Decline of Ranid Frog Species in Western North America: Are Bullfrogs (Rana catesbeiana) Responsible?

Abstract

The decline of ranid frog species native to western North America is a pattern alluded to by many workers. We review the factors cited as having caused these declines, using, as primary examples, some of the ranid species native to California. We present explicit statements of four major alternative hypotheses: 1) bullfrog introduction, 2) habitat alteration, 3) predation by intro- duced fishes, and 4) commercial exploitation. Additionally, we review data relating to four other factors suggested as having caused declines: 1) toxicants, 2) pathogens and parasites, 3) acid rain, and 4) catastrophic mortality. Notably, data do not exist that suggest that the often-invoked bullfrog hypothesis is most compelling. Some factors, like commercial exploitation, are untestable because the putative causal conditions no longer exist, whereas others, like catastrophic mortality, are dif- ficult to test because of their unpredictability. Perhaps the most neglected but potentially important alternative is predation by introduced fishes. Existing data cannot distinguish adequately among three major testable alternatives: bullfrogs, habitat alteration, and introduced fish predation. In the absence of satisfactory data, the chronological priority of fish introductions over those of bullfrogs and the greater access fish may have to earlier ranid life stages make the fish predation hypothesis more compelling. Several alternative hypotheses are confounded because existing correlative data support at least two alternatives equally well. Manipulations of testable alternatives are imperative to distinguish causal factors. Over a century ago, Lockington (1879) observed that populations of Rana au- rora in the vicinity of San Francisco were diminishing. Since that time, investi- gators have repeatedly suggested that nearly every ranid frog species native to western North America has experi-