Alien birds may be providing native plants a critical service

A sad fact of Hawaiian forests is that we often don't know the impact of the loss of an insect, a plant or a bird.

Ecosystems are intricate webs, in which one bird may be a plant's pollinator, and the plant a source of nectar to the bird. Another bird might feed on the plant's fruit, in return for providing the plant with seed distribution services.

Many of the Hawaiian fruit-eating (and therefore seed-dispersing) birds have become extinct. And without their seed distributors, the plants themselves could quickly go the same way.

But in a few situations, researchers are finding that alien birds have stepped up to the plate.

Researchers Jeffrey Foster of the University of Illinois and Scott Robinson of the Florida Museum of Natural History, in an October 2007 article in the journal “Conservation Biology,” report that the Japanese white-eye or meijiro and the red-billed leiothrix may be filling the seed-distributing niche opened up by the extinction of some native fruit-eating birds.

Their article, “Introduced Birds and the Fate of Hawaiian Rainforests,” found from stomach contents that the birds ate widely of native fruit, and they used seed traps to show that the seeds were then being distributed rather widely.

“The Hawaiian Islands have lost nearly all their native seed dispersers, but have gained many frugivorous (fruit-eating) birds and fleshy-fruited plants through introductions. Introduced birds may not only aid invasions of exotic plants but also may be the sole dispersers of native plants,” the authors said in the abstract to their paper.

The alien birds are particularly important in forests that have already been dominated by alien species, and the authors say they are actually responsible for helping as many as six native understory re-establish themselves.

“Some native plant species are now as common in exotic forest understory as they are in native forest,” they said.

The birds also distribute the seeds of alien plants, but in their study, the authors said they found that more than 85 percent of the seeds were from native plants.

“Without suitable native dispersers, most common understory plants in Hawaiian rainforests now depend on introduced birds for dispersal, and these introduced species may actually facilitate perpetuation, and perhaps in some cases restoration, of native forests,” they said.

There is still the problem that there do need to be some surviving native plants to be a fruit and seed source for the birds.

Foster and Robinson said they believe that aggressive weed control is still critically important in forests that still have mostly native plants.

© 2007 Jan W. TenBruggencate