Grasshopper outbreak challenges conservation status of a small Hawaiian Island

Abstract

A tiny (63.1 ha) and uninhabited Nihoa Island within the Hawaiian Archipelago is situated 250 km NW of Kauai. It is a part of Papahānaumokuākea National Marine Monument established in 2006 and jointly administered by NOAA, USFWS, and the State of Hawaii (Department of Land and Natural Resources). The island’s known terrestrial biota include 26 vascular plant species, 27 bird species, and 243 arthropod species. Approximately half of the species are endemic to Nihoa or indigenous to Hawaii. Four plant species and two resident bird species are federally listed as threatened or endangered species. Gray bird grasshopper Schistocerca nitens has occurred on the main Hawaiian Islands since 1964 and was first reported from Nihoa in 1977. In 2002–2004, there was an outbreak of this grasshopper that aggravated the drought and denuded most of the island’s vegetation. Since then, grasshopper numbers crashed, most probably due to insufficient soil moisture for embryonic development. With subsequent rains, the island’s vegetation recovered. During the USFWS expedition to Nihoa in October 2006, grasshopper population assessments were undertaken. Based on 18, 300 × 2 m transect counts, the Nihoa grasshopper population was estimated at 19,430 ± 10,360 individuals. Laboratory rearing of S. nitens revealed that its development occurs without diapause. Potentially, the grasshopper can produce as many as four annual generations on Nihoa, although it is likely that only two generations occur. This article reviews the implications of fluctuations in S. nitens population dynamics for island flora and entomo- and avifauna, in particular, for the endangered endemics, the insectivorous Millerbirds. Potential threats to the island’s biota and challenges for conservation are discussed.