Monitoring Population Trends of Eleutherodactylus Frogs

Like many Neotropical frogs, a number in the genus Eleutherodactylus have declined or gone extinct in the past two decades. However, the extent of Eleutherodactylus population declines is unknown. Our objective was to identify a good method for monitoring the density of Eleutherodactylus populations to assess the extent of declines. We did this in two ways. First, we compared two methods of directly estimating density, closed population capture-recapture analysis and distance sampling, and one method of indirectly estimating density, visual encounter surveys, for multiple Eleutherodactylus species at three sites in Ecuador. We then conducted a power analysis to estimate the power of our current sampling design to detect declines. Distance sampling estimates of density were biased low compared to capture-recapture estimates. When we corrected this bias, distance sampling estimates became imprecise. Estimates of density from visual encounter surveys were also imprecise. In contrast, capture-recapture estimates were fairly precise and most likely unbiased. Moreover, capture-recapture analysis had the most power to detect declines, although even with capture-recapture analysis, power was low with only five years of sampling. We conclude that capture-recapture analysis is a good method for monitoring Eleutherodactylus density over time, but the sampling area and/or the number of sampling occasions should be increased from the area and number of occasions used here in order to increase sample sizes and therefore power.