Alien Bird Species Can Help Native Plants Move Around, Say Surprised Scientists

Evolving together as a community is not necessary for non-native species to fill critically important ecosystem roles, such as dispersing seeds — tasks originally performed by now-extinct native species on the Hawaiian island of Oʻahu

by GrrlScientist for Forbes | @GrrlScientist

Red-billed Leiothrix (Leiothrix lutea), sometimes known to aviculturists as the Pekin Robin or as the Pekin Nightingale, is one of many non-native species that can be found on the Hawaiian Isles. Native to lowland forests of India, Bhutan, Nepal, Burma and parts of Tibet, it was introduced to Hawai‘i in 1918, and subsequently spread throughout all the islands except Lanai.
(Credit: Dibyendu Ash / CC BY-SA 3.0)

Native bird species on the Hawaiian island of O‘ahu have been almost completely wiped out and replaced by a mélange of non-native birds from around the world, creating a never-before-seen type of ecosystem. As a result, the local plants now depend almost entirely on introduced species to disperse their seeds, according to a pioneering study. Previous studies reported that interactions between plants and animals are especially vulnerable to extinctions or to introduced species. But this surprising discovery highlights how ecological communities dominated by non-native species can be just as stable as native communities, with non-native species carrying out critically important ecosystem functions.

Hawai‘i is “the extinction capital of the world”

“[W]hen I first got to the island it was impressive to realize that some places that look like a tropical forest have absolutely no native species,” said community ecologist, Jeferson Vizentin-Bugoni, a postdoctoral fellow at the University of Illinois at Urbana-Champaign and lead author of the groundbreaking new study, in email.

“When I joined the project I knew the ‘Hawaiian Islands are known as both the extinction and invasive species capitals of the world’,” Dr. Vizentin-Bugoni added.

Landscape on Kahanahāiki, one of the sampling sites on Oʻahu Island. This tropical rainforest is comprised almost exclusively of introduced plants and animals.
(Credit: Sean MacDonald.)

“[T]here is no other environment like Hawaii,” said study co-author Corey Tarwater, an assistant professor of zoology at the University of Wyoming, in email. “You have this isolated archipelago that is more than 3000 km from the nearest continental land mass and is home to many endemic species. Despite representing only 0.2% of the land area of the U.S., almost one-third of the species on the U.S. Endangered and Threatened Species list are in Hawaii.”

According to the American Bird Conservancy, 95 of Hawai‘i’s 142 bird species were driven extinct on this island archipelago since humans arrived (more here) sometime between 124 and 1120 AD. Currently, 33 of Hawai‘i’s remaining 44 endemic birds are included on the Endangered Species Act, although ten of those have not been seen for decades and are probably extinct, too.

Native Hawaiian plants have fared similarly. More than 100 plant taxa have already been driven extinct, and over 200 species have 50 or fewer representatives remaining in the wild, according to Hawai‘i’s Division of Forestry and Wildlife. Officially, 366 of Hawai‘i’s plant taxa are listed as Endangered or Threatened by Federal and State governments, and an additional 48 species are proposed to be listed as Endangered.

Adult male red-crested cardinal (Paroaria coronata). Native to subtropical or tropical dry shrubland and heavily degraded former forest in South America, this species was introduced to O‘ahu, Maui, and Kauai.
(Credit: Dick Daniels / CC BY-SA 3.0)

“Then you have introduced species from almost every continent and this mix of native species and non-native species from all over are interacting with each other and forming new communities,” Professor Tarwater added.

“I knew beforehand the dataset of this project had great potential, but at that moment I started to realize that we have a lot to learn from the Oahu case about what other ecosystems may become if the high rates of extinctions and invasions keep increasing during the Anthropocene,” Dr. Vizentin-Bugoni said.

“We wanted to study this extreme novel ecosystem because this can hopefully teach us about what may happen to many communities as more and more species are introduced all over the world and we wanted to aid in the conservation of this amazing and unique place,” Professor Tarwater said.

Identifying who’s who in O‘ahu’s novel seed dispersal networks

Traditionally, scientists sought to elucidate how mutually-beneficial interactions actually work within a community by focusing on native-dominated communities, where relationships between plants and animals are complex, having evolved over long time periods. But how do native communities compare to novel communities composed of non-native species that are popping up all around the world? No one knows.

To address this complete lack of knowledge, Dr. Vizentin-Bugoni and his collaborators investigated seed dispersal, an important ecosystem role originally performed for millenia on O‘ahu by a variety of now-extinct native bird species.

To what extent are introduced bird species integrated into local plant seed dispersal networks? The team asked. Are introduced seed-dispersing species filling ecological roles previously performed by extinct natives?

Fruits from introduced plant species collected at the Kahanahāiki site, Oʻahu Island.
(Credit: Jef Vizentin-Bugoni)

Collecting the data was straightforward, although time-consuming: Dr. Vizentin-Bugoni and his collaborators identified more than 100,000 seeds in poop samples collected from birds at seven sites across Oʻahu. Together, these sites encompassed the broad environmental variation across the island.

“We collected 3,278 fecal samples from 21 bird species,” said Dr. Vizentin-Bugoni.

That is a lot of poop.

“Yes, it was a LOT of poop,” agreed Dr. Vizentin-Bugoni.

How did they know which bird made which poop?

“Each bird was placed in a paper bag for a few minutes and ‘voilà’, most of them leave a sample in the bag, which we then label and take to the lab to see which seeds are in the sample,” Dr. Vizentin-Bugoni explained. The poop samples, which were collected over a period of three years, were visually examined under stereoscopes and the 109,424 viable seeds they contained were sorted and identified.

