The larval stage of amphibians is often used as a model for ecological processes. However, the demography, ecology, and life history of amphibians after their larval stage is not well known. The article I read focuses on describing the life history and demography of the California Tiger Salamander as well as the effects the environment has on both. The California Tiger Salamander is found mainly in the grasslands of the Central Valley as well as in the surrounding foothills and valleys.
The data presented by the authors dealt with variations in the timing and magnitude of the breeding migrations. The size and age structure of breeding adults as well as the production of juveniles were also observed. Mortality rates were recorded from the metamorphic stages to adulthood. The data was collected over a seven-year time period in Carmel Valley located in Monterey County.
The test area was a pond that was fenced off from larger animals. The pond would dry up each summer and refill during the fall. There were additional ponds near the test area which provided other breeding grounds. The distance of the ponds from the test area varied from 550-3000m.
The data was collected by using drift fences surrounding the pond which would direct the salamanders into irregularly placed pitfalls. The pitfalls were placed on the outside and inside of the pond in order to record the amount of salamanders entering and leaving the pond. The pitfalls were checked every morning and captures were recorded and tagged.
The authors also used skeletochronology to estimate the individual ages of the salamanders. Skeletochronology is a technique which estimates the salamander’s age by counting lines of arrested growth (LAG) in thin cross-sections of elongate bones mostly located in the toe. It should be noted that few investigators have validated the assumptions formed by skeletochronology.
The data showed that the researchers captured and observed approximately 20% of the population. The data also showed that the males were usually the first to arrive when the pond began to fill up and stayed much later than the females. The ratio of sex usually stayed around 1:1, except for years in which the pond was not completely filled by rainfall. This was attributed to surviving females skipping breeding opportunities in years with later rainfall. When breeding occurred, the females bred an average of 1.4 times and produced an average of 8.5 young.
The data also showed that the mortality for the metamorphs in their first summer was extremely high. This was due to a low percentage of first year metamorphs being recaptured in comparison to mature salamander recapture rates. The overall estimate of juveniles that reached maturity was approximately 5%.
The conclusions that the authors came to were that an isolated breeding pond on its own is not able to sustain a long term population. In order for a population to remain stable, a female must reproduce at least one male and female offspring. Calculating the numbers, at least 18.2% of the juvenile population would have to survive in order to maintain population stability.
Because less than 50% of the population bred at the pond more than once, the authors also considered the test area as a sink habitat rather than a source habitat meaning the population at the pond was not a major one.
Although the data was well prepared and the paper well written, there are some problems with this paper. First, the authors did not study the salamanders in their terrestrial habitats. They even say that a greater focus should be put on the time salamanders spend in terrestrial areas as the population greatly decreases during that time. There could be many different evolutionary factors which play into the overall fitness of the salamanders as their size was proven to be a neglible factor. These factors could be selected for or against in the terrestrial habitats.
Another problem was that the drift fence was ineffective at times. During flooding, the salamanders were able to climb over the fence, effectively escaping capture and observation. This could have skewed the mortality, return, trespassing and breeding rates that the authors gathered over the seven year period. Mortality and survivorship were also hard to determine because adults chose to skip breeding in certain years, which could have also yielded incorrect data.
Even when they were captured, the authors had to use skeletochronology at times when the salamanders were unmarked or immigrants from the other ponds. Although the authors found that there was little error, this is still considered a new form of data analysis and is still unproven.
The authors chose to use skeletochronology because it was quicker and tagging the salamanders seemed to cause an increase in mortality. This assumption was made because the authors found they recaptured a greater amount of salamanders that were untagged in comparison to tagged salamanders.
That being said, this paper is still well done. Because less attention is given to amphibians in juvenile and adult stages of life as well as in their terrestrial habitats, there was not much precedent for the authors to draw from. By looking at what was done correctly and incorrectly in this study other researchers can formulate better experiments. This paper was a step in the right direction for the study of amphibians in the later stages of life and helped to show that all facets of life as well as all habitats should be studied in order to better understand a species.
Trenham, P.C., Bradley Shaffer, H., Koenig, W.D., Stromberg, M.R. (2000). Life History and Demographic Variation in the California Tiger Salamander (Ambystoma californiense). Copeia, 2000(2), 365. DOI: 10.1643/0045-8511(2000)000[0365:LHADVI]2.0.CO;2