Four introduced bird species show significant changes in size and beak morphology after a few decades on the island.
On the Hawaiian island of O’ahu you can find mostly non-native bird species. The native seed-dispersing birds have been replaced by many fruit-eating birds through introductions. These birds not only facilitate the invasion of exotic plants, they may also be the only dispersers of native plants. The non-native species have become integrated into the native ecosystem (see this Science paper for more details). But have these introduced bird species changed since they took over the island? A recent study in the journal Evolution tried to answer this question by studying four species that were introduced since the 1920s.
Jason Gleditsch and Jinelle Sperry focused on the following species: the Japanese White-eye (Zosterops japonicus), the Red-billed Leiothrix (Leiothrix lutea), the Red-wiskered Bulbul (Pycnonotus jocosus) and the Red-vented Bulbul (P. cafer). They compared museum specimens from these species’ native ranges with the living birds on O’ahu.
These comparisons revealed significant changes since the birds arrived on O’ahu. In general, the birds became shorter with more robust beaks. The changes in beak morphology are probably related to a more generalized diet, including many types of fruit.
These clear changes took place in just a few decades, showing that under particular circumstances evolution can proceed rapidly. It does, however, raise the question of whether these changes are due to natural selection or non-adaptive processes. The researchers used Lande’s F statistic, a measure from quantitative genetics to determine the role of genetic drift (i.e. a non-adaptive process) in evolutionary changes. The analyses suggested that the observed patterns are due to a mixture of adaptive and non-adaptive processes.
However, the test does not address the role of founder effects or population bottlenecks, when a population is reduced to a small number of individuals . These events can have profound effects on the consequent evolutionary trajectory of a species. For example, the Red-billed Leiothrix went to a population bottleneck due to competition with another introduced species, the White-rumped Shama (Copsychus malabaricus). This event probably selected for certain traits that provided the starting point for further evolutionary change.
The morphological changes in these non-native frugivores have important implications for the future of this ecosystem. The authors conclude that “because all of the native frugivores have been extirpated from O’ahu, the ability of the nonnative community of frugivores to effectively disperse fruiting plants is crucial for the long‐term stability and functioning of the novel forest ecosystems on the island.”
Gleditsch, J. M., & Sperry, J. H. (2019). Rapid morphological change of nonnative frugivores on the Hawaiian island of O’ahu. Evolution.