Population structure and dynamics of the western spotted frog, Rana p. pretiosa Baird & Girard, in Yellowstone Park, Wyoming

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1) The growth rate of the Lake Lodge frogs is very low compared to rates revealed by studies of other anurans. The pre-reproductive life span is almost 4 yrs in males, 5-6 yrs in females. The annual growth increment decreases each year so that the average growth during the first full year after transformation is about 10 mm, during the second year about 7 mm, during the third about 6 mm, and during the fourth about 3 mm. The rate of growth of older frogs is very low-probably the annual increments are on the order of 1 or 2 mm or less. There is seasonal variation in the rate of growth and the maximal growth occurs during a 2-3 week period in July. The population is sexually dimorphic with respect to body length; females are larger. 2) The Lake Lodge population inhabits a marginal area and replacement is spotty, even virtually nil in some years because of almost complete larval mortality. The age composition of the population varies from year to year and the size of the various year-classes reflects varying degrees of reproductive success in past seasons. The mortality rate is approximately the same for all year-classes (about .39 ± .10 over the period of time studied) and survivorship curves for males and females are, insofar as can be determined, Type II or diagonal. The sex-ratio is about 1:1 through the 5th year after transformation but there are approximately 3.5 times as many older females as older males in the population (or there is a persistent sampling bias creating an illusion of such a situation). The inconsistency between the observed numbers of old males and females and the apparent absence of differential mortality is not explained, though a hypothesis postulating increased mortality in males at the end of 5 yrs is advanced. Predation pressure is very light, at least insofar as adult frogs are concerned. Marsh hawks, occasional vagrant garter snakes, and possibly a few mink are the only members of the community which emerge as likely predators. Tadpoles are eaten by dytiscid larvae and several passerines (robin, grey jay) but the conditions which promote predation by birds also result in tremendous larval mortality, owing to desiccation, so that avian predation on tadpoles probably does not affect population size markedly. The size of the Lake Lodge population is estimated at around 1587 in 1953. The mortality-emigration (.39 ± .10) of about 600 between 1953 and 1954 was approximately offset by recruitment of young (about 555) from the 1953 breeding effort. The mortality between 1954 and 1955 is estimated as approximating that of 1953-54 (estimates of .47 ± .18 and .30 by two different methods). The 1954 reproductive effort contributed from 35 to 50 young so the population is estimated to have declined to about 1000 individuals in 1955. 3) The population exhibits marked changes in dispersion during the summer and these changes are associated with migrations of the population to upland situations in May and a return to permanent water in July. All members of the population participate in these movements and there are no differences between the sexes or age-groups with respect to rate or magnitude of movements. However there are differences in the types of upland habitats utilized. Sexually mature frogs occupy ponds where oviposition is to occur while younger frogs occupy other habitats, e.g. small puddles in the meadows and intermittent stream courses. Emigration to adjacent stream systems occurred; a maximal movement of 4200 ft was recorded. The location of physiographic features in the area determines the pattern of movements and influences the size of activity ranges. It was found that frogs occupying three different sections of the study area exhibited activity ranges differing significantly in extent.

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Reference Code: A60TUR01IDUS

Full Citation: Turner, F. B. 1960. Population structure and dynamics of the western spotted frog, Rana p. pretiosa Baird & Girard, in Yellowstone Park, Wyoming. Ecological Monographs 30(3): 251-278.

Location: ANIMAL EF: RANA PRETIOSA