The researchers made seed traps out of screen and PVC pipe from the hardware store.
Click on image for full size
Courtesy of Nash Turley, University of Washington

Can Forests Survive Without Birds?
News story originally written on January 30, 2009

Can forests that have lost all of their birds still function normally? This is an important question for the now bird-less forests on the island of Guam, an island in the western Pacific.

How did Guam lose its birds? In the mid-1940s, the brown tree snake was accidentally introduced to what was then snake-free Guam. This snake became Guam's new top predator and ate its way through a buffet of the island's bird community. As a result, 10 of the island's 12 forest bird species are now extinct on Guam, and the two surviving forest bird species remain only in tiny, localized populations where snakes are controlled. Guam's now silent forests currently hold about 13,000 snakes per square mile.

I started to think about the potential ecological impacts of bird loss in 2002, when, two years out of college, I was hired by the U.S. Geological Survey to develop a "Rapid Response Team" that would identify and eradicate new populations of brown tree snakes on U.S.-associated Pacific Islands. Although I had heard the snake story in my college conservation biology course, I did not know where Guam was when I applied for the job. Yet, three weeks later, I was on a plane headed there.

Bird loss and seed movement

As I worked on Guam during the next three years, I often wondered why no one was studying how Guam's bird losses impacted the forests' remaining organisms. So in 2005, I began a Ph.D. program in biology at the University of Washington to investigate how bird loss changes the movement of seeds around Guam's forests.

This spring, I--along with my co-advisers, Joshua Tewksbury and Janneke Hille Ris Lambers, our collaborator at the University of Guam, Ross Miller, and my field assistant, Theresa Feeley-Summer--began to examine whether the loss of birds had caused changes in how the seeds the birds typically eat are distributed.

The study is funded by the Budweiser Conservation Scholarship and the National Science Foundation's (NSF) Integrative Graduate Education andResearchTraineeship (IGERT)and Graduate Research Fellowship programs. In this study, we set seed traps at various distances from fruiting False Elder (Premna obtusifolia) trees in the forests of Guam and Saipan, a nearby island with birds, and then counted the number of seeds that fell into each trap. This shows us how far the seeds of fruiting trees are traveling in Guam's bird-less forests, as compared to Saipan's forests with birds.

Screen-door netting and a mile of PVC

The first step in our research was to design traps to catch falling seeds using locally available materials. This task required many trips to the new Home Depot on Guam; we purchased the store's entire supply of screen-door netting, flexible PVC piping and PVC connectors.Believe me, I got some strange looks when I asked the Home Depot sales person for 2,000 feet of screen-door netting and a mile of PVC.

Although our study is still ongoing, we have already produced some important results. We found that all of the seeds from the fruiting trees on Guam remained near their parent trees and maintained intact seed coats. By contrast, many more of the seeds from the fruiting trees on Saipan were found without seed coats away from their parent tree.

The differences between the distributions of the seeds on Guam and Saipan can be attributed to the differences in their bird populations. In Saipan's forests, birds stop at fruiting trees, eat fruit, swallow the seeds and then fly to the next tree, where they defecate, effectively moving seeds away from where they are produced. We believe the handling of seeds by birds removes the seed coat and promotes the germination of seeds. In the bird-less forests of Guam, however, fruits ripen, fall off of the tree and settle at tree bases without being eaten or moved by birds.

Unfortunately, our results do not bode well for the future of Guam's fruit-producing trees. Research from around the world has shown that seeds from fruits falling under parent trees (like fruits in Guam) tend to experience higher mortality from predators and fungal infections than seeds that are moved away from their parent trees. In addition, for many (tree) species, seeds that are not handled by birds are less likely to germinate than seeds that are handled by birds.

More on seed dispersal

Where will our research go from here? I hope that Home Depot has restocked its screen-door netting and PVC piping, because we plan to construct about 1,000 more seed traps. We will use them to study seed dispersal for 14 more species of trees. This will give us a community perspective on seed dispersal patterns.

We will also investigate how the lack ofseed handling and seed dispersal by birds impacts the germination and growth rate of seeds. In addition, we will evaluate the impacts of bird loss on local people by interviewing people who extract forest products for traditional uses. Several tree species used for medicinal purposes or as carving wood have seeds dispersed by birds, and thus may be experiencing population declines noticed by local people.

Although the introduction of a non-native snake caused Guam's bird loss, other factors are causing bird losses in forests around the world. The ecological impacts of all of these declines--no matter what the cause--are likely to be similar. Therefore, the complete loss of Guam's birds provides an extreme example that can inform us about the ecosystem impacts of bird losses around the world. The results of our research may be used by conservationists to develop and apply timely management approaches that will minimize the ecological impacts of bird loss.

Text above by Haldre Rogers, courtesy of NSF

Last modified February 20, 2009 by Lisa Gardiner.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Tropical Rain Forest Birds

The birds of the rain forest are the most beautiful in the world. A wide range of colors can be seen darting through the trees as the forest tops come to life. Many species of tropical birds are kept...more

Tropical Rainforests

Tropical rainforests are home to thousands of species of animals, plants, fungi and microbes. Scientists suspect that there are many species living in rainforests have not yet been found or described....more

Biomes and Ecosystems

Biomes are large regions of the world with similar plants, animals, and other living things that are adapted to the climate and other conditions. Explore the links below to learn more about some of the...more

Triggers of Volcanic Eruptions in Oregon's Mount Hood Investigated

A new study has found that a mixing of two different types of magma is the key to the historic eruptions of Mount Hood, Oregon's tallest mountain, and that eruptions often happen in a relatively short...more

Oldest Earth Mantle Reservoir Discovered

Researchers have found a primitive Earth mantle reservoir on Baffin Island in the Canadian Arctic. Geologist Matthew Jackson and his colleagues from a multi-institution collaboration report the finding--the...more

Its Not Your Fault A Typical Fault, Geologically Speaking, That Is

Some geologic faults that appear strong and stable, slip and slide like weak faults. Now an international team of researchers has laboratory evidence showing why some faults that 'should not' slip are...more

Extended Period of Lower Solar Activity Linked to Changes in Sun's Conveyor Belt

A new analysis of the unusually long solar cycle that ended in 2008 suggests that one reason for the long cycle could be a stretching of the sun's conveyor belt, a current of plasma that circulates between...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA