The relative contributions of species richness and species composition to ecosystem functioning

How species diversity influences ecosystem functioning has been the subject of many experiments and remains a key question for ecology and conservation biology. However, the fact that diversity cannot be manipulated without affecting species composition makes this quest methodologically challenging. Here, we evaluate the relative importance of diversity and of composition on biomass production, by using partial Mantel tests for one variable while controlling for the other. We analyse two datasets, from the Jena (2002–2008) and the Grandcour (2008–2009) Experiments. In both experiments, plots were sown with different numbers of species to unravel mechanisms underlying the relationship between biodiversity and ecosystem functioning (BEF). Contrary to Jena, plots were neither mowed nor weeded in Grandcour, allowing external species to establish. Based on the diversity–ecosystem functioning and competition theories, we tested two predictions: 1) the contribution of composition should increase with time; 2) the contribution of composition should be more important in non-weeded than in controlled systems. We found support for the second hypothesis, but not for the first. On the contrary, the contribution of species richness became markedly more important few years after the start of the Jena Experiment. This result can be interpreted as suggesting that species complementarity, rather than intraspecific competition, is the driving force in this system. Finally, we explored to what extent the estimated relative importance of both factors varied when measured on different spatial scales of the experiment (in this case, increasing the number of plots included in the analyses). We found a strong effect of scale, suggesting that comparisons between studies, and more generally the extrapolation of results from experiments to natural situations, should be made with caution.

 

Sandau N, Fabian Y, Bruggisser OT, Rohr RP, Naisbit RE, Kehrli P, Aebi A and Bersier LF (published online) The relative contributions of species richness and species composition to ecosystem functioning Oikos (doi: 10.1111/oik.03901)

Potential for exploitative competition, not intraguild predation, between invasive harlequin ladybirds and flowerbugs in urban parks

Capture d_écran 2017-05-11 à 16.19.26

Abstract In aphidophagous insect communities invaded by the harlequin ladybird Harmonia axyridis Pallas (Coleoptera: Coccinellidae), intraguild predation (IGP) is widely implicated in the displacement of native predators, however, indirect trophic interactions are rarely assessed. Using molecular gut-content analysis, we investigated the relative frequencies of IGP by H. axyridis on the predatory flower bug Anthocoris nemoralis Fabricius (Heteroptera: Anthocoridae) and prey overlap for a shared prey, the lime aphid Eucallipterus tiliae L. (Hemiptera: Aphididae), in Tilia 9 europaea crowns in urban parks. The frequency of IGP by H. axyridis was low: 2.7 % of larvae and 3.4 % of adults tested positive for A. nemoralis DNA. The presence of lime aphid DNA in predators was higher: 56.5 and 47.9 % of H. axyridis larvae and adults, respectively, contained E. tiliae DNA, whereas 60.8 % of adult A. nemoralis tested positive for aphid DNA. Incorporating insect densities revealed that the density of H. axyridis larvae had a strong negative effect on the likelihood of detecting aphid DNA in A. nemoralis. Prey overlap for E. tiliae was widespread in space (2–13 m height in tree crowns) and time (May–October 2011) which suggests that interspecific exploitative competition, mediated by predator life-stage, more so than IGP, is an important indirect trophic interaction between cooccurring H. axyridis and A. nemoralis. Whether exploitative competition equates to displacement of A. nemoralis populations requires further investigation. Our results emphasize the need to incorporate indirect interactions in studies of insect communities following invasion, not least because they potentially affect more species than direct interactions alone.

Keywords: Indirect effects, Molecular gut content, Anthocoris nemoralis, Harmonia axyridisEucallipterus tiliae, Predator–prey interactions

Howe AG, Ravn HP, Bressen Pipper C and Aebi A (2016) Potential for exploitative competition, not intraguild predation, between invasive harlequin ladybirds and flowerbugs in urban parks. Biological Invasions 18:517-532 (doi:10.1007/s10530-015-1024-y)

Pister la pyrale: un projet de sciences citoyennes

La pyrale du buis fait des ravages dans les jardins privés et publics de la ville de Neuchâtel. Un groupe interdisciplinaire de chercheurs collabore avec des citoyens pour suivre sa trace et pour réfléchir à des solutions innovantes à ce problème.

Pour en savoir plus, visiter la page du projet.

Chenille de pyrale du buis. Photo: A. Aebi

Chenille de pyrale du buis.
Photo: A. Aebi