How important are birds for maintaining the integrity of ecological communities?
Birds provide many services to the forest, but we usually think of them only as pollinators, seed dispersers, or eaters of mosquitoes. However, birds have other important relationships with forest plants and insects.
For example, a study in Missouri found that young trees experienced high levels of herbivory from insects when birds were experimentally kept away from them. The damage was severe enough to slow the growth of these young trees when measured over 2 years. Thus, birds that eat insects can make an important difference in the structure of forests.
When bird populations decrease, we can expect the effects to reverberate throughout the forest community. With fewer birds, insect populations increase, and the insects do more damage to the trees.
The decline of insect-eating birds has been shown to produce this effect in temperate forests, but, until recently, the phenomenon had never been studied in a tropical forest. The question is particularly interesting in tropical forests because of their high diversity of birds and insects.
Some ecologists predicted that predators would not effectively limit the densities of insects in a tropical forest, where there is very high insect diversity.
In other words, are birds indirectly helping out trees by eating herbivorous insects? I examined this question through experiments, observations and comparisons between different forest types.
In one experiment, I used canopy cranes to access exclosures placed in the forest canopy. Exclosures are structures designed to keep birds away from certain branches in the canopy. Because birds could not forage on these branches, the insects were free to chew away with impunity. Other branches were designated as the control branches and were open to bird foraging.
A construction crane in the middle of a forest allows researchers to access the canopy of the forest. The Smithsonian Tropical Research Institute built the first canopy crane, pictured here, in Panama. This crane was used by Sunshine Van Bael for her research.
A comparison of the damage to leaves and the numbers of insects on exclosure and control branches demonstrated that birds limited insect density. At one site, the damage by herbivores increased by 50% inside of the exclosures than outside. This suggests that birds are important because they indirectly help the trees by cleaning off their herbivores.
Interactions between birds, herbivorous insects and tropical canopy trees may suffer ecological change as a result of human disturbance. As habitat destruction continues in tropical regions, forest canopies will become increasingly reduced in area and fragmented. Severe fragmentation will undoubtedly affect bird populations, whose decreasing numbers could have a cascading effect through the community.
The result of this cascade would be an increase in insect populations and resultant greater damage to trees. On a larger scale, global climate change, the most massive human-induced change, may affect photosynthetic mechanisms of trees. Changes in leaf properties will reverberate throughout the assemblage of species feeding in tropical forest canopies. For example, leaves could become more palatable to herbivores after a drought, encouraging insect outbreaks.
In the face of such changing dynamics, it is imperative that we have baseline knowledge of how herbivory is limited in forest canopies. Without this knowledge, we will lack the necessary tools to maintain the integrity of forest canopy communities and tropical ecosystems.