Experimental evidence that the effectiveness of conservation biological control depends on landscape complexity

1. 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. 2. 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. 3. 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. 4. 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.

1. 集约化农业的扩张是生物多样性以及人类赖以生存的生态系统服务供给的主要威胁。旨在保护生物多样性的农业环境计划（agri-environment schemes）这类局地尺度的保护管理策略，已被广泛采用以减轻农业集约化带来的负面影响。然而，此类局地管理措施的有效性，大概率取决于周边景观的结构与组成。2. 我们开展了实验，检验在从中度简单到高度复杂的景观梯度中布设花带资源时，其对作物害虫局地生物防控的效用。3. 研究发现，在中度简单的景观中，实验性添加花带资源可提升两种全球重要作物害虫的寄生率，但在高度复杂的景观中则无此效果；这一结果进而降低了害虫种群丰度，提高了作物产量。4. 综合与应用。我们的研究结果为“中间景观复杂度假说（intermediate landscape complexity hypothesis）”提供了实验支撑——该假说预测，局地保护管理在中度简单的农业景观中效果最佳，而在缺乏响应能力的极简单景观，或是响应潜力已达饱和的高度复杂景观中效果较差。基于对景观依赖过程的深入理解，该研究成果可指导更具针对性且成本效益更高的保护生物防控项目实施，从而减轻农业集约化带来的负面影响。