Growing conditions, such as water and nutrient supply, are the major determinates of tree growth, but insectivorous birds can also play an important role, say scientists in a study published in the January issue of Ecology. Under the right conditions, birds contribute to whole tree growth by preying on herbaceous arthropods, such as leafhoppers, caterpillars and grasshoppers.
While it may be conventional wisdom that birds help tree growth by controlling insect infestations, previous research showed this relationship to be much more dynamic and complex than it appears on the surface. For example, conditions, such as the plant and insects’ natural defenses against predators, can dampen the effects of top-down predators like birds.
William Bridgeland from Northern Arizona University and his co-authors investigated which ecological conditions can strengthen the top-down effects of predators enough to influence bottom-up processes. In other words, what conditions, if any, permit birds to regulate tree growth driven by more basic conditions, like water and nutrient supply.
The scientists excluded birds from 35 individual mature cottonwood trees at two sites in northern Utah with nets that allowed most insects to pass through freely; they then compared tree growth, arthropod numbers and species between exclosed trees and open trees which were fully accessible by birds. They found that the birds consistently reduced insect richness, number and biomass; however, only when certain conditions were met did that translate to tree growth.
It turns out that the interplay between the growing conditions and the predator effects had to be just right: when trees received plenty of rain one year they grew vigorously and their insects multiplied because they also benefited from the rain. The rains supplied the trees with enough water in this typically arid place, but their growth then became limited by increased herbivory from the insects. This set the stage for the birds to lower herbivory by eating the insects.
In addition, of the hundreds of species of herbivorous insects on the trees, only a small subset were actually targeted as prey by the birds—this was due to chemical defenses in other insects, effective predator avoidance behaviors or simply being too small to be worth eating. From the tree’s perspective, birds were beneficial only when the most abundant herbivores happened to also be the bird’s preferred prey.
This only occurred at one site in one year when three species of grasshoppers were abundant on the trees. These few species that were great bird food, as well as important herbivores, connected the birds to the trees; under the right conditions, then, birds can act as tree growth regulators.
Bridgeland, W., Beier, P., Kolb, T., & Whitham, T. (2010). A conditional trophic cascade: Birds benefit faster growing trees with strong links between predators and plants Ecology, 91 (1), 73-84 DOI: 10.1890/08-1821.1