Researchers have documented precipitous declines in Cascades Frog (Rana cascadae) populations in the southern portion of the species' range, in the Lassen region of California. Reasons for the declines, however, have not been elucidated. In addition to common, widespread causes, an understanding of local community interactions may be necessary to fully understand proximal causes of the declines. Based on existing literature and observations made during extensive aquatic surveys throughout the range of R. cascadae in California, we propose that a proliferation of freshwater leeches (subclass Hirudinida) in the Lassen region may be adversely affecting R. cascadae populations. Leeches may affect R. cascadae survival or fecundity directly by preying on egg and hatchling life stages, and indirectly by contributing to the spread of pathogens and secondary parasites. In 2007, we conducted focused surveys at known or historic R. cascadae breeding sites to document co-occurrences of R. cascadae and leeches, determine if leeches were preying on or parasitizing eggs or hatchlings of R. cascadae, and identify the leech species to establish whether or not they were native to the region. We found R. cascade at 4 of 21 sites surveyed and freshwater leeches at 9 sites, including all sites with R. cascadae. In 2007 and 2008, the predatory leech Haemopis marmorata frequented R. cascadae egg masses, was observed probing or tearing at eggs on 24 occasions, and was 10 times more common in 1-m2 plots centered on egg masses than in similar plots without egg masses. Six species of leech were identified from the Lassen region, only 3 of which have been documented from the region prior to this study. Given our documentation of a diverse freshwater leech fauna in direct association with a precipitously declining species, we believe a better understanding of the biology and ecology of this poorly studied taxon is needed, in addition to studies of the effects of freshwater leeches on survival and recruitment of co-occurring amphibians.
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Vol. 91 • No. 1