Habitat use and conservation implications for Akikiki (Oreomystis bairdi) and Akekee (Loxops caeruleirostris), two endangered Hawaiian honeycreepers

Abstract

Limited resources for biodiversity conservation warrant strategic science-based recovery efforts, particularly on islands, which are global hotspots of both endemism and extinction. The diverse assemblages and the extreme isolation of the islands of the Hawaiian archipelago make them a unique laboratory for development of a coherent strategy for recovery of rare species and for large-scale systems conservation efforts. The Akikiki (Oreomystis bairdi) and the Akekee (Loxops caeruleirostris) are critically endangered honeycreepers endemic to the Hawaiian island of Kauai. Recent declines and range contraction spurred this study, the first systematic scientific investigation of these little known species. I conducted occupancy sampling for Akikiki and Akekee and vegetation surveys at plots within five study areas on the Alakai plateau of Kauai to assess range-wide occupancy and habitat use. Occupancy rates for both species increased from west to east along the plateau (Akikiki: Ψ = 0.02 ± 0.07 to 0.55 ± 0.21 Akekee: Ψ = 0.03 ± 0.10 to 0.53 ± 0.33), but were low throughout the ranges of both species. Canopy height was correlated with occupancy for both species, which suggests the damage done by hurricanes in 1982 and 1992 may be restricting these birds to the most intact forest remaining. Vegetation surveys revealed several key differences in forest composition and structure between areas, indicative of broader changes occurring across the plateau. Invasive plants such as Himalayan ginger (Hedychium gardnerianium) were dominant in the western portion of the plateau, where there was a corresponding decline in native plant cover. Conversely, ground disturbance by feral ungulates was higher in more eastern native-dominated plots. These results highlight the need to control ungulates and limit habitat degradation in the regions with the highest occupancy of Akikiki and Akekee. Without significant investment to address these threats and protect and restore suitable habitat for these species, it is unclear how long these birds will persist.