Diffuse competition can be reversed: a case history with birds in Hawaii

Abstract

Diffuse competition, based on niche overlap of an invader with most native species in a community, is considered a mechanism preventing its successful establishment in the community. However, native species must themselves have extensive niche overlap, implying that diffuse competition can be reversed if some external force increases numbers of the introduced species while native community numbers remain unchanged. Reverse diffuse competition, defined as an invader that outcompetes most of the native community, can then lower numbers of native species. We illustrate these principles by documenting diffuse competition between the introduced Japanese White‐eye (Zosterops japonicus) and eight species of Hawaiian forest birds at Hakalau Forest National Wildlife Refuge followed by reverse diffuse competition. Competition was based on extensive overlap of multiple foraging substrates with native species. The external force which reversed diffuse competition was a restoration area contiguous with much of the old‐growth forest where diffuse competition previously occurred. Propagule pressure from that area increased white‐eye numbers in the old‐growth forest, without initial change in numbers of native birds, resulting in greater than sixfold increase in adult white‐eyes on our study site. This increase resulted in stunted skeletal growth of young birds, and diverse changes in molt of both young and adults of native birds and white‐eyes. While native birds declined from stunted growth, white‐eyes increased adult survival. Changes in condition of native birds in time and space matched associated changes in white‐eye density. Normalizing natural selection of lower mass and shorter bills resulted in loss of almost half of native birds in a 3373 ha area of the refuge, adjacent to the restoration area, and 10% of native birds in a lower 1998 ha area. Competition with female native birds was initially stronger than with males as indicated by more male‐biased adult sex ratios in five species. Structure of the forest, small size of arthropods available, greater use of the understory, and lower minimum air temperature resulted in successful white‐eye competition with the entire native community. Reverse diffuse competition is a process in community ecology and a way an additional species can be packed into a community.