Figure 1.17 A manipulative experiment. Manipulative experiments provide the strongest tests of hypotheses. (a) In a study that tested if birds are an important factor in determining the number of insects on oak trees in Missouri, ecologists placed cages around some white oak saplings to exclude birds, and left other oak saplings uncaged to serve as a control. (b) From this experiment, the researchers measured the number of insect herbivores per leaf and the amount of leaf tissue that was consumed in each of the two treatments.

After R. J. Marquis and C. J. Whelan, Insectivorous birds increase growth of white oak through consumption of leaf-chewing insects, Ecology 75 (1994): 2007–2014. Photo by Chris Whelan, University of Illinois.