Our field work takes place on a chain of islands in the Western Pacific called the Mariana Islands. The Brown Tree Snake (Boiga irregularis) was introduced to the island of Guam in the 1940's and has has since caused the functional extirpation of all native forest bird species on the island, resulting in the only contemporary case of whole-island forest bird loss in the world. This work will provide the first landscape-level assessment of the impacts of bird loss on an entire forest community. Read about our research sites. Our research involves the following areas:
|Mariana Fruit Dove|
Avian seed dispersal impacts trees in several ways: 1) by handling fruit and seeds, birds often change rates of seed germination, 2) by transporting seeds away from the parent trees, birds provide a mechanism for seeds to escape areas of low survival due to density-dependent mortality, 3) by transporting seeds to specific microsites (e.g. treefall gaps), birds help seeds reach particular environments that are suitable for germination, and 4) by transporting seeds to new areas (e.g. degraded forest, open fields), birds help tree species colonize new habitats.
Previous research: Our previous research has compared seed dispersal distance between islands with birds and Guam, measured the impact of birds on seed germination for two tree species, and measured the strength of distance-dependent mortality (and thus the importance of being moved away from a parent tree) for six species. Another research project compared seed rain in degraded forest areas between Guam and Saipan, and showed that native seeds do not reach seed traps in degraded areas on Guam, likely limiting regeneration of native forests in these areas.
|Papaya tree in gap|
Food Web: Seven insectivorous bird species form the top of the terrestrial food web in the Mariana Islands (all are now functionally absent from Guam) and consume a wide variety of insect prey. If the top predator in this system, birds, are able to limit the abundance of prey populations, their loss on Guam may cascade down to impact lower trophic levels. In forests, this could result in increased herbivory on plants. Similarly, in agricultural areas around the world, birds are often considered important predators of pests, and farmers are encouraged to attract insectivorous birds expressly for their pest control services. Yet birds are often regarded as pests rather than bio-control agents, and our understanding of the situations in which they provide pest control services is still limited.
Previous research: We have tested the role of birds in food webs by setting up bird exclosures alongside paired control areas in native forest on Guam, Saipan, Tinian, and Rota, and following the fate of planted seedlings of 6 forest tree species. We also compared spider abundances between islands, and found that Guam has 2 to 40 times more spiders than Saipan, Tinian, and Rota likely as a result of bird loss. We hypothesize that the increase in spiders, which are also predators of many insects, may buffer the system from impacts of the loss of bird loss. Finally, we have conducted one study on the impact of bird loss on agriculture. We conducted an exclosure experiment on farms on the island of Rota to understand the role of birds in controlling herbivory and affecting production of long beans and eggplant, two common local crops.
Active research: An ongoing study aims to determine the mechanism causing the increase in spider abundances on Guam- is it release from predation, release from competition for shared prey, reduced physical damage caused by birds flying through or stealing web material, or something else? Another study examines the impact of birds on butterfly populations through comparative surveys on Guam, Saipan, and Rota, along with an experimental comparison of caterpillar predation on each island.
Pollination: There are only two bird species that likely pollinated trees in the native forests of Guam- the Micronesian Honeyeater and the Guam Bridled White-eye- and there are very few tree species with flowers that clearly follow the bird pollination syndrome. In a recent project, we examined the role of birds in pollinating trees with smaller, white flowers typically thought to attract insects. A pilot study showed that birds visited our five focal tree species quite frequently, suggesting that they may play an important role in pollination.
Rodent Seed Predation: Several species of introduced rodents are seed predators on Guam, Saipan, Tinian and Rota, and these species are suspected to have an effect on recruitment and survival of native forest plants. Yet predation of introduced rodents by the Brown Treesnake has led to lower densities on Guam as compared to the other islands. We examined rates of rodent seed predation across a suite of native and exotic tree species on islands with and without treesnakes, to determine the indirect impact of the snake on seed predation.
Ungulates: We are collaborating with Ann Gawel, previously a Masters student at the University of Guam, to investigate the impact of introduced ungulates (pigs and deer) on the survival of six species of forest tree seedlings.
Photos courtesy Isaac Chellman.