On the Big Island of Hawaii, hundreds of thousands of ‘ohi’a trees, whose scarlet blooms color the archipelago and sustain many endemic nectar-feeding birds, are swiftly succumbing to a fungal disease. Now biologists have discovered that the fate of the ‘ohi’a (Metrosideros polymorpha) and the birds that depend on its flowers are even more uncertain due to climate change.
“This study showing how ‘ohi’a phenology is connected to climatic cycles and how important ‘ohi’a flower resources are to birds shows we need to start thinking of outside-the-box solutions to conserve not just ‘ohi’a but also native bird species,” said Jared Wolfe, first author on the paper published in Ecology.
A research ecologist with the U.S. Forest Service, Wolfe and his colleagues investigated data collected between 1976 and 1982 at a 40-acre site on Keauhou Ranch, just north of Hawaii Volcanoes National Park on the island of Hawaii.
“To start teasing apart associations between climate, plant phenology and bird communities,” Wolfe said, they analyzed precipitation and other climatic factors, flower and fruit numbers and the reproduction and abundance of endemic birds — including the nectar-eating ‘i’iwi (Vestiaria coccinea) and ‘apapane (Himatione sanguinea).
“Wet seasons would trigger flowering among ‘ohi’a, and the native nectarivores — ‘i’iwi and ‘apapane — would structure breeding around the availability of these flowers,” Wolfe said. “They would breed when there was a larger proportion of ‘ohi’a trees flowering.”
These results illuminate a later study conducted between 1998 and 2013, he said, when drought-like conditions coincided with a drop in ‘i’iwi populations from 2000 to 2011.
“Our findings are showing the mechanism behind how changes in climate can reduce food resources and potentially lead to population crashes in nectar-feeding native birds in Hawaii,” Wolfe said. “Climatic projections suggest an increased severity of dry seasons in the archipelago, so this might have further impacts on native species.”
Wolfe’s team plans to return to the study area to examine how the plant and bird communities have changed with the climate since researchers recorded the initial dataset.
“We’re unique in the sense that we’re measuring multiple trophic levels within the food web and seeing how climate can resonate through one trophic level at a higher trophic level,” Wolfe said. That’s important, he said, “to further our understanding of how climate change might be influencing not just the phenology of fruits, flowers and other food resources for wildlife, but also how wildlife subsequently responds to those changes.”
The research emphasizes long-term monitoring and encourages “studies measuring multiple aspects of the food web simultaneously across time to get a better handle on how climate can change communities as they respond to bottom-up processes,” Wolfe said.