Cannibalism and predation by western toad (Bufo boreas boreas) larvae in Oregon, USA

Larval amphibians have been widely used as model organisms in studies of community ecology of freshwater systems (Morin 1983, Alford 1999). Much of this work has assumed that trophic effects of larval anurans are focused on periphyton and planktonic algae (Dickman 1968, Seale 1980, Duellman and Trueb 1986), a view that has recently been questioned. Recent experiments suggest that anuran larvae can occupy broader trophic roles than previously believed and may function as important predators in some pond communities (Petranka et al. 1994, Petranka and Kennedy 1999). Reports of larval cannibalism and predation among anurans have been restricted to the genera Rana (family Ranidae) and Scaphiophus (Pelobatidae) (Bleakney 1958, Bragg 1964, Petranka et al. 1994, Alford 1999, Petranka and Kennedy 1999). Several members of the genus Bufo (Bufonidae) are known to cannibalize eggs (Bufo calamita, Banks and Beebee 1987; B. arenarum , Crump 1992; B. terrestris, Babbitt 1995) or to consume invertebrate or larval fish prey (DiazPaniagua 1989, Nguenga et al. 2000). However, published reports of larval cannibalism and predation on amphibian prey in the family Bufonidae are limited to a single laboratory study (B. bufo, Nagai et al. 1971). Field observations of cannibalism or interspecific predation by larval bufonids have not been reported. Here we report field observations of cannibalism by larval western toads (Bufo boreas boreas), as well as predation by B. b. boreas larvae on larval and metamorphosing Pacific chorus frogs (Pseudacris regilla) and Cascades frogs (Rana cascadae) in the Cascade Mountains of Oregon, USA. On 12 July 2001, DJJ and CJR observed high densities of anuran larvae of 3 species (B. b. boreas, P. regilla, and R. cascadae) at West Snow Lake (43°02.05'N, 122°29.13'E; elev. 1460 m). The lake had dried down to a pool ca. 40 X 20 m with a maximum depth of ca. 1.5 m. Our visual survey of the site yielded the following abundance estimates: 9,000 B. b. boreas larvae (mean snout-vent length (SVL) = 12.0 mm, n = 10); 34,000 P. regilla larvae, metamorphs, and juveniles (mean SVL = 10.0 mm, n = 10); and 26,000 R. cascadae larvae, metamorphs, and juveniles (mean SVL 21.0 mm, n = 10). Most feeding B. b. boreas tadpoles were engaged in typical benthic scouring, with lesser numbers grazing the underside of the surface film. During our survey we observed 5 B. b. boreas tadpoles feeding on live, apparently uninjured conspecifics in shallow (25 cm) water ca. 45 cm from shore. We observed 2 additional B. b. boreas larvae (SVL ca. 12 mm) pursue and attack a conspecific that was swimming weakly in 20-cm-deep water. Within 30 seconds, another ca. 10 B. b. boreas larvae were feeding on the injured larva. Viscera and recently erupted rear limbs were consumed first, and the entire larva was consumed within ca. 90 seconds. We observed 12 similar cannibalistic aggregations of B. b. boreas larvae in the pool, all of which were located 35-45 cm from shore in water 20-30 cm deep. We (DJJ and CJR) also observed B. b. boreas larvae prey upon 1 live R. cascadae larva (SVL ca. 22 mm) and 1 live R. cascadae metamorph (SVL ca. 18 mm). The R. cascadae larva and metamorph were both initially attacked by 1 B. b. boreas larva. In both cases we observed small aggregations of B. b. boreas (5-7 larvae)