Our Livinsi Kc.ujiiiics — Nun-nalive Species 



■155 



native species and those that lasted the longest 

 in the wild all exhibited extreme sizes (i.e.. 

 larger or smaller than those most affected) or 

 some other trait that has minimized their \ul- 

 nerability to snake predation. 



Examples of these traits include large size: 

 Mariana flying fox {Pteroptis iiuiiiannus). 

 Marianas crow iCorvus kiiluiiyi). and Indian 

 monitor lizard (Varaniis indicus): urban 

 dwellmg: Micronesian starling (Aplonis opacci). 

 mourning gecko {Lepidodactyliis lugubris). and 

 stump-toed gecko (Gehyni niiililata): cavity 

 nesting: Micronesian starling and Micronesian 

 kingfisher {Halcyon cinmimomimiY. cave ceil- 

 ing roosting: gray swiftlet {Aewdrainiis vaniko- 

 rensis); and extremely small size: mourning 

 gecko and Marianas blue-tailed skink (Eiiioici 

 caendeovauda). All surviving endotherm popu- 

 lations (birds and mammals) consist of fewer 

 than 1.000 individuals, and long-term popula- 

 tion \iabilities are in doubt for most of these 

 groups on Guam. 



Small lizards are much more numerous and 

 have better long-term prospects even though 

 evidence exists of localized extinctions caused 

 by temporary surges in snake populations. The 

 big tree gecko {Gehyni occaiiicci) has virtually 

 disappeared since 1985, but its smaller con- 

 gener (species in the same genus), the stump- 

 toed gecko, persists in forested habitats in low 

 numbers (Rodda et al. 1991 ). Some small intro- 

 duced lizard species (mourning gecko, common 

 house gecko. Hemidactyhts fremitus, and brown 

 four- fingered skink, Ccuiia cf. fiisca) have 

 expanded into new habitats in the absence of 

 other species; they therefore maintain larger 

 population levels on Guam even though they 

 experience heavy predation by snakes. 



The relative abundance of the Marianas blue- 

 tailed skink has dropped markedly as the brown 

 four-fingered skink increased after its arrival in 

 Guam in the early 1950"s (Fig. 2). Effects of pre- 

 dation by the snake and interactions between 

 introduced lizards are evident in the relative 

 abundances of lizard families, with the primari- 

 ly arboreal gekkonids declining while the pri- 

 marily terrestrial and more predation-resistant 

 skinks have increased. Even introduced rodents 

 and shrews show declines due to predation by 

 snakes; trapping success for rodents and shrews 

 was significantly reduced in 1984-85 compared 

 to that of 1962-64 (Savidge 1987). 



Risks of Dispersal from Guam 



The many brown tree snakes on Guam make 

 it probable that they may disperse as passive 

 stowaways in ship and air traffic to other islands 

 and the U.S. mainland (Fritts 1987, 1988; 

 McCoid and Stinson 1991). To date, stowaway 

 brown tree snakes have airived in the northern 



Marianas Islands (Saipan, Rota, and Tinian); 

 Marshall Islands (Kvvajalein Atoll); Cocos 

 Island near Guam; Okinawa; Diego Garcia in 

 the Indian Ocean; Oahu Island. Hawaii; and 

 Coipus Chrisli. Texas (Fritts 1988; unpublished 

 manuscript). Verified and probable sightings of 

 brown tree snakes span 1949-94 and show that 

 dispersal of the brown tree snake is not uncom- 

 mon. The apparent surge in the I990"s probably 

 reflects better reporting of stowaway incidents 

 rather than increased dispersal. 



Risks of Damages from Further 

 Colonizations 



The islands adjacent to Guam are the north- 

 ern Marianas, which have vertebrate faunas that 

 are similar to Guam's, including some of the 

 same introduced species. Like Guam, the north- 

 em Marianas have no native snakes. Thus, prey 

 bases similar to those on Guam and capable of 

 supporting high population levels of brown tree 

 snakes exist in the northern Marianas, and 

 species losses can be anticipated if the snake 

 becomes established. For example, of 27 native 

 resident bird species on the main islands of the 

 northern Marianas (Saipan, Tinian. and Rota). 

 20 are shared with the original fauna of Guam 

 and an additional 7 species are closely related to 

 birds known from Guam. Guam and the north- 

 em Marianas also share five introduced bird 

 species (Engbring et al. 1986). Six species of 

 birds are federally listed as endangered or 

 threatened in the northern Marianas, and all of 

 these are conspecific (of or relating to the same 

 species) or closely related to birds that have dis- 

 appeared from Guam or declined significantly 

 there (Engbring and Ramsey 1984; U.S. 

 Department of the Interior 1990). Of 20 species 

 of terrestrial amphibians and reptiles presently 

 or fomierly known from Guam and Cocos 

 Island, 15 are shared with the northern 

 Marianas, 8 native and 7 introduced (Rodda et 

 al. 1991). Thus, the northern Marianas not only 

 share the ecological vulnerabilities that led to 

 mass extirpations on Guam, but also the bulk of 

 the remaining habitat for Marianas' native 

 species is on islands that have received stow- 

 away snakes from Guam. 



Hawaii suffered major losses in its verte- 

 brate fauna after the arri\'al of the Polynesians 

 and again after contact with Europeans. The 

 state originally had 59 passerine bird species, 

 but only 38 survived into historical times. Fifty 

 species of passerines have been introduced in 

 Hawaii, and those birds make up most of the 

 land birds present today. At least 30 species of 

 birds native to Hawaii are federally listed as 

 threatened or endangered. One bird species 

 native to Guam, the gray swiftlet. is established 

 on Oahu (Moulton and Pimm 1986). Of the 14 



Birds 



Mammals 



Reptiles 



■ Due to snake 



^ Safe O Endangered 



I Extirpated 



Fig. 1. Status in 1993 of Guam's 

 native forest vertebrates (those pre- 

 sent in 1950) with estimates of the 

 degree to which dechne was due to 

 the introduction of the brown tree 

 snake. 



1940 50 60 70 80 90 



Fig. 2. Changes in the proportions 

 of specimens of two common 

 skinks in museum collections from 

 Guam in four samples spanning 

 1945-90: Marianas blue-tailed 

 skink {Emoki caendeocauda) and 

 brown four-fingered skink (Carlia 

 cf. fiisca). 



