454 



Non-notive Spt'tics — Oitr Liviiti; Resources 



For further information: 



Philip C. Rosen 



University of Arizona 



Department of Ecology and 



Evolutionary Biology 



Tucson. AZ 85721 



Hayes, M.P., and M.R. Jennings. 1986. Decline of ranid 

 frog species in western North America: are hullfrog.s 

 (Riimi cauwhciiiiid) responsible ,' .loumal of Hcrpetology 



2n:490-.';(;». 



Hayes. M.P.. and MR. Jennings. 1988. Habitat correlates of 

 distnbution of the California red-legged frog {Rana auro- 

 ra drayumii) and the foothill yellow-legged frog {Rana 

 boylei): implications for management. Pages 144-138 in 

 R.C. Szaro. K.E. Severson. and D.R. Patton, eds. 

 Management of amphibians, reptiles, and small mam- 

 mals in Noilh Amenca. U.S. Forest Service Gen. Tech. 

 Rep. RM-166, Fort Collins, CO. 



Jennings, MR., and M.P. Hayes. 1994. Decline of native 

 ranids in the desert southwest. In PR. Brown and J.W. 

 Wright, eds. Proceedings of the Conference on the 

 Herpetology of the North American Deserts. 

 Southwestern Association of Herpetologists Special 

 Publ. 5. Los Angeles. CA. In press. 



Leonard, W.P, H.A. Brown, L.L.C. Jones, K.R. McAllister, 

 and R.M. Stonn. 199.^. Amphibians of Washington and 

 Oregon. Seattle Audubon Society, Seattle, WA. 168 pp. 



Minckley, W.L., and J.E. Deacon, eds. 1991. Battle agamst 

 extinction: native fish management in the American 

 West. University of Arizona Press, Tucson. 517 pp. 



Platz, J.E.. R.W. Clarkson, J.C. Rorabaugh, and D.M. Hillis. 

 1990. Rana herlantlieri: recently introduced populations 

 in Arizona and southeastern California, Copeia 

 1990:324-.\^.1. 



Schwalbe, C.R., and PC. Rosen. 1988, Preliminary report 

 on effects of bullfrogs on wetland herpetofauna in south- 

 eastern Arizona. Pages 166-173 in R.C. Szaro, K.E. 

 Severson, and D.R. Patton eds. Management of amphib- 

 ians, reptiles, and small mammals in North America. 

 U.S. Forest Service Gen. Tech. Rep, RM-166, Fort 

 Collins, CO. 



Sjogren. P. 1991. Extinction and isolation gradients in 

 metapopulations: the case of the pool frog {Rana lesson- 

 ae). Pages 133-147 in M.E. Gilpin and L Hanski, eds. 

 Metapopulation dynamics. Academic Press, London, 

 UK, 



Vial. J.L., and L. Saylor 1993. The status of amphibian pop- 

 ulations: a compilation and analysis. International L'nion 

 for the Conservation of Nature and Natural Resources, 

 Species Survival Commission, Declining Amphibian 

 Populations Task Force Working Document I, 98 pp, 



Vitt, L.J., and R.D. Ohmart. 1978. Herpetofauna of the 

 lower Colorado River: Davis Dam to the Mexican border 

 Proceedings of the Western Foundation of Vertebrate 

 Zoology 21^3-72. 



Invasions of 

 the Brown 

 Tree Snake 



by 



T.H. Frim 



G.H. Rodda 



National Biological Service 





^§.^. 



Brown tree snake [boii^a irregu- 

 laris). 



Counesy G,H RoMa 



A found 1950, populations of the biovvn tree 

 snake (Boii^a invi;uluris) were introduced 

 on Guam, a previously snake-tree island. This 

 introduction was the result of post-World War II 

 tiaffic carrying military materials from the 

 South Pacific region (Savidge 1987; Rodda et 

 al. 1992). It resulted in major ecological 

 changes and the loss of several bird and lizard 

 species from the island starting in the I970"s 

 and e.xtending to the late 1980"s, The severity of 

 ecological damages resulting from this intro- 

 duced snake may have been increased by the 

 presence of other nonindigenous species, which 

 served as alternative prey as native species 

 declined. 



The brown tree snake dispersed throughout 

 Guam in the 1950"s, 196(rs. and 1970"s, reach- 

 ing high populations that resulted in devastating 

 levels of predation on most native and intro- 

 duced vertebrates (Savidge 1987: Engbring and 

 Fritts 1988; Rodda et al, 1992). At the peak of 

 the snake's in'uption on Guam, densities proba- 

 bly exceeded 100 snakes/lia (40 snakes/acre). 

 but following depletion of many of Guam's 

 birds and mammals, snake densities appear to 

 have fallen to 20-50 snakes/ha (8-20 

 snakes/acre; Rodda et al. 1992). 



In the face of the loss of native forest birds 

 and drastic reductions in other bird, mammal, 

 and reptile species, the snake subsisted on 

 smaller lizard prey and on introduced species, 

 including lizards (Hemidactybts fremitus and 

 Carlia cf. fusca), domestic poultry and cage 

 birds, rodents (Rattus spp. and Mus miisculiis). 

 house shrews {Suncits iiiurinus). Eurasian tree 

 sparrows {Passer luontaniis). and Javanese tur- 

 tle doves iStreptopelia bitorquata). Thus, the 

 reduction of snake densities that might have 



been expected after the loss of native prey 

 species was limited because the snake could 

 subsist on alternative introduced prey. 



Species Lost from Guam 



Since the airival of the snake on Guam, the 

 island has lost most of its indigenous forest ver- 

 tebrates (Fig, 1). Too few baseline data are 

 available to unequivocally determine the degree 

 to which the snake is responsible for these loss- 

 es, but several kinds of evidence create a strong 

 case for the snake's role in the e.xtiipation of 

 many bird species (Savidge 1987, 1988; Corny 

 1988; Engbring and Fritts 1988) and several 

 lizard species (Rodda et al. 1991 ), Additionally, 

 some evidence exists that the snake played a 

 role in the disappearance and decline of Guam's 

 native mammals, three bat species (Wiles 

 1987). but no direct information is available for 

 the two bat species that disappeared before 

 1980, The evidence clearly shows, however, 

 that Guam has experienced a remarkably com- 

 plete loss of its vertebrate fauna. 



Even with all of the vertebrates at risk from 

 the snake, the pattern of species' losses has fol- 

 lowed a size gradient that is consistent with the 

 snake's dietary habits (Engbring and Fritts 

 1988; Fritts 1988). Small birds, small mam- 

 mals, and medium-sized lizards disappeared 

 first and seem to have been most heavily affect- 

 ed. Contrary to what might have been expected, 

 the most abundant bird species were affected 

 first. We cannot determine if the abundance of 

 the prey led to more effective search images for 

 the snakes or if the ecological characteristics of 

 the species and the habitats occupied con- 

 tributed to this prey difference. The surviving 



