We conducted an 8.5 year mark-recapture study of the endangered California tiger salamander (Ambystoma californiense) to determine which aspects of the salamanders’ life history have the greatest influence on population-level demographic parameters. Salamanders were captured using a drift fence array that included 2.2 km of fencing, and were “marked” with a combination of visual implant alphanumeric tags, visual implant elastomer, and photographs processed through a pattern recognition program. This recapture data was used to create an integral projection model consisting of eight demographic functions relating body size to growth, survival, maturity, and fertility for three different age classes (larvae, metamorphs, and juveniles/adults). These functions include 24 estimated parameters, and we conducted an elasticity analysis to determine which of these parameters has the greatest influence on both population growth rate and carrying capacity. Finally, we added stochasticity to the model in terms of the percentage of adult females that choose to breed in each year and the probability of the breeding pond holding water long enough for larvae to successfully metamorphose. Using this stochastic model, we conducted a population viability analysis, looking at the probability of extinction in the next 100 years if varying fractions of the terrestrial habitat are developed.
Results/Conclusions
We had a total of 29,763 salamander captures of which 12,986 were marked and 3,257 were recaptures. Our survival functions show that survival probability is highly correlated with body size, ranging between 1% and 80% for the smallest and largest individuals. This creates a strong argument for using the integral projection model approach, since it allows demographic parameters to vary continuously with body size rather than being constant within each age/size class. Our study is the first to create an amphibian integral projection model, and thus provides valuable insights into demographics of this vertebrate class. The demographic parameter with the greatest impact on population growth rate is the slope of the metamorph growth curve, and the parameter with the greatest impact on carrying capacity is the slope of the juvenile/adult survival curve. These results indicate that early rapid growth allowing maturation at a young age allows populations to recover quickly from a small size, while greater survival during the long adult stage allows greater maximum population size. Results of the population viability analysis indicate that for the 100-year extinction probability to remain below 5% for the typical breeding population, all land within 1100 m of the shoreline must be conserved.