Jan. 11 (UPI) -- On Oahu, the third largest of the Hawaiian islands, most of the airborne seed dispersal is now being carried out by non-native birds, according to a new study published Monday in the journal PNAS.
Not only have non-native species taken over seed dispersal networks, researchers found, but they're also mostly dispersing the seeds of non-native plants.
"Hawaii is one of the most altered ecosystems in the world, and we are lucky enough to examine how these nonnative-dominated communities alter important processes, such as seed dispersal," study co-author Corey Tarwater, an assistant professor of zoology at the University of Wyoming, said in a news release.
"What we have found is that not only do nonnative species dominate species interactions, but that these nonnative species play a greater role in shaping the structure and stability of seed dispersal networks than native species," Tarwater said. "This means that loss of a nonnative species from the community will alter species interactions to a greater extent than loss of a native species."
Scientists used data from field surveys, as well as extensive footage from motion-sensor game cameras, to determine which birds species on Oahu are spreading seeds.
The findings offer a stark reminder of the extent to which Hawaii's ecosystems have been transformed by the loss of native species and proliferation of invasive plants and animals. Over the last 700 years, an estimated 77 bird species and subspecies native to Hawaii have gone extinct.
The study is one of the first to show non-native species can come to dominate seed-dispersal networks, doing so almost exclusively to the benefit of non-native plants.
"This forms what has been called 'ecological meltdown,' which is a process occurring when nonnative mutualistic partners benefit each other and put the system into a vortex of continuous modification," said lead study author Jeferson Vizentin-Bugoni, postdoctoral researcher at the University of Illinois at Urbana-Champaign.
Seed dispersal has a tremendous impact on plant population dynamics and biodiversity. As seed dispersal networks have been reshaped, so too have Hawaii's plant communities.
"The Hawaiian Islands have experienced major changes in flora and fauna and, while the structure of seed dispersal networks before human arrival to the islands is unknown, we know from some of our previous work, recently published in Functional Ecology, that the traits of historic seed dispersers differ from the traits of introduced ones," said co-author Sam Case.
"For instance, some of the extinct dispersers were larger and could likely consume a greater range in seed sizes compared to the current assemblage of seed dispersers," said Case, an ecologist and doctoral student at Wyoming.
The latest data showed 93 percent of all seed dispersal events involved non-native species. Though non-native birds still disperse the seeds of some native plants, their seed-dispersal services mostly benefit non-native plants.
"Nonnative birds are a 'double-edged sword' for the ecosystem because, while they are the only dispersers of native plants at the present, most of the seeds dispersed on Oahu belong to nonnative plants," Vizentin-Bugoni said. "Many native plant species have large seeds resulting from coevolution with large birds. Such birds are now extinct, and the seeds cannot be swallowed and, thus, be dispersed by the small-billed passerines now common on Oahu."
Though the species of the dispersers have changed, the research showed that the traits that help predict the dynamics of seed dispersal networks remain the same.
The niche-based traits, like dietary preferences, proved more important than neutral-based traits like abundance.
Researchers found bird species that eat most fruit are more likely to disperse the seeds of a wide range of plant species.
Additionally, plants that put out fruits for longer periods of time -- especially fruits with small seeds -- are more likely to have their seeds dispersed.
"Land managers can use these ecological traits to identify species that can be removed or added to a system to improve seed dispersal," Tarwater said. "For example, removal of highly important nonnative plants or the addition of native plants with traits that increase their probability of dispersal, could aid in restoration efforts."