Temporal trends in agile frog Rana dalmatina numbers: results from a long-term study in western France.
Reports of amphibian declines have highlighted the urgent need for long-term data sets to increase understanding of population changes. To detect population changes in the agile frog Rana dalmatina in Vendée, western France, counts were made of spawn masses over 16 years and road mortalities over 13 years. Long-term trends were evaluated using regression analysis of the logarithmic transforms of annual mortalities and egg masses as dependent variables against year as the independent variable. Tests of the regressions against a 0 hypothetical coefficient, indicative of population stability, gave coefficients that were positive for road mortalities and negative for spawn counts. However, neither was significantly different from 0, indicating a stable population. Further analysis using jackknifing produced a series of pseudo-regression coefficients, which agreed with the true regressions. Results from both datasets were therefore congruent and indicated wide annual fluctuations, with a major increase in numbers between 2009 and 2014. Data from spawn deposition in a recently established pond suggested that the presence of invasive crayfish Procambarus clarkii influenced both deposition sites and long-term population changes.