Abstract: Palmyra Atoll, in the Line Islands of the equatorial Pacific, supports one of the largest remaining native stands of Pisonia grandis forest in the tropical Pacific Ocean. In 2003, we surveyed terrestrial arthropods to document extant native and introduced species richness, compare these lists with historical records, and assess potential threats to native species and ecosystem integrity. In total, 115 arthropod taxa were collected, bringing the total number of taxa recorded since 1913 to 162. Few native species were collected; most taxa were accidental introductions also recorded from the Hawaiian Islands, the presumed main source of introductions to Palmyra. The overlap with previous historical surveys in 1913 and 1948 was low (<40%), and new species continue to establish, with one species of whitefly reaching pest status between 2003 and 2005. We observed numerous dead or dying large Pisonia grandis, and the green scale Pulvinaria urbicola (Coccidae) was particularly abundant on trees of poor health. Abundant introduced ants, particularly Pheidole megacephala, tended this and other hemipterans feeding on both native and introduced plants. We hypothesize that the Pheidole-Pulvinaria facultative mutualism is causing the decline of Pisonia grandis. Because of the unique properties of Pisonia grandis forest on oceanic atolls, its importance for nesting seabirds, and its alarming global decline, immediate conservation efforts should be directed at controlling introduced Hemiptera and disrupting their mutualisms with nonnative ants on Palmyra Atoll.

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AS BIOGEOGRAPHIC BARRIERS break down in the face of increasing global commerce and travel, introduced species have become a dominant component of global environmental change (Vitousek et al. 1999, Mack et al. 2000). Oceanic islands are among the systems most vulnerable to biological invasion and homogenization (D'Antonio and Dudley 1995, Cowie 2001). Their small size allows coexistence of fewer total species in small populations relative to continents, and their native biota evolved in geographic isolation from many functional forms of predation and competition (Carlquist 1974, Wilson 1996, Whittaker 1998). Although the replacement of endemic species by introduced species may actually increase local species and functional diversity on islands (Sax and Gaines 2003), regional homogenization of biotic communities causes decline in total global diversity (Samways 1999, Olden et al. 2004).

Palmyra is a remote atoll in the northern Line Islands (Figure 1), approximately 2,000 km south-southwest of the Hawaiian Archipelago (5° 53' 6" N, 162° 6' 11" W). The atoll consists of roughly 50 small, low islets, with a total land area of less than 5 [km.sup.2] and a maximum elevation of 2 m, all surrounded by a barrier reef. With an average annual rainfall of more than 4 m, Palmyra is a wet atoll within the Inter-tropical Convergence Zone (Mueller-Dombois and Fosberg 1998). This high rainfall, coupled with huge nutrient influx in the form of guano from thousands of resident seabirds and migratory shorebirds, supports thickly vegetated rain forest that is unique even in comparison with nearby, more arid islands such as Kiritimati (Wester 1985). The atoll is home to a relatively intact and diverse marine ecosystem; 29 species of...