Research on top-down and bottom-up forces in terrestrial systems has advanced beyond documenting the mere existence of trophic cascades to elucidating ecosystem characteristics that contribute to their strength. We conducted an experiment to examine how arthropod community structure mediates the ability of insectivorous birds to influence host tree growth. We built whole-tree bird exclosures around 35 mature cottonwood trees at two sites in northern Utah to measure the effect of bird predation on arthropod herbivore and predator species richness, abundance, composition, biomass, and tree growth. We maintained bird exclosures over two growing seasons, and conducted time-constrained, non-destructive arthropod surveys, in which all morphospecies seen on the trees were recorded, three times each season. Leaf damage, shoot growth, trunk growth, total non-structural carbohydrate concentration in shoots, and seed production were measured on each tree. We characterized the bird community by species composition and minimum study site population size.

Results/Conclusions

We found a strong trophic cascade (trunk growth reduced by bird exclosure) only at one site in one year. This combination of site and year had the highest precipitation, overall tree growth, and arthropod abundance and richness compared to other site-year combinations. The trophic cascade was weak or not evident when tree growth and insect populations were low due to drought. We conclude that when the limiting factor for tree growth switched from water availability to herbivory, the avian predators’ role became important by reducing herbivory.  

Bird predation reduced arthropod herbivore and predator abundance, richness, and biomass. Biomass was reduced proportionately more than abundance, indicating that birds selected larger than average prey. This overall pattern was driven largely by three herbivore groups (namely, non-aphid sap-feeders, folivores, and insects without chemical defenses), suggesting that birds targeted these groups. Structural equation modeling suggested these groups constituted a trophic pathway from the birds to the trees. We hypothesize that arthropod communities dominated by these guilds increase the likelihood of a strong trophic cascade.