INSECTIVOROUS BIRDS INCREASE GROWTH OF WHITE OAK THROUGH CONSUMPTION OF LEAF-CHEWING INSECTS

We examined the indirect effects of insectivorous birds on plant growth through consumption of leaf-chewing insects in a Missouri Ozark deciduous forest. Over a period of 2 yr, we compared insect numbers, leaf damage, and resultant plant growth for control saplings of white oak (Quercus alba), vs. saplings that we caged to limit access by insectivorous birds but not herbivorous insects. In a third treatment, we sprayed insecticide on young white oak trees to determine the impact of the insect herbivores on plant growth in the presence of birds. The total number of insects encountered on plants inside of cages was twice that on control plants for both years. Insecticide spraying reduced herbivore numbers substantially but did not eliminate them. As a result of the treatments, cage plants suffered 25% leaf area loss, control plants 13%, and spray plants 6% at the end of the first season (34, 24, and 9%, respectively, for the 2nd yr). As a result of the differences in damage, cage plants produced one-third less total aboveground biomass compared to insecticide-treated plants, with control plants producing intermediate values. Differences in biomass production were due mainly to decreased leaf biomass, which in turn was associated with decreased leaf size in subsequent years as a result of high damage during the previous year. This is the first terrestrial ecosystem study to demonstrate a significant impact of insectivorous birds on plant growth. Our results suggest that over the long term observed declines in North American populations of insectivorous birds may reduce forest productivity because of potentially higher numbers of leaf-chewing insects and the concomitant negative effect on plant growth.