Survival of embryos, tadpoles, and adults of the frogs Rana aurora aurora and Rana pretiosa pretiosa sympatric in southwestern British Columbia

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Abstract
Field survival of embryos, tadpoles, and adults of Rana aurora and Rana pretiosa sympatric in marshes near Vancouver, British Columbia, was studied.Embryonic survival for R. aurora was 90% and better, whereas for R. pretiosa it was about 70%. As well, in dry periods during breeding, R. pretiosa embryos face the danger of desiccation and extensive or complete mortality.Survival of tadpoles of both species in a pond breeding site studied was less than 1% in 1968. By the end of the season of transformation, there was about 5% survival of young-of-the-year frogs of both species from the initial number of eggs deposited in river breeding sites.After the end of the first full year of life (1969), there was a minimal survival of 2.5% for R. aurora and 3.5% for R. pretiosa from the eggs laid the year before. By the end of 1969, there was a 52% survival of R. aurora and 67% of R. pretiosa which metamorphosed in 1968.For R. pretiosa adults, there was a 64% survival between 1968 and 1969; males suffered higher mortality than females. The survival of adult R. aurora was 69% between 1968 and 1969.Factors influencing mortality are discussed, and the conclusion is reached that predation (and chance climatic events for R. pretiosa embryos) on all life-history forms is the strongest factor limiting frog population numbers.

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