Negative Effects of an Introduced Bird Species on Growth and Survival in a Native Bird Community

Abstract

Exploitative competition is a major determinant of community structure in natural assemblages [1, 2], but, introduced species are rarely competitors that lead to extinction of native species [3, 4]. Here we document strong community-wide competition from the Japanese white-eye (Zosterops japonicus) on native Hawaiian passerine birds. Introduced in 1929 [5], white-eye successfully invaded old-growth forest and coexisted with eight native species [6], overlapping multiple foraging substrates with each but evidencing no agonistic interactions [7]. The endangered Hawaii akepa (Loxops coccineus coccineus) was viable during 1987-1999 but became nonviable during 2000-2006 in association with an abrupt increase in white-eyes [7]. We show that after 2000, juveniles of every native bird species measured had lower mass and shorter bills and tarsi. For most species, lower mass led to decreased juvenile survival, and shorter bills to decreased survival of second-year and older adults. Lower survival of smaller birds represents normalizing selection that is restoring previous size means to future generations [8]. Birds at a nearby site with fewer white-eyes had normal size. White-eye had less stunting of bills and did not suffer the survival consequences of native species. Exploitative competition for food between native birds and an introduced species requires intensive management to prevent further declines.