Flood initiates bottom-up cascades in a tri-trophic system: host plant regrowth increases densities of a leaf beetle and its predators

1. Following the passing of a typhoon over central Japan in late August 2001, a large-scale flood occurred owing to the heavy precipitation. Willow trees on the banks of the Yasu River were badly damaged. In the following year, we examined the effects of the flood on the regrowth response of a willow, Salix eriocarpa (Franch), and its subsequent effects on the densities of a leaf beetle, Plagiodera versicolora (Laicharting), and its predators. 2. We used 10 randomly selected pairs of heavily and lightly damaged trees. Field observations showed that the number of sprouting shoots was significantly greater on heavily damaged trees than on lightly damaged trees. The sprouts continued to grow until August, producing lateral shoots and leaves. 3. The numbers of leaf beetles at all developmental stages (egg, larva, pupa and adult) significantly increased on heavily damaged trees as compared to numbers on lightly damaged trees. Adults of the leaf beetle aggregated predominantly on sprouting shoots throughout the growing season. 4. As a result of the increase in the number of leaf beetles, leaf herbivory on heavily damaged trees was significantly greater than on lightly damaged trees. 5. Two arthropod predators, the larvae of the ladybird Aiolocaria hexaspilota (Hope) and the web-building spider Agelena opulenta (L. Koch), also increased significantly on heavily damaged trees. These findings indicate that the flood caused by the typhoon initiated bottom-up cascading effects from the willow to arthropod predators through herbivorous insects by increased foliage sprouting.