Nonnative Birds Take Charge in Oahu Seed Dispersal

Researchers discovered that nonnative birds in the island of Oahu, Hawaii, plays a greater role in seed dispersal - with most of the seeds found belonging to plant species also nonnative to the island.

"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," explains Corey Tarwater, assistant professor from University of Wyoming's Department of Zoology and Physiology, in a news release from UW.

She adds that aside from the large role these nonnative species play in species interactions, they also play a larger part in shaping the seed dispersal networks in the island compared to its native species. Dr. Tarwater also notes that the loss of these nonnative species would have an effect on species interactions as compared to the loss of a native species.

The University of Wyoming research team, led by Dr. Tarwater, published their findings, titled "Ecological correlates of species' roles in highly invaded seed dispersal networks," set to appear in the January 26, 2021 issue of the Proceedings of the National Academy of Sciences (PNAS).

Nonnative Species Helping in Seed Dispersal

According to Jeferson Vizentin-Bugoni, lead author and postdoc researcher at UW and the US Army Research Lab at the time of the study, their work is among the first studies that demonstrate nonnative species taking over crucial roles in an ecosystem's seed dispersal network.

He notes that this suggests the large effect of species extinctions and invasions on Oahu's ecosystems that most of seed dispersal activities have been conducted by nonnative birds, with the seeds belonging to nonnative plants.

Furthermore, Vizentin-Bugoni adds that these events creates what is known as an "ecological meltdown," which occurs in ecosystems where nonnative species form a mutualistic relationship that puts the system "into a vortex of continuous modification."

Seed dispersal is an important ecosystem function, directly affecting plant population dynamics and structure, which in turn affects biodiversity within the system - even spelling an ecosystem's chances of regeneration and recovery. Aside from natural factors such as wind and running water, seed dispersal is often facilitated by birds and animals.

New Agents in Hawaii's Ecological Recovery

According to the UW news release, Hawaii formerly enjoyed diverse ecological communities before turning into "the extinction and species invasion capital of the world." Additionally, experts estimate that for the last seven centuries, 77 bird species and subspecies across the Hawaiian archipelago have been extinct, equivalent to 15 percent of global bird extinctions.

"The Hawaiian Islands have experienced major changes in flora and fauna," noted Sam Case, co-author of the paper and a UW PhD student in Tarwater's team. He added that while the nature of seed dispersal in Hawaii before humans remains largely unknown, there are details showing that the historic seed dispersal agents have different traits compared to the introduced ones.

Tarwater also explains that invasive dispersers fill the role of extinct native dispersers, albeit incompletely. This leads to a number of native plants not being dispersed. On Oahu alone, 11.1 percent of bird species and up to 46.4 percent of plant species are native. Meanwhile, up to 93 percent of all seed dispersal events include nonnative species.

Vizentin-Bugoni calls nonnative birds as a "double-edged sword," explaining that while they serve as the only dispersal agent of native plants in the island, most of the seeds they carry are from nonnative plants.