Top-down trophic cascade in a willow (Salix warburgii) food web

Background/Question/Methods

Food webs play a crucial role in community and ecosystem ecology. Previous studies have shown that both direct and indirect trophic interactions can be important in maintaining food web dynamics. Trophic cascade, in particular, has been found to occur in a variety of communities and ecosystems. For example, trophic cascade effects from secondary consumers are often found to benefit plant biomass. However, studies on “secondary consumer – plant” trophic cascade in terrestrial ecosystems are somewhat biased toward “insect consumer – herbaceous plant” interactions. Evidence on trophic cascade in “vertebrate consumer – woody plant” systems are scarce. In this study, we examine the effects of bird predation on herbivorous arthropods (aphids) and willow (Salix warburgii) trees along a riparian zone in northern Taiwan. Due to logistic difficulties and considering that individual heterogeneity is high in woody species, we set the experiment unit at branch level. This allows us to use branches from the same tree as control and treatment. The treatment branches were enclosed with netting to exclude birds, and the control branches were not. We measured the following three sets of variables: (1) plant growth (e.g. leaf number, leaf damage and bud number); (2) plant reproduction (i.e. flower number); (3) herbivore abundance (i.e. aphid density).

Results/Conclusions

After five months of bird exclusion (September 2010 to January 2011), we found no treatment effects on either plant growth or herbivore abundance. However, flower numbers on bird-exclusion branches were 31% less than control branches, suggesting a positive trophic cascade from the birds on willow reproduction. One possible explanation for the lack of response in plant growth might be that bird-exclusion branches were investing more in chemical defense, which helped the plants to keep the same level of growth in the expense of future reproductive investment. Nevertheless, it is still not clear why then aphid abundance was not different between bird-exclusion and control branches. Our next steps are to investigate chemical defense levels in the leaves and to include additional herbivore species in the analysis.