After years of fighting the threats posed by foreign species on the Hawaiian islands, conservationists have discovered that invasive birds may now be the only hope left for the survival of some native plants.
Hawaii is one of the most invaded places in the world, in terms of foreign species. More than 4,600 plant and 140 bird species have been introduced by human activity, with at least 58 types of bird establishing permanent breeding populations there. Most land birds in Hawaii are now exotic.
Biologists Jeff Foster at the University of Illinois, Urbana-Champaign, and Scott Robinson at the University of Florida, Gainesville, took a look at seed dispersal in Hawaiian forests — both those with native plants, and those composed of exotic species that had been planted in the 1960s for the timber industry.
The team captured native and exotic bird species throughout both types of forest and analysed the contents of their stomachs and faeces.
Most of the seeds that were found belonged to native plants rather than invasives — out of proportion to these plants' relative abundance. That's good news for the restoration of native plants. But, interestingly, it was the exotic birds that were doing all the work.
Although many native birds still exist in these forests, they almost exclusively feed on invertebrates or nectar. Only one native bird, the Hawaii Amakihi, was found eating the fruit of a native plant species, but this eating habit was so uncommon as to have practically no effect on seed dispersal. Four exotic bird species, in contrast, were eating native fruits and dispersing seeds.
Restoration
All this bird activity is obviously having an impact: when the team looked at forest undergrowth they found native plant saplings sprouting up everywhere. Surprisingly, most of the understory plants in the exotic forest were also native trees.
This discovery, due to be published in the October issue of Conservation Biology1, complicates the goal of habitat restoration programmes aiming to return the islands to their natural state.
"Is it best to have all invasives removed, even if that means severing the link between seeds and their current dispersers? Or is it best to keep invasive birds and all the trouble they cause? I don't know," says Foster. "Returning Hawaii to her natural state is not attainable, but we're doing the best we can and, ironically, exotic birds appear to be helping."
"This research clearly points out that we have a dilemma," says ecologist Donald Drake at the University of Hawaii in Manoa, Honolulu. "Because of the loss of native birds, both native and alien plants now appear to depend on alien birds for dispersal in some areas. But we must be cautious about actions taken based on the observations of any one study," he says.
1 Foster, J. T. & Robinson, S. K., et al. Conserv. Biol. 21, 1248-1257 (2007).
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