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 60 percent to 70 percent of tree species in the native forests are dispersed, at least in part, by birds, she said.
To test the effects of the loss of native birds on seed distribution, Rogers devised seed traps that look a bit like satellite dish receivers, with tubing bent into a circular shape and covered with fine mesh screen-door netting. She set 119 traps beneath and near Premna obtusifolia, or false elder, trees on Guam and the nearby island of Saipan, which does not have brown tree snakes. For each tree sampled, she set two traps directly beneath the tree's canopy, two about 3 feet away, three at 16 feet, three at 33 feet and seven at 65 feet.
On Saipan, Rogers and her colleagues found seeds in nearly every trap at each distance, though more seeds were found in closer traps and fewer farther away. However, on Guam the seeds appeared only in traps directly beneath the canopy. What's more, most of the farther-dispersed seeds from traps on Saipan had the seed coats removed, a factor that could speed seedling germination and the growth of new trees and something that likely could only be accomplished in the gut of a bird. None of the seeds foun
|Contact: Vince Stricherz|
University of Washington