“We have reference libraries we work from and for some rare seeds, we had to contact local experts to identify the seeds,” Dr. Vizentin-Bugoni said.

Dr. Vizentin-Bugoni and his collaborators found that O‘ahu’s seed dispersal network included 15 bird and 44 plant species connected by 112 distinct links (Figure 1).

Fig. 1. Structure of the island-wide seed dispersal network on Oʻahu and illustration of two emblematic interactions. (A and B) The novel network was nested [specialist species interacting with proper subsets of partners of the most generalist species (wNODF) = 48.67; 95% confidence interval (CI) = 34.24 to 46.66] (A) and modular [subsets of species interacting preferentially with each other, forming modules of highly connected species (QW) = 0.24; 95% CI = 0.07 to 0.09] (B). Species and links from distinct modules are depicted by different colors (blue, orange, and green), and gray links are interactions connecting modules. ( C ) Japanese white-eye feeding on Pipturus albidus, the most commonly consumed native plant. (D) A red-billed leiothrix feeding on Clidemia hirta, the most widely consumed and widespread introduced plant (Illustration credit: P. Lorenzo).

By meticulously documenting these seed dispersal networks, Dr. Vizentin-Bugoni and his collaborators found they are comprised almost exclusively of non-native birds (86.7%) and plants (65.9%), and that 93.3% of dispersal events were introduced plants. Surprisingly, the team did not record any interactions between a native bird and a native plant.

“Because all native fruit-eating birds are extinct on Oahu, we knew chances were low that we’d find native birds consuming native seeds,” said study co-author, Jinelle Sperry, a wildlife biologist for the U.S. Army Engineer Research and Development Center and an adjunct professor at University of Illinois Urbana-Champaign, in email.

“But to have caught that many birds over three years and not have found a single native interaction is pretty astounding.”

Non-native birds prefer to eat fruits of non-native plants

“Even though we did find some invasive birds eating native plants, the vast majority of interactions are between invasive birds and invasive plants,” Professor Tarwater said.

“These birds are really flexible in what they’ll consume, but across sites, they’re primarily eating invasive plant species,” Dr. Sperry agreed.

Nevertheless, non-native birds are not a perfect fit for their new role, having been present for only one hundred years or less, suggesting that the native plant community is undergoing seismic shifts.

“Many Hawaiian communities now lack birds with the appropriate beak shapes or sizes to consume seeds of certain native plants. Those plants are out of luck with regards to dispersal,” said co-author, community ecologist Jason Gleditsch, a doctoral student at the University of Illinois at Urbana-Champaign.

A red-whiskered bulbul (Pycnonotus jocosus), a non-native bird species on O‘ahu, eats the fruit of a common guava (Psidium guajava), a non-native plant species on O‘ahu.
(Credit: Jeferson Vizentin-Bugoni)

“So birds play a ‘double-edged sword’ role for conservation: while the native plants need these invasive birds to disperse their seeds since they are the only dispersers out there, these birds are also dispersing lots of invasive plants,” Professor Tarwater explained.

These non-native birds’ clear preference for non-native fruits has critical implications for conservation.

“Because of that, in restored sites, we need to really entice birds to eat native fruits,” Dr. Sperry pointed out.

Convincing non-native birds to distribute seeds of native plants is not easy, but it can be done.

“This means, from a restoration perspective, unless the invasive plants are removed from the landscape, the birds will continue to disperse them,” Professor Tarwater said. “On the other hand, it also means that restoration efforts can be made to encourage these birds to eat native plants.”

“The specialization we detected also contradicts long-standing predictions about generalization of islands colonizers,” Dr. Vizentin-Bugoni elaborated. “In ecological theory, we use the term ‘niche broadening’ which means, for example, birds that colonize islands are expected to eat whatever they can find. Therefore, the considerably high specialization we detected in the communities is surprising, and challenges what is expected to be the outcome of ‘niche broadening’.”

The Japanese white-eye (Zosterops japonicus), is an omnivore that was originally introduced to O‘ahu in 1929. It rapidly expanded its range and can now be found in every forest on every island in the Hawaiian archipelago.
(Credit: Laitche / CC BY-SA 4.0)

Surprisingly, Dr. Vizentin-Bugoni and his colleagues found that the novel ecological communities on O‘ahu are structured just like native-dominated communities around the world. This suggests that novel communities arise quickly and independently of the species involved, and it demonstrates that non-native species can quickly fill gaps in an ecosystem network after native species are driven extinct.

“Oahu in particular is the extreme example of the loss and gain of species; all native dispersers have gone extinct and all that is left to disperse these endemic and threatened plants are a suite of introduced dispersers,” Professor Tarwater said.

“Because Hawaii is so isolated, there were endemic species you could only find there. These species that were like diamonds in the tree of life now have been replaced by … soda cans. The surprise for us is how these soda cans are rearranging and it’s still working, but we lost almost all the diamonds,” Dr. Vizentin-Bugoni said.

“Once they’re gone, they’re gone. And we lose not only biodiversity but also the potential of those species to benefit humans, such as production of new medicines from plants, for instance.”


Jeferson Vizentin-Bugoni, Corey E. Tarwater, Jeffrey T. Foster, Donald R. Drake, Jason M. Gleditsch, Amy M. Hruska, J. Patrick Kelley, and Jinelle H. Sperry (2019). Structure, spatial dynamics, and stability of novel seed dispersal mutualistic networks in Hawai‘i, Science, 364(6435):78–82 | doi:10.1126/science.aau8751

Originally published at Forbes on 7 April 2019.