Research

Boiga irregularis

The Ecology of Bird Loss Project investigates the ecological importance of birds for forests and the consequences of losing birds on ecosystem processes that maintain biodiversity, support functioning ecosystems, and provide ecosystem services to people.

Our field research takes place on the Mariana Islands in the western Pacific Ocean. The Brown Tree Snake (Boiga irregularis) was introduced to the southernmost island in this chain, Guam, in the 1940s. Since then, it has caused the functional extirpation of all native forest bird species, resulting is the world’s only contemporary case of whole-ecosystem bird loss in the world. With nearby islands that are otherwise similar, this system provides a unique “natural experiment” to understand the importance of birds for forests. Because Guam also represents a severe example of the global phenomenon of vertebrate decline and extinction, it can offer insights into the scope and severity of the impacts of this phenomenon that are occurring globally. Our research focuses on the following areas:

Mariana Fruit Dove

Seed Dispersal: Roughly half of the world’s plant species are dispersed to some extent by vertebrates, and up to 90% of trees in the world’s most diverse ecosystems – tropical forests – rely on animals for seed dispersal. In Guam, birds disperse approximately 70% of the tree species, and the loss of seed dispersal could strongly reduce the regeneration ability of Guam’s forest species.

Seed dispersal can benefits plants in several ways: 1) by handling fruits and seeds, birds can increase the probability and pace of germination, 2) by moving seeds away from parent trees, birds help seeds escape the area of high predation, disease, and competition near members of its same species, 3) by moving seeds to specific microsites (e.g., treefall gaps), birds help seeds reach areas that are favorable for germination and growth, and 4) by transporting seeds to new areas (e.g., deforested areas or ), birds help tree species colonize new habitats.

We have found that the loss of seed dispersal can severely reduce recruitment of tree species on Guam, with large negative impacts for plants that cannot escape areas of high mortality near members of their same species and for seeds that are not removed from fruit pulp. Dispersal also strongly affects the biodiversity of plants across the landscape, with the biodiversity hotspots of regeneration within forests – treefall gaps – strongly reduced in the absence of dispersal.

Papaya tree in gap

Although vertebrates can benefit plants in many ways, plants vary in their dependence on seed dispersal and therefore vary in the negative consequences of losing seed dispersers. Our ongoing work focuses on assessing how tree species vary in their reliance on each of these dispersal benefits. By incorporating this understanding into forest processes simulations, we are working toward predicting the long-term fate of an ecosystem without seed dispersers.

Predation: Vertebrates take a top role in food webs in ecosystems across the world. On Guam, seven insectivorous bird species formed the top of the terrestrial food web, but are now absent. The loss of top predators can reverberate across food webs, with negative cascading impacts on lower trophic levels. In forests, the loss of insect predators could result in increased insect abundance and therefore increased herbivory on plants, linking bird declines to negative impacts on plants. This is relevant to agricultural systems if birds no longer are present to control agricultural pests.

Our research on trophic changes following bird decline has revealed large responses of other important insect predators – spiders – following bird loss. We found that spider abundances on Guam range from 2 to 40 times greater than spiders on islands with birds. Ongoing projects explore the hypotheses that the increase in spiders represents a form of ecological resilience, buffering the ecosystem from insect abundance and herbivory increases that would otherwise result from bird loss.

Pollination: Two bird species likely pollinated trees in the native forests of Guam: the Micronesian Honeyeater and the Bridled White-eye, and the loss of these pollinators could reduce regeneration of plant species that rely on birds for pollination. However, there are very few tree species with flowers that clearly follow the bird pollination syndrome. We examined the potential for a lack of bird pollination to mediate plant population decline on Guam, but found that although birds visit the flowers of many species, experimentally excluding birds did not strongly affect the number of seeds they were able to produce. Therefore the loss of birds’ role as pollinators appears to be a weaker driver of ecosystem change following bird loss on Guam.

Introduced mammals: Introduced mammalian predators and herbivores have caused pervasive and often negative impacts in ecosystems where they were formerly absent, especially oceanic island ecosystems. Our work has focused on understanding the consequences of novel species interactions with mammals on Guam, where the only native mammals are bats. 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. We are investigating the impact of introduced ungulates (pigs and deer) on the survival of forest trees and the potential for these introduced species as seed dispersers.

Photos courtesy Isaac Chellman.