Decline in native birds is linked to tiny mejiro

The Japanese white-eye, known as Zosterops japonicus in zoological circles and more commonly in Hawaii as the mejiro, is a small songbird whose delicate outward appearance suggests nothing in the way of ecological invasion or intense competition for survival.

Yet, as biologists at the University of Hawaii at Manoa have demonstrated in a recently published study, the introduction of the little bird to Hawaii forests had far-reaching implications for native species, demonstrating a complex process through which an outside species can overcome defensive forces of nature to create its own space within an existing system.

The findings by UH professors Leonard Freed and Rebecca Cann were published in the November edition of the scientific journal Ecosphere.

As Freed and Cann explain in "Diffuse Competition Can Be Reversed: A Case History With Birds in Hawaii," diffuse competition is a natural mechanism that prevents outside species from successfully establishing themselves in an ecological community on the basis of "niche overlap," or competition between species for necessary resources.

Some level of niche overlap already exists between native species. Thus, it is difficult for a new species to enter a community in which competition already exists for the resources it needs to survive and thrive.

However, the authors write, this principle implies that "diffuse competition can be reversed if some external force increases numbers of the introduced species while native community numbers remain unchanged."

The authors documented diffuse competition and reverse diffuse competition between the introduced Japa­nese white-eye (originally introduced to Hawaii in 1929 to control insects) and eight species of Hawaii forest birds in the Haka­lau Forest National Wildlife Refuge on Hawaii island.

As the authors noted, diffuse competition for sustenance was reversed by the creation of a restoration area adjacent to an old-growth forest where diffuse competition had occurred.

An influx of Japa­nese white-eyes in the old-growth forest occurred with no initial change in the number of native birds. This resulted in increased survival of white-eyes and adverse physical changes in the native bird population — including stunted growth, changes in molt and shorter bills — that ultimately contributed to higher mortality.

"This is the first case where an introduced bird has its greatest effect in a natural habitat from competitive superiority," the authors wrote in their conclusion. "Replacement of native species makes the bird a driver of ecological change."

In the article, Freed and Cann called on the Haka­lau Forest National Wildlife Refuge to revise its 15-year conservation plan to include management of white-eyes to prevent further declines in native bird populations.