Abstract
Agricultural intensification (AI) is currently a major driver of biodiversity loss and related ecosystem functioning decline. However, spatio-temporal changes in community structure induced by AI, and their relation to ecosystem functioning, remain largely unexplored. Here, we analysed 16 quantitative cereal aphid–parasitoid and parasitoid–hyperparasitoid food webs, replicated four times during the season, under contrasting AI regimes (organic farming in complex landscapes vs. conventional farming in simple landscapes). High AI increased food web complexity but also temporal variability in aphid–parasitoid food webs and in the dominant parasitoid species identity. Enhanced complexity and variability appeared to be controlled bottom-up by changes in aphid dominance structure and evenness. Contrary to the common expectations of positive biodiversity–ecosystem functioning relationships, community complexity (food-web complexity, species richness and evenness) was negatively related to primary parasitism rates. However, this relationship was positive for secondary parasitoids. Despite differences in community structures among different trophic levels, ecosystem services (parasitism rates) and disservices (aphid abundances and hyperparasitism rates) were always higher in fields with low AI. Hence, community structure and ecosystem functioning appear to be differently influenced by AI, and change differently over time and among trophic levels. In conclusion, intensified agriculture can support diverse albeit highly variable parasitoid–host communities, but ecosystem functioning might not be easy to predict from observed changes in community structure and composition.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-012-2366-0) contains supplementary material, which is available to authorized users.
Highlights
Agricultural intensification (AI) on a local and a landscape scale is a major cause of biodiversity loss (Foley et al 2005)
We found distinct differences in aphid, parasitoid and hyperparasitoid communities between fields with low and high AI and over time
Intensified fields were mainly colonised by leaf-colonising aphids (M. dirhodum, R. padi), which may have benefited from the higher nitrogen levels (Honek 1991; Hasken and Poehling 1995) that arise from the high amounts of inorganic fertilisers applied in conventionally managed fields
Summary
Agricultural intensification (AI) on a local and a landscape scale is a major cause of biodiversity loss (Foley et al 2005). Organic farming has been suggested to oppose such changes and to increase components of biodiversity such as species richness (Hole et al 2005) and evenness (Crowder et al 2010). Biodiversity may increase and stabilise overall ecosystem function (Tilman et al 2006), but characteristics of particular species and food web structure are important factors influencing the response of communities to human-induced habitat loss and alteration (Melian and Bascompte 2002; Sole and Montoya 2006; Brose et al 2006; Laliberteand Tylianakis 2010). Even when species richness in a trophic guild remains constant, the frequency of their interactions can change greatly due to changes in habitat quality (Tylianakis et al 2007). The effects of AI on interaction diversity in parasitoid–host food webs have so far been inconclusive, with both negative
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