivores present a specialized behaviour to circumvent the presence of latex in the host leaves. Although more vigorous plant modules should be better defended compared with the less vigorous modules, Danaus species were able to bypass host defences, and feed on healthy, rapidly growing and vigorous plant modules of C. procera, hence causing more damage to these modules.

In a climate change context, the buildup of CO2 will affect plant communities worldwide. This study evaluated the effects of CO2 enrichment on the development and defense of two Cerrado native species Baccharis dracunculifolia and B. platypoda and their associated endophytic fungi richness. The study took place in Open-Top Chambers, two with ambient CO2 concentration (~400 ppm) and two in an enriched environment (~800 ppm). Baccharis platypoda developed 20% more leaves under enriched CO2 conditions, whereas B. dracunculifolia was 30% taller and showed 27% more leaves than those under ambient conditions. In both species, leaf polyphenol concentration did not differ between treatments. Nevertheless, polyphenol content had a positive correlation with plant height on both species' individuals grown under CO2 enriched conditions. Endophytic fungi richness and colonization rate on both plant species did not differ between ambient and enriched conditions. Our results show the positive effect of CO2 fertilizer in at least one of the measured growth parameters. An important new finding was a synergistic increase in growth and chemical defense in both studied species under enriched CO2 conditions, suggesting higher carbon assimilation and accumulation. This study suggests that the effects on primary productivity and secondary metabolites of Baccharis species will potentially reflect on the diversity and distribution of Cerrado plants and their associated animal communities.

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