No abstract

Meadows are often managed by mowing to prevent succession and brushwood encroachment. This management practice is efficient to maintain plant diversity, but the effect on arthropod groups is less well known. We sampled spiders in the Grande Caric¸aie (Switzerland), a site of nature conservation importance, in two types of wet meadows, an unmanaged site and one 2-years-old mown conditions. Numbers of individuals for the most abundant families, diversity measures, and indicator taxa were compared among vegetation and treatment types. The results indicate that the less mobile spiders and species linked to litter or dead reeds, including rare species, are reduced by mowing. Present management consists in triennial mowing of 2-4 ha non-contiguous sectors. We investigate the conflict between the need to mow the meadows to maintain them, and the negative effect of mowing on spider communities. We therefore propose a new management scheme aimed at maintain the vegetation while lessening its negative effects on spider communities by providing refuges.

Abstract. We describe here the phenological patterns of the 25 most common ballooning species of spiders caught by a 12.2 m suction trap during an eleven year survey in Switzerland. We aimed at identifying and quantifying the number, position, spread, and relative weight of activity periods for the whole community. Further, we explored the possible link between phenological patterns and habitat use. For this purpose, we used bump-hunting approaches and fitted mixtures of normal distributions to the abundance data. The phenologies can be grouped in four categories, from uni-to quadrimodal. The specific peaks in the timing of ballooning were found between February and November, with most ballooning activity occurring in summer and autumn. For some taxa, it was possible to analyze the data for young instars and adults. For the majority of taxa, the adults' peak appeared between the early and late peaks of immature individuals. Species inhabiting the ground level of open areas, often disturbed by agricultural practices, were clearly dominant in the multimodal categories; spiders living in more closed and stable habitats, such as tree-shrub and herb layers, typically had a single peak of adult dispersal. This discrepancy in phenology may simply reflect different numbers of generations, but may also result from an adaptation to maximize the persistence of populations in unstable habitats.

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 webbuilding 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. bruennichi was 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. ZusammenfassungDie 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 1 Published in Basic and Applied Ecology 13, issue 8, 706-714, 2012 which should be used for any reference to this work 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 Vegetationsst...