Type: report

Article abstract: In 2003, the USGS implemented a new monitoring program for the endangered arroyo toad (Anaxyrus californicus) on Marine Corps Base Camp Pendleton (MCBCP). To address problems associated with large variations in adult toad activity, we employed a spatial and temporal monitoring approach to track arroyo toad breeding populations by documenting the presence of egg strings and larvae. We survey sites within three major watersheds up to four times per year to calculate and account for imperfect detection probabilities. We also conduct night surveys for adult toads, following a monitoring program initially implemented by Dan Holland in 1996. This report details results and analyses specific to the 2006 surveys, as well trends in occupancy, breeding and adult activity from 2003 to 2006. Wide variations in seasonal rainfall have marked the past several years of arroyo toad monitoring at MCBCP. In 2006, we received only half the normal average rainfall (138 mm). Consequently, the largely ephemeral San Onofre and San Mateo Creeks remained partially dry throughout the 2006 breeding season. Over all watersheds, 64% of potential toad breeding habitat contained water during our survey efforts. However, 82.7% (se= 7.4) of the available wet habitat was occupied by breeding arroyo toads. We recorded the highest occupancy in the Santa Margarita Watershed (85.0%), followed by the San Mateo (22.3%) and San Onofre (0.0%) Watersheds. Even in the wetted areas, San Mateo and San Onofre Creeks had unexpectedly low occupancy of arroyo toads. In particular, we found a significant 74.1% decrease in occupied breeding habitat within San Mateo Creek from 2005 to 2006. We hypothesize that this low occupancy for San Mateo and San Onofre Creeks may be the result of reduced sand cover, which dropped from an average of 26-50% in 2005 to only 11-25% in 2006. In general, the creek beds became rockier with few sandy areas available for arroyo toad breeding. Another possibility for the low occupancy of arroyo toads in San Mateo and 2 San Onofre Creeks could be the cool temperatures and relatively late rainfall of 2006. These factors can result in the absence of arroyo toad breeding activity. Even though surface water availability was highly variable (44-95%) from 2003 to 2006, the overall extant of breeding toads in wetted areas was relatively stable (77-95%) with no significant change over the four year period. The night survey count data from 1996 to 2006 also showed