The status of breeding seabirds and herons at Ashmore Reef, off the Kimberley coast, Australia

Headnote

Abstract

Ashmore Reef is situated on the edge of Sahul Shelf, off the Kimberley coast, Australia. Surveys in 2010 indicate the three small islands within Ashmore Reef support approximately 100,000 breeding seabirds of 16 species and four heron species on an annual basis. That such a diversity and abundance of tropical seabirds utilize a total land area of just 55 ha for breeding purposes is exceptional in an international context. In this paper we review population sizes of breeding seabirds and herons at Ashmore Reef. Bayesian change-point models applied to count data spanning a 60 year period demonstrate that populations of breeding seabirds have increased at this location. Large ground-nesting seabirds display positive step changes in population size since the late 1980s whilst populations of shrub-nesting congeners display similar step changes since the late 1990s. We discuss the potential reasons for these abrupt population increases.

Keywords: seabirds, herons, Ashmore Reef, Kimberley, population size

Introduction

Ashmore Reef lies within Australian waters at 12°20'S, 123°0'E, some 630 km north of Broome and 325 km off the Kimberley coast (Fig. 1). The Ashmore Reef National Nature Reserve serves to protect marine ecosystems with high biological diversity (Commonwealth of Australia 2002). Ashmore Reef contains four vegetated islands (West, Middle and East islands and a recently vegetated cay; total land area ~55 ha), and several additional sandbanks that rise above the high water mark (Fig. 2) (Russell et al. 1993).

The available terrestrial habitats at Ashmore Reef support a large population of seabirds, including some of the most important seabird rookeries on the Sahul Shelf (Serventy 1952, Milton 2005, Bellio et al. 2007). The diversity of seabirds across the three main islands is exceptional in an Australasian context. It has been speculated that this diversity may have arisen because of the isolated nature of this island group, opportunities for both ground and shrub-nesting species on the islands and the proximity of Ashmore Reef to the Indonesian Through Flow - a potentially nutrient rich current linking the Pacific and the Indian Oceans that is active in the vicinity (Commonwealth of Australia 2002; Milton 2005; Potemra 2005; Bellio et al. 2007).

Many seabirds at Ashmore Reef are breeding visitors and are thus present in large numbers on a seasonal basis....