The aim of habitat augmentation in farmed landscapes is to conserve biological diversity and support beneficial animals that reduce pests and pollinate crops. The effectiveness of local biodiversity improvements and beneficial services can be mediated by landscape complexity. To test the effectiveness of local and landscape habitats for increasing pest reduction by birds, we quantified the avian predator community and performed a sentinel prey exclosure experiment in walnut orchards of the Sacramento Valley, California, USA. We compared predation rates between 10 orchards with woody hedgerows or riparian vegetation maintained in their margins and 10 orchards with conventionally managed bare margins. We quantified landscapes around orchards to test the effectiveness of woody margins and avian predators in reducing sentinel prey along a gradient of 0-38% semi-natural landscape cover. Our experimental prey were cocooned diapausing larvae of Cydia pomonella (L.) (Lepidoptera: Tortricidae; codling moth) which overwinter in walnut orchards and produce larvae that damage walnuts in the subsequent growing season.
Results/Conclusions
Excluding birds reduced cocoon predation by 38 percentage points, and predation of uncaged cocoons increased with increasing proportions of semi-natural landscape cover. Predation also increased as the size of walnut trees and the depth of bark furrows increased, likely because these characteristics were also associated with increasing abundance of avian predators with traits specific to consuming tree-dwelling cocoons. A woodpecker abundance × semi-natural cover interaction predicted pest reduction contrary to Intermediate Landscape Complexity Hypothesis predictions: the effect size of woodpecker abundance on cocoon predation was large in landscapes with intermediate cover, but greatest in landscapes with < 1% cover. This suggests that orchards supported woodpeckers in landscapes with little semi-natural vegetation, with positive effects on pest reduction. Woody orchard margins supported higher avian predator abundance and species richness than conventionally managed margins, but woody patch presence alone did not increase pest reduction. Winter predation rates were not associated with growing season walnut insect damage or pesticide applications, possibly reflecting unmeasured treatment decisions by farmers. We provide evidence that increasing the abundance of avian predators with traits specific for consuming target pests —by retaining old trees locally and semi-natural cover regionally— can increase orchard pest reduction services in an intensive agricultural region.