EFFECT OF INTRODUCED CRAYFISH AND MOSQUITOFISH ON CALIFORNIA NEWTS

One goal of conservation biology is to explain population declines. We present field survey data and experimental evidence that implicate in troduced predators as a possible cause of decline in the California ne wt (Taricha torosa). In 1994 and 1995 we surveyed 10 streams in the Sa nta Monica Mountains of southern California for amphibians. These stre ams contained California newts when surveyed between 1981 and 1986. Of the 10 streams surveyed in 1994, three contained introduced mosquitof ish (Gambusia affinis) and/or crayfish (Procambarus clarkii). These th ree streams contained no California newt eggs, larvae, or adults. The seven streams without introduced predators contained California newts. We conducted laboratory and field experiments to determine if Califor nia newt larvae and egg masses are susceptible to predation by mosquit ofish and crayfish. Results from these experiments indicate that crayf ish consume California newt egg masses and that both mosquitofish and crayfish consume larval newts. In 24-hour field experiments, no newt l arvae survived in crayfish enclosures, and only 13% of the larvae surv ived with mosquitofish. Newt larvae are known to have antipredator ada ptations for native predators. Apparently, these adaptations are not a dequate for coexistence with introduced crayfish or mosquitofish. Heav y rains in 1995 removed introduced crayfish from one stream. We found newt egg masses, larvae, and adults in that stream the following sprin g. This same stream showed no evidence of California newts when crayfi sh were present in matched-date surveys in 1994. These experiments are surveys present evidence that predation by mosquitofish and crayfish may cause localized decline of newts in mountain streams of southern C alifornia. