Modeling two decades of elevational change in endemic bird densities in Hawaiʻi’s Kaʻū Rainforest
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Protected areas are vital to the recovery of endangered species. Of the 24 remaining endemic passerines in Hawaiʻi, 16 species are endangered or critically endangered. Yet protected areas in the archipelago are changing as a result of climate change and biological invasions. For example, the non-native southern house mosquito ( Culex quinquefasciatus ), the primary vector of avian malaria ( Plasmodium relictum ), has been encroaching upward as higher elevations warm. How the ranges of endemic birds have also shifted their range upward in the Kaʻū Rainforest, the largest native forest on the Island of Hawaiʻi, is not well understood. We used hierarchical distance sampling to characterize how population density has changed from 2002 to 2024 for eight endemic bird species in the Kaʻū Rainforest. For five species, the most parsimonious model included an interaction between year and a quadratic elevation effect. Species that were most common at lower elevations in 2002, e.g., ʻApapane ( Himatione sanguinea ), tended to be most common at mid- elevations by 2024. Species that were already most common at higher elevations, such as ʻIʻiwi ( Drepanis coccinea ) and Hawaiʻi ʻĀkepa ( Loxops coccineus ), tended to decline in density. For example, ʻIʻiwi density at 1,530 m declined from 3.31 birds per ha (95% confidence interval [CI]: 2.67-4.11) to 1.34 birds per ha (95% CI: 1.05-1.71), and Hawaiʻi ʻĀkepa were completely extirpated from elevations below 1,530 m by 2024. Our results demonstrate how the elevational ranges of endemic species have shifted in response to climate change, biological invasions, and habitat degradation. Translating these results into conservation measures may require a more thorough investigation of the causal mechanisms of these range shifts with finer scale habitat data, under the same hierarchical modeling framework.
LAY SUMMARY
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Protected areas such as National Parks, private lands managed for conservation, and state managed reserves have been essential to conserving Hawaiʻi’s native forest birds.
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Climate change and the cold-intolerant, invasive southern house mosquito, the primary vector of avian malaria, both threaten protected areas in Hawaiʻi. As the climate warms, avian malaria has been encroaching upwards to higher elevations.
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We found evidence that five of eight native forest bird species were shifting upward in elevation, perhaps in response to these dual threats, in Kaʻū Rainforest, the largest native forest in the Hawaiian Islands. The endangered Hawaiʻi ʻĀkepa has been extirpated from elevations below 1530m. Further, the vulnerable ‘I‘iwi has been extirpated from elevations below 1200m, and their density has declined by half at 1530m.
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Although more research could enhance understanding the mechanisms of this shift, our study supports wildlife managers in decisions related reducing the impact of mosquitos and climate change on native forest bird species within Hawaiʻi’s protected areas.
