Invasive birds spread seeds of invasive plants, study finds

On Oahu, where native birds have been largely displaced by their introduced cousins, native plants depend almost entirely on invasive birds to spread their seeds and thrive, a new study has found.

Unfortunately, they don’t do a great job of it, and it appears many native plants will need a helping hand if they are to continue to survive, according to a study published this month in the journal Science.

The study, by University of Illinois biologists with help from researchers from the University of Hawaii and other institutions, is billed as the first comprehensive study on species interactions in an ecosystem almost entirely made up of introduced plants and birds.

During the three-year project, more than 40 researchers and field assistants collected more than 3,200 fecal samples from 21 bird species at seven sites across Oahu, identifying more than 100,000 viable seeds from the droppings.

An analysis of the seeds revealed that Oahu’s invasive birds have developed complex patterns of often specialized interactions with plants that are strikingly similar to native-dominated ecosystems elsewhere.

This was surprising, according to the study, because while such complex patterns are seen in ecosystems that have evolved over millenniums, the introduced species on Oahu have coexisted for less than 100 years.

To see complex yet stable networks in such a degraded ecosystem challenges the widely held perception that coevolution is required for the emergence of those complex networks in nature.

“This realization was astonishing and one of the most memorable ‘eureka moments’ of my career to date,” said Jeferson Vizentin-Bugoni, the study’s lead author, from the University of Illinois at Urbana-Champaign.

The researchers tested the stability of non-native bird-plant interactions by simulating the extinction process.

Vizentin-Bugoni said he tried to determine how quickly the birds would go extinct by removing plants in a given sequence. What he found were rates of population collapse “very similar” to native communities, giving them a surprising stability.

While that might sound like good news, Vizentin- Bugoni said, the study didn’t find any evidence of a single native bird consuming a native seed.

Vizentin-Bugoni said that while he didn’t have a lot of optimism for finding interactions between pairs of native species, “it is still shocking to see not a single interaction between a native bird and a native plant.”

It turns out the invasive birds are eating and dispersing mostly non-native plant seeds, according to the study.

Why aren’t Oahu’s birds dispersing more native plant seeds?

“We don’t know precisely why yet, but likely it has multiple causes,” he said. “Birds do eat fruits of some native plants, but most birds have small bills and cannot disperse seeds that are too large. Such large seeds are probably adapted to dispersal by some large-bodied native animals which are now extinct.”

These native plants with larger seeds are like ecological “ghosts” waiting for a disperser that never comes and now depend entirely on humans to do this service, Vizentin-Bugoni said.

The researchers said that while some work is occurring in the forests of Oahu to perpetuate native species, more is needed to help these species survive.

“Without active management, Oahu’s forests will continue to be transformed into an invasive-dominated landscape, with only a few native plants holding on,” said paper co-author Corey Tarwater, a University of Wyoming assistant professor.

Vizentin-Bugoni said the study was undertaken on Oahu essentially because the Hawaiian Islands are the extinction and invasive- species capital of the world.

“The Hawaiian archipelago — and the island of Oahu in particular — has a long story of extinctions and species invasions. It started when the first Polynesians colonized the archipelago. The fossil evidence suggests that several large-bodied birds were extinct around this period as a result of hunting,” he said.

“But most of the extinctions are more recent. Both extinctions and colonization of exotic species increased dramatically with the increase of the human population on the island and — more recently — with tourism: People bring pets, seeds and diseases which occasionally are accidentally spread and harm the native biodiversity.”

Vizentin-Bugoni said researchers don’t know whether the new ecosystem works as well as it did before extinction and invasion started because “we don’t know precisely how the ecosystem was in the past.”

“This is one of the sad aspects of extinctions: When species are lost, we also lose the information on what roles they used to play in the ecosystem, and we can only reconstruct an incomplete picture of this based on few fossils when those exist,” he said.