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

Abstract

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...

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