Brown tree snake

From YenraWiki

In the last sixty years, brown tree snakes have become the embodiment of the bad things that can happen when invasive species are introduced in places where they have few predators. Unchecked for many years, the snakes caused the extinction of nearly every native bird species on the Pacific island of Guam.

A variety of other damage has been directly attributed to brown tree snakes, including large population losses among other native animal species in Guam's forests, attacks on children and pets, and electrical power outages.

But new research by University of Washington biologists suggests that indirect impacts might be even farther reaching, possibly changing tree distributions and reducing native tree populations, altering already damaged ecosystems even further.

Birds typically make up a small part of the life of a forest, but they are important for pollination, spreading seeds around the forest and controlling insects that feed on plants. Guam, an island thirty miles long and five to fifteen miles wide about 3,800 miles west of Hawaii, lost most of its native birds after the brown tree snake was introduced by accident from the Admiralty Islands following World War II. The snake has few predators on Guam, so its population density is quite high – estimated at more than three thousand per square mile – and some individuals there grow to an unusual size of ten feet long.

Before introduction of the brown tree snake, Guam had twelve species of native forest birds. Today ten of those are extinct on Guam, and the other two species have fewer than two hundred individuals. Though Guam has some non-native bird populations, few other birds moved in when native species died out, and none of them live in the forest. That leaves few birds to consume tree seeds and then drop them away from the trees.

That could have two possible negative impacts on the native forests, Rogers said. First, some plant species need birds to handle their seeds to ensure effective germination. In addition, seed predators and fungi that kill seeds are often found in high density directly beneath a parent tree, so the trees rely on birds to disperse seeds beyond the range of those negative effects. If native birds performed those functions on Guam, tree populations could suffer from the loss of birds. It appears sixty percent to seventy percent of tree species in the native forests are dispersed, at least in part, by birds, said Haldre Rogers, a University of Washington doctoral student in biology.

Altered seed scattering might transform the remaining forest from a diverse mixture of tree species to clumps of trees of the same species, separated by open space. That could have serious consequences, including extinction, for plant and animal species that still live in the forests.