Interaction Between the Effects of pH and Density on Growth and Development in Rana Temporaria L. Tadpoles

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The common frog (Rana temporaria L.) occurs in areas where waters are naturally acid and where breeding sites are susceptible to acidification from anthropogenic sources. Acid conditions have been shown to cause embryo mortality. They can also delay metamorphosis and reduce size at metamorphosis among individual tadpoles. Time of metamorphosis and size at metamorphosis may also be influenced by crowding effects. It is conceivable that reductions in tadpole density arising from embryo mortality will offset depressant effects of acid conditions on individual tadpoles. Furthermore, interactions among tadpoles may be modified by individuals' responses to acid conditions. To investigate such possible effects, common frog tadpoles were raised to metamorphosis in the laboratory at three densities (4, 8 and 16 tadpoles litre-1) and two levels of pH (nominally pH 4 and pH 7) on a limiting ration. Overall, the effects of density on size at metamorphosis and time of metamorphosis far outweighed those of low pH. The depressant effect of low pH on mean development rate and mean size at metamorphosis decreased with increasing tadpole density. Low pH accentuated hierarchical effects within density replicates, and dominant tadpoles suffered little or no net retardation of growth or development due to low pH. Tadpoles which grew and developed rapidly at low pH suffered debilitating limb deformities. The implications of these interactive effects of density and pH are discussed in the context of freshwater acidification and its possible effects on frog populations. The response of individuals in a group is likely to be more important than the mean response of the group. Key-words: Amphibian, frog, Rana temporaria, tadpole, pH, density, interaction, growth, metamorphosis, acidification, deformity