| As the invasive frog, Eleutherodactylus coqui, continues to expand in Hawai'i, attempts to control its populations are in progress, and there is a need to measure frog abundance quickly, inexpensively and with reasonable accuracy. Current methods for assessing coqui frog populations include mark-recapture analysis of population density and census counts of active frogs. Mark-recapture analyses are more accurate, but they are labor-intensive and expensive. Male coqui frogs call at night and their loud continuous chorusing commonly reaches levels of 70 decibels in dense populations. Sound pressure level (SPL) is relatively simple and inexpensive to measure, but its accuracy as a measure of frog abundance or activity is not known. I used parameters of an overnight chorus SPL model to examine variation in SPL in separate populations of frogs. I repeatedly recorded coqui choruses in lowland wet forest of southeast Hawai'i Island in 2006--2007. Results showed that the chorus model parameters are subject to large variability, and dense populations of frogs ranging from 1,070--4,700 adult frogs per hectare (mark-recapture analysis) were not readily distinguished by the peak magnitude of night time SPL. Lower temperatures significantly reduced SPL of population vocalization. Leaf wetness significantly affected SPL rise characteristics. Relative humidity, over the narrow range (83--100%) found in East Hawai'i forests, had no significant effect on SPL of frog vocalization. Use of other SPL model parameters for estimating population size requires knowledge of effects of temperature and moisture on frog activity. |
