The ecology of coexistence in two closely related species of frogs (rana)

The red-legged frog, Rana aurora aurora and the western spotted frog, Rana pretiosa pretiosa, were found coexisting in southwestern British Columbia. The locality - the upper Little Campbell River near White Rock - is the only area in the Pacific Northwest in which the two species are known to be sympatric. Little was known about the biology of the frogs, which are closely related and resemble each other in habits; finding a sympatric locality provided a unique opportunity to study how they manage to coexist and avoid competitive exclusion. The study involved finding the ecological requirements during the nonbreeding season and finding the reproductive isolating mechanisms. The study revealed that R. pretiosa is much more aquatic than R. aurora. The affinity of R. aurora for land and R. pretiosa for water is the basic ecological difference permitting them to coexist. The morphology, ecology, and behavior of the frogs are adapted to their preference for land or water. R. aurora has relatively longer hind limbs than R. pretiosa; but the feet of R. pretiosa are more extensively webbed. The eyes of R. aurora face laterally; those of R. pretiosa face upwards. The skin of R. pretiosa is covered with a thick mucous coating; that of R. aurora is smooth. There is much overlap in the diet of these species; they share as much as 88% of the kinds of foods most commonly eaten, and 75% of the food items most abundantly eaten. At times they feed within close proximity of each other; however, R. aurora feeds predominantly on land, whereas R. pretiosa feeds predominantly from water. Body temperatures of wild frogs in the field were significantly different, R. pretiosa attains higher body temperatures than R. aurora, indicating divergent preferences in habitat requirements. R. pretiosa has a higher temperature tolerance than R. aurora. R. pretiosa uses water to thermo-regulate, while R. aurora uses shade and sun on land. The rate of evaporative water loss and water loss leading to lethal desiccation were the same in both species, indicating no obvious physiological basis for their water or land preferences. To escape from predators, R. aurora uses land and R. pretiosa uses water. R. aurora is a strong jumper, and jumps in a nearly straight path at an angle of 45°; R. pretiosa is a weaker jumper and jumps in a circle at an angle of about 10° to the ground, but normally escapes by submerging to the bottom of the nearest water. Both species breed during the same two to four weeks in February and March within a few feet of one another in the same bodies of water. They avoid interbreeding, however, by means of such premating isolating mechanisms as differences in mating call, male calling behavior, and microgeographic choice of spawning sites. Embryos of each species have different thermal adaptations and requirements which are correlated with adult breeding habits. For example, R. pretiosa females deposit eggs in very shallow water exposing them to relatively high daytime temperatures. R. aurora deposits eggs beneath several feet of water buffering them from heat stress and wide thermal fluctuations. Factors underlying their rare occurrence in sympatry and observed geographic distributions are discussed in terms of their habitat requirements and the present existence of other species of ranid frogs in the Pacific Northwest.Science, Faculty ofZoology, Department ofGraduat