Natural Mortality of Tadpoles in a Population of Rana Aurora

Abstract

The Survivorship, distribution, growth rates, and natural predation rates on the population of Rana aurora tadpoles in Marion Lake, British Columbia, were studied during the summers of 1969 and 1970. Natural survivorship appeared to have two phases: a rapid decline in numbers during the first 4 weeks after hatching was followed by a less rapid decline, until approximately 5% of the population remained at metamorphosis, after 11—14 weeks. The size attained by tadpoles living at any depth in the lake was strongly correlated with the accumulated degree—days since hatching (r = .93). The regression equation (degree—days vs size) obtained from the data on animals in the lake could accurately predict the growth rate of tadpoles kept in the laboratory at maximum food supply. It was concluded that animals in the lake grew at the maximum rate permitted by the temperature. Alterations of the density of tadpoles and predatory salamanders were performed in enclosures in the lake. The percentage of tadpoles surviving in the enclosures containing no salamanders was twice as high as it was in the natural population. The percentage of tadpoles surviving in the experimental enclosures was independent of density, at least up to 75 tadpoles/m2. Except at the lowest density (5/m2), no significant effects were noted at any density, on the rate of growth or time to metamorphosis. These experimental observations support the notion that food is not limiting to the tadpoles at their normal densities. Most of the mortality of tadpoles observed in the natural population was attributed to predation because: (1) Tadpoles kept in the laboratory on starvation diets survived for many weeks, even though they did not grow. (2) Tadpoles living at densities up to 100 times normal could survive and grow to metamorphosis in the absence of predators. (3) Predation rates observed in the field, and simulated on the basis of laboratory experiments, were sufficient to account for much of the mortality observed in the natural population. Laboratory studies of salamanders preying on tadpoles in multi—prey systems showed that the number of tadpoles eaten was proportional to the density of tadpoles present except at very low density, where fewer tadpoles than expected were eaten. The tadpoles also had a refuge from some predators after they had grown to a certain size. Salamanders were observed to move into areas of high tadpole density, thereby increasing locally the intensity of predation. It was concluded from the laboratory and field observations that mortality of tadpoles is density—dependent. However, the limiting factors for the Rana aurora population are unknown, since the frogs were not studied after they left the lake. The numbers of eggs laid in the 2 years of the study were very similar, but no social mechanisms for controlling the breeding population were observed.