Plant diversity in a nutshell: testing for small-scale effects on trap nesting wild bees and wasps

Declining plant species richness in agro-ecosystems and thus reduced habitat quality can have cascading effects on ecosystem functioning, leading to reduced pollination and biological control. Here we test if plant diversity can affect arthropod diversity and abundance on a very small scale, manipulating plant species richness (2, 6, 12 and 20 sown species) in small adjacent subplots (63 9 m) in 10 wildflower strips in an agricultural landscape. We simultaneously analyzed the effect of plant species richness, vegetation structure, and plant composition on the species richness and abundance of cavity-nesting wild bees, wasps, their prey and natural enemies, and on the structure of their food webs. By separating the trap-nesting species into functional groups according to their prey, we aimed to understand the underlying patterns for the effects of plant diversity. Increasing plant species richness had a significant effect only on spider-predating wasps, the group of wasps trophically most distant from plants. In contrast, bees and food-web structure were unaffected by plant diversity. Spider-predating wasp abundance negatively correlated with the abundance of spiders, suggesting top-down control. Interestingly, the abundance of spiders was the only variable that was strongly affected by plant composition. The hypothesis that the effect of plant diversity decreases with increasing trophic level is not supported by our study, and the mobility of species appears to play a greater role at this small spatial scale.

Fabian, Y., N. Sandau, O. T. Bruggisser, A. Aebi, P. Kehrli, R. P. Rohr, R. E. Naisbit, and L.-F. Bersier. 2014. Plant diversity in a nutshell: testing for small-scale effects on trap nesting wild bees and wasps. Ecosphere 5(2):18. http://dx. doi.org/10.1890/ES13-00375.1 PDF

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The importance of landscape and spatial structure for hymenopteran-based food-webs in an agro-ecosystem

Capture d’écran 2013-08-26 à 10.55.331. Understanding the environmental factors that structure biodiversity and food webs among communities is central to assess and mitigate the impact of landscape changes.

2. Wildflower strips are ecological compensation areas established in farmland to increase pollination services and biological control of crop pests and to conserve insect diversity. They are arranged in networks in order to favour high species richness and abundance of the fauna.

3. We describe results from experimental wildflower strips in a fragmented agricultural landscape, comparing the importance of landscape, of spatial arrangement and of vegetation on the diversity and abundance of trap-nesting bees, wasps and their enemies, and the structure of their food webs.

4. The proportion of forest cover close to the wildflower strips and the landscape heterogeneity stood out as the most influential landscape elements, resulting in a more complex trap-nest community with higher abundance and richness of hosts, and with more links between species in the food webs and a higher diversity of interactions. We disentangled the underlying mechanisms for variation in these quantitative food web metrics.

5. We conclude that in order to increase the diversity and abundance of pollinators and biological control agents and to favour a potentially stable community of cavity-nesting hymenoptera in wildflower strips, more investment is needed in the conservation and establishment of forest habitats within agro-ecosystems, as a reservoir of beneficial insect populations.

Fabian Y, Sandau N, Bruggisser OT, Aebi A, Kehrli P, RE and Bersier LF (in press) The importance of landscape and spatial structure for hymenopteran-based food webs in an agro-ecosystem. Journal of Applied Ecology The importance of landscape and spatial structure for hymenopteran-based food webs in an agro-ecosystem dpi:10.1111/1365-2656.12103

Direct and indirect bottom-up and top-down forces shape the abundance of the orb-web spider Argiope bruennichi

Picture: O. Bruggisser

Species abundance in local communities is determined by bottom-up and top-down processes, which can act directly and indirectly on the focal species. Studies examining these effects simultaneously are rare. Here we explore the direct top-down and direct and indirect bottom-up forces regulating the abundance and predation success of an intermediate predator, the web-building spider Argiope bruennichi (Araneae: Araneidae). We manipulated plant diversity (2, 6, 12 or 20 sown species) in 9 wildflower strips in a region of intensive farmland. To identify the major factors regulating the distribution and abundance of A. bruennichi, we quantified three characteristics of vegetation (species diversity, composition and vegetation structure) as well as the spider’s prey community and natural enemies. The distribution and abundance of A. bruennichiwas regulated by combined bottom-up and top-down processes as well as by direct and indirect interactions between trophic levels. Four main factors were identified: (1) the strong direct effect of vegetation structure, (2) the positive effect of plant species diversity, which affected spider abundance directly and indirectly through increased densities and size of flower-visiting prey species, (3) the positive or negative direct effects of different plant species, and (4) the strongly negative direct effect of predacious hornets. The advantage of taking a global approach to understand the regulation of species abundance is highlighted first by the quantification of the relative importance of factors, with a surprisingly strong effect of hornet predators, and second by the discovery of a direct effect of plant diversity, which raises intriguing questions about habitat selection by this spider.


Zusammenfassung

Die Abundanz einer Art in einer lokalen Gemeinschaft wird durch bottom-up- und top-down-Prozesse bestimmt, die direkt oder indirekt auf die betrachtete Art wirken können. Studien, die beide Effekte gleichzeitig untersuchen, sind selten. Hier untersuchen wir die direkten top-down- und die direkten und indirekten bottom-up-Effekte, die die Abundanz und den Fangerfolg der Radnetzspinne Argiope bruennichi(Araneae: Araneidae) bestimmen. Wir manipulierten die Pflanzendiversität (2, 6, 12 bzw. 20 ausgesäte Arten) in neun Blühstreifen in einer Region mit intensiver landwirtschaftlicher Nutzung. Um die wichtigsten Faktoren, die die Verteilung und Abundanz von A. bruennichi bestimmten, zu identifizieren, quantifizierten wir drei Eigenschaften der Vegetation (Artendiversität, Zusammensetzung und Struktur der Vegetation) sowie das Beutespektrum der Spinne und ihre natürlichen Feinde. Die Verteilung und Abundanz von A. bruennichi wurde von kombinierten bottom-up- und top-down-Prozessen sowie durch direkte und indirekte Interaktionen zwischen den trophischen Ebenen gesteuert. Vier Hauptfaktoren wurden identifiziert: (1) der starke direkte Einfluss der Vegetationsstruktur, (2) der positive Effekt der Pflanzendiversität, der die Abundanz der Spinnen direkt und indirekt durch erhöhte Dichte und Größe der blütenbesuchenden Arten beeinflusste, (3) der positive bzw. negative Effekt von bestimmten Pflanzenarten und (4) der stark negative Effekt der räuberischen Hornissen. Der Vorteil eines globalen Ansatzes, um die Regulation der Abundanz dieser Art zu verstehen, wird herausgestellt durch die Quantifizierung der relativen Bedeutung der Faktoren (mit einem überraschend starken Effekt der Hornissen) sowie durch die Entdeckung eines direkten Effekts der Pflanzendiversität, wodurch sich interessante Fragen zur Habitatwahl dieser Spinnenart ergeben.

The paper is available here

Diversity protects plant communities against generalist molluscan herbivores

Wildflower strips are used to increase natural enemies of crop pests and to con- serve insect diversity on farmland. Mollusks, especially slugs, can affect the vege- tation development in these strips considerably. Although recent theoretical work suggests that more diverse plant communities will exhibit greater resistance against herbivore pressure, empirical studies are scarce. We conducted a semi-natural experiment in wildflower strips, manipulating trophic structure (reduction in herbivorous mollusks and reduction in major predators) and plant diversity (2, 6, 12, 20 and 24 sown species). This design allowed us to assess the effect of plant diversity, biomass and composition on mollusks, and vice versa, the effect of mollusc abundance on vegetation. Seven species of mollusks were found in the strips, with the slugs Arion lusitanicus, Deroceras reticulatum and Deroceras panormitanum being most frequent. We found a negative relationship between plant diversity and mollusk abundance, which was due predominantly to a decrease in the agricultural pest species A. lusitanicus. These results are consistent with the hypothesis that plant diversity can reduce the impact of her- bivores. However, plant identity also had an effect on mollusks, and accounted for a much larger fraction of the variation in mollusk communities than biodi- versity effects. While overall plant diversity decreased during the 3 years of the study, in the final year the highest plant diversity was found in the plots where mollusk populations were experimentally reduced. We conclude that selective feeding by generalist herbivores leads to changes in plant community composi- tion and hence reduced plant diversity. Our results highlight the importance of plant biodiversity as protection against generalist herbivores, which if abundant can in the long term negatively impact plant diversity, driving the system along a “low plant diversity – high mollusk abundance” trajectory.

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