Trophic cascade effects of avian predation on a willow in an urban wetland

Trophic cascades play a crucial role in ecosystem functioning. In this study, we tested the effects of avian predation on willows (Salix warburgii) and associated arthropods in an urban wetland. We excluded birds by netting around willow branches for 20 months from September–November 2010 to June 2012. We compared the leaf count, leaf area, leaf biomass, bud count, catkin (flower) count and herbivory from pairs of bird-exclusion and no-exclusion branches on 11 trees. Simultaneously, we compared herbivorous and predatory arthropod abundances associated with bird-exclusion and no-exclusion branches. Another nine trees were used as reference branches to assess whether the bird exclusion impacted other branches of the same trees (i.e., no-exclusion branches). Bird exclusion resulted in increased herbivory 1 year after the treatment, followed by a reduced leaf count, leaf area, leaf biomass, bud count and catkin count in the second year. The bird-exclusion branches exhibited greater spider abundance than the no-exclusion branches. However, herbivorous arthropod abundances were similar between the branch types. The reference branches had similar values in all plant traits and for all arthropod abundances to those of the no-exclusion branches. This study demonstrated the branch-level effects of trophic cascades on willows via the exclusion of birds and a resulting reduction in herbivory. However, whether and how the arthropods mediate such effects require further investigation. This study adds to the limited empirical data demonstrating the effects of trophic cascades on plant reproduction. Our findings highlight the importance of bird conservation in urban wetlands.