Microevolution and Ecology of Salamanders and Frogs
Sources and sinks theory and population dynamics of amphibians from the Lesser Caucasus mountains
My publications on this subject
In 1989, Pulliam formulated the theory of "Sources and Sinks", currently assumed to be among keystones of population ecology. No one population lives in a spatially homogeneous area. Some habitats (sources) are favourable, and reproduction rates overcompensate mortality. When carrying capacity of such habitats is exhausted, young individuals disperse in surrounding territories, where conditions are not so good and reproduction cannot compensate mortality (sinks). Dynamics of a population is determined by an equilibrium of number increase in "sources" and number decrease in "sinks".
Long-term studies of demography of amphibian populations in central Georgia (Borjomi district) included the analysis of number dynamics, age structure and its dynamics, fecundity, survival of eggs and larvae, the distribution of reproductive efficiency in space and time. Most of studied species had the sources-sinks type of number dynamics (except for Caucasian salamander, which is very sporadically distributed and intensively use all available habitats). Usually only little percent of breeding sites used by a population of frogs, mud-divers or toads provide vast majority of metamorphs. In the rest of habitats tadpoled die completely or almost completely. In the brown frog population ( Rana macrocnemis ) only one to five of over hundred breeding sites participate in the formation of a new generation.
It is an interesting inverse correlation between attractiveness of individual breeding sites and their effectivity.
Frogs prefer to spawn in shallow, well-warmed pools located at large clearings. In some of them, hundreds of egg clumps are found. However, such pools usually rapidly desiccate and eggs and larvae extinct. In less attractive, deep and cold pools, where usually not many individuals deposit their eggs, larvae survive much better. The following situation is common: hundreds of clumps in a shallow, large, warm pool and only few clumps in cold creek or pond nearby, deposited possibly by frogs which came to breeding sites too late; but just these few clumps often survive. It appears to be that there is some conflict between individual comfort of a female, which preferentially goes for spawning in warmer water, and reproductive success of a frog which is higher if one breeds in deep, cold pools with low probability of desiccation.
Interestingly, shallow warm pools play an important role in the long-term dynamics of the frog population - it "sunks" surplus of new generation and makes the entire population very stable throughout years of study.
1. Tarkhnishvili, D.N. & I.A.Serbinova, 1993. The ecology of the Caucasian salamander ( Mertensiella caucasica Waga) in a local population - Asiatic Herpetological Research, 5: 147-165
2. Gokhelashvili, R.K. & D.N.Tarkhnishvili, 1993. Peliminary data on age structure of amphibian populations from Georgia. -Proceedings of the Academy of Sciences of Georgia. 19(5-6): 414-423 (Georgian, with Russian and English summary).
3. Tarkhnishvili, D.N.,1994. Breeding of the toad Bufo verrucosissimus : sexual dimorphism and shifting spawning sites. - Amphibia-Reptilia (E.J.Brill, Leiden), 1994(3/4)
4. Tarkhnishvili, D. N. and Gokhelashvili, R. K., 1994. Preliminary data of the age structure of a Mertensiella caucasica population. -Ibid: 327-334.
5. Gokhelashvili, R.K. & D.N.Tarkhnishvili, 1994. Age structure of six Georgian anuran populations and its dynamics during two consequtive years (Anura). -Herpetozoa (Wien), 7(1/2): 11-18.
6. Chubinishvili, A. T., Gokhelashvili, R. K. and Tarkhnishvili, D. N., 1995. Population ecology of the Caucasian parsley frog ( Pelodytes caucasicus Boulenger) in the Borjomi canyon. -Russian J. Herpetol. 2 (2): 79-86.
7. Tarkhnishvili, D. N., Gokhelashvili, R. K., 1999. Spatial structure and regulation of the population of brown frog Rana macrocnemis in Georgia. Herpetological Journal 9: 169-177.