Abstract
Audial surveys of anuran amphibians (frogs, toads and similar) are cost-effective and allow for the coverage of large areas, but they are usually regarded as unsuitable for population size estimations due to imperfect detection. Our study demonstrated a method for obtaining minimum adult population size estimates from vocalising anuran counts by using sex ratios, life history and vocalising behaviour parameters from other studies. We collected data from 2016 to 2018 for seven taxa on 65 plots (each 25 km2) representing the entirety of Latvia. Among taxa, average breeding waterbody audible detection probabilities ranged from 0.56 to 0.88 per plot, minimum adult frog density (MAFD) estimates were from 12.0 to 51.7 individuals per km2, but the estimated fraction of population covered by MAFD varied from 57 to 86%. The least accurate density estimates were in taxa with brief calling activity and quiet mating calls (Rana temporaria), and in taxa with a calling activity dependent on the numbers of males in a pond (Bufo bufo). Our study suggests that lek-breeders would be more suitable than explosive-breeding taxa for minimum population size estimates from audial data. The use of MAFD allowed for coarse minimum population size estimates for the entire country from the audial monitoring data, these ranged from 3.7 ± 0.5 thousand (Bombina bombina) to 1.64 ± 0.47 million (B.bufo) adults.
Highlights
Audial surveys of anuran amphibians are cost-effective and allow for the coverage of large areas, but they are usually regarded as unsuitable for population size estimations due to imperfect detection
Several studies that paired acoustic abundance estimates with mark-recapture surveys supported the assertion that audial surveys are well-correlated with abundance[8,9,10], whereas other studies indicated that they are not informative at low densities[11] or that the calling index is weakly related to anuran abundances[12]
This means that the real number of males present most likely will be higher than those estimated from the calling male counts and, we will estimate only the minimum adult population size
Summary
Audial surveys of anuran amphibians (frogs, toads and similar) are cost-effective and allow for the coverage of large areas, but they are usually regarded as unsuitable for population size estimations due to imperfect detection. Fraction of waterbodies with audial records for population size estimates, it is possible to count calling males at peak breeding activity and use average peak detection probabilities and sex ratios from other studies Such an approach may lead to incorrect results for each particular waterbody due to natural variation of parameters; for instance, sex ratios may vary between sites and years[5,17,18]. During a monitoring survey with many sites and relatively few visits to each site, it is unlikely that all the sites will be visited at the perfect time since the maximum activity period can be brief and the frog calling activity can be unpredictable This means that the real number of males present most likely will be higher than those estimated from the calling male counts and, we will estimate only the minimum adult population size. If the sampling is representative, it is possible to extrapolate the data and to acquire minimum population density or population size for a larger territory
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