Responses to competition effects of two anuran tadpoles according to life-history traits Alex Richter-Boix, Gustavo A. Llorente and Albert Montori Richter-Boix, A., Llorente, G. A. and Montori, A. 2004. Responses to competition effects of two anuran tadpoles according to life-history traits. / Oikos 106: 39  /50. Experimental manipulations of the densities of two larval anurans, Pelodytes punctatus and Bufo bufo , showed that these species compete asymmetrically in semi-natural conditions. Growth, mass at metamorphosis, date of metamorphosis, and survival were used as measures of response to interspecific competition. A mechanistic approach was used to collect information on the behaviourof the two species in different conditions. The competitive superiority of Pelodytes at individual level was correlated with a larger body, faster growth rate, increased per capita competitive impact on conspecifics, and greater reduction in the availabilityof trophic and spatial resources. In the presence of Pelodytes, Bufo showed slower growth, smaller size at metamorphosis and reduced survival. In the interspecific treatments Bufo individuals modified their behaviour by increasing activity and use of the water column while Pelodytes did not change their foraging activityor space use in the aquaria. However, the presence of Bufo resulted in a reduced larval period and smaller size at metamorphosis. We hypothesise that the presence of Bufo act as a signal of environmental degradation and shorten the larval period of Pelodytes, a typical temporal pond breeder . The smaller Bufo tadpoles are potentially stronger competitors at population level because they use relatively large amounts of energy (greater densities and higher metabolic rates). Consequently, they use larger proportions of the shared resources than their larger competitor. A possible evolutionary response for larger tadpoles is the development of interference mechanisms or ‘‘escaping’’ from ephemeral ponds where mortality by drying represent a high risk. A. Richter-Boix, G. A. Llorente and A. Montori, Dept Biologia Animal, Facultat de Biologia, Univ. de Barcelona. Av. Diagonal 645, ES-08028 Barcelona, Spain (herpetologia@bio.ub.es). A wide spectrum of interactions between species occurs in natural systems. Competition and predation are among the most significant forces that structure com- munities (Sih et al. 1985, Morin 1999, Gurevitch et al. 2000). The perceived role of competition in community organization is considerable (Connell 1983, Schoener 1983, Goldberg and Barton 1992, Gurevitch et al. 1992), especially asymmetric intra- and interspecific competi- tion (Lawton and Hassell 1981, Persson 1985, Morin and Johnson 1988, Weiner 1990). Asymmetric competi- tion is of special interest because it does not necessarily lead to the evolution of weak interactions among species, as one might expect through divergent character dis- placement. Thus, natural selection generated by this kind of competition is likely to be a persistent and continuous phenomenon in communities. There are three kinds of studies on asymmetric competition, distinguished by the temporal scale used (Law et al. 1997). The microscopic scale deals with encounters between individuals that depend on beha- vioural mechanisms of competition between animals. The mesoscopic scale addresses population dynamics, and often involves the manipulation of densities of pairs of species in a reciprocal manner. Finally, the macro- Accepted 7 November 2003 Copyright # OIKOS 2004 ISSN 0030-1299 OIKOS 106: 39  /50, 2004 OIKOS 106:1 (2004) 39