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  • Author or Editor: S. Lavorel x
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Herbivory is a major driver of plant communities. Most herbivores preferentially consume dominant species and slow down plant succession, but it remains unclear in which ways different herbivore species have contrasting effects on plant communities. In this study, we investigated the extent to which closely related insect herbivores with slight differences in their feeding behaviour induce contrasting reactions in plant communities. We studied the impact of herbivory by two Gomphocerinae grasshopper species, Chorthippus scalaris and Euthystira brachyptera, on the outcome of competition between the plant species Dactylis glomerata and Festuca paniculata. Under a controlled choice experiment, C. scalaris preferentially consumed D. glomerata while E. brachyptera preferred F. paniculata, but in an experimental plant community (mesocosm) both species consumed the plant species proportionally to their abundance and the amount of herbivory marks detected on the leaves did not depend on the specific grasshopper species. The herbivory pressure of both grasshopper species significantly reduced the vegetative height and the number of tillers of F. paniculata, with C. scalaris showing a stronger effect. As a consequence, herbivory by E. brachyptera did not significantly affect interspecific plant competition, whereas C. scalaris enhanced the dominance of D. glomerata. Our study shows that closely related herbivores that only slightly differ in their feeding behaviour can induce contrasting effects on interspecific plant competition, and that the dominant species D. glomerata is more tolerant to herbivory than F. paniculata. The specific plant and herbivore traits responsible for contrasting herbivory effects on plant competition remains to be explored.

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Measurements of trait community composition are known to be sensitive to the way species abundance is assessed, but not to what extent. This was investigated by considering two of the most commonly used indices of community trait composition, trait averages and functional diversity, in bee communities along a post-fire environmental gradient. The indices were computed using three different species abundance measurements (log and unlog number of individuals and species occurrence only) and 5 traits. For certain traits, the responses of the indices to fire varied according to how species abundance was measured. The measurements that took species abundance into account in the most distinct way (e.g., occurrence vs. unlog data) produced the least similar results for all traits. Species were then grouped into different classes on the basis of their relative abundance (i.e., dominants, subdominants, and rare species). As a result, the measure that attaches the highest importance to the abundance of species (unlog data) related mostly to the dominant species traits, while the measure attaching the lowest (i.e., species occurrence) related more to rare species traits. Species diversity was mostly independent of trait averages and functional diversity, regardless of the measure of species abundance used. We also quantified functional redundancy (i.e., the potential minus the observed functional diversity in each community). When more weight was attached to species abundance, redundancy decreased and tended to be less correlated with species diversity. Overall, the way species abundance is taken into consideration in indices of functional composition offers promising insights into the way community assembly mechanisms respond to environmental changes.

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