The loss and fragmentation of semi-natural habitats are a major threat to biodiversity in agricultural landscapes. Through the loss of biodiversity, important ecosystem services such as the biological control of pest insects can also be reduced. As habitat loss and fragmentation usually occur together, most existing studies have examined only their combined effect. While habitat loss has large, consistently negative effects on biodiversity, the effect of fragmentation per se is little known. Based on the small number of relevant empirical studies, it appears even that positive effects of habitat fragmentation on biodiversity would prevail.
The proposed study will differentiate between the effect of local fragmentation and landscape composition on biodiversity and pest control in agricultural landscapes. The following hypothesis will be tested:
(i)The amount of semi-natural habitats in the surrounding landscape haslarge, positive effects on local biodiversity, especially for species and groups with high rates of aerial dispersal (e.g. fully winged plant- and leafhoppers, ballooning spiders).
(ii)Local fragmentation has negative effects mainly on habitatspecialists with low dispersal abilities (e.g. land snails, flightlessplant- and leafhoppers, cursorial spiders).
(iii)The control of apple aphids is reduced by local fragmentation and/orenhanced by high amounts of semi-natural habitats in the surrounding landscape.
We will test these hypothesis using two focal habitats in agricultural landscapes as models: traditional orchards and sown wildflower strips.Other semi-natural habitats, which can provide source populations of flora and fauna for the focal habitats will be considered as well. In a paired sampling design, we will select landscape situations which have a similar amount of semi-natural habitat within a 500 m radius of the focal habitat.In one member of the pair, the focal habitat (orchard, wildflower strip) will be isolated 100 m or more from the nearest semi-natural habitat, and in the other member of the pair the focal habitat will be well connected to semi-natural habitats. We will repeat this paired sampling along a gradient from low to high amounts of semi-natural habitats. Vascular plants, land snails, grasshoppers, plant- and leafhoppers and spiders will be investigated as biodiversity indicators. In addition, exclusion experiments will be conducted in the field to quantify the influence of spiders on apple aphids. The intensity of this ecosystem service will be analysed with respect to local habitat fragmentation and landscape composition.
The proposed project will be one of the first that differentiates between the effects of local habitat fragmentation and landscape-scale habitat loss on biodiversity and ecosystem services. This is also of practical relevance, as the Swiss by-law on ecological quality currently supports farmers for connecting semi-natural habitats. If proven to be effective, this innovative, spatially explicit agri-environment scheme could obtain a guiding function for other countries.