Experimental evidence that the effectiveness of conservation biological control depends on landscape complexity.
The expansion of intensive agricultural practices is a major threat to biodiversity and to the delivery of ecosystem services on which humans depend. Local-scale conservation management strategies, such as agri-environment schemes to preserve biodiversity, have been widely adopted to reduce the negative impacts of agricultural intensification. However, it is likely that the effectiveness of these local-scale management actions depend on the structure and composition of the surrounding landscape. We experimentally tested the utility of floral resource strips to improve local-scale biological control of crop pests, when placed within a gradient of moderately simple through to highly complex landscapes. We found that experimental provision of floral resources enhanced parasitism rates of two globally important crop pests in moderately simple landscapes but not in highly complex ones, and this translated into reduced pest abundances and increased crop yield. Synthesis and applications. Our results lend experimental support for the 'intermediate landscape complexity hypothesis', which predicts that local conservation management will be most effective in moderately simple agricultural landscapes, and less effective in either very simple landscapes where there is no capacity for response, or in highly complex landscapes where response potential is already saturated. This knowledge will allow more targeted and cost-effective implementation of conservation biological control programs based on an improved understanding of landscape-dependent processes, which will reduce the negative impacts of agricultural intensification.