High Islands Versus Low Islands: A Comparison of Fish Faunal Composition of the Palau Islands

Abstract

High islands, with potentially greater habitat diversity, are expected to have greater species richness and diversity compared to low islands, typically atolls and coral islands of lower habitat diversity, within the same geographical area. Patterns of species similarity, richness, and diversity were compared among coral reef fishes between the low island of the Southwest Palau Islands (SWPI), and the low and high islands of the Main Palauan Archipelago (MPA). Data from diurnal visual transects accounted for approximately 64% and 69% of the shorefish faunas known from the SWPI and MPA, respectively. Two distinct fish faunas were representative of low and high islands. The first was confined to the coral islands of the SWPI. The second was partitioned into both low and high islands of the MPA, and Helen Reef, a large atoll in the SWPI. The second type was clustered into atolls, low islands with atoll-like barrier reef systems, a coral island, and three high island systems, one with an extensive barrier reef system. Contrary to the prediction that high islands, with relatively greater habitat diversity, would have greater species richness and diversity, species richness and diversity were greatest at Kossol, a large atoll-like ‘low island’ locality at the northern end of a high island in the MPA, followed by two atolls, Kayangel (MPA, north of Kossol) and Helen Reef. In contrast, species richness and diversity were lower at high island localities and lowest at small coral islands. These results suggest that habitat diversity for reef fishes increases as a function of increasing area regardless of whether the locality is a high or low island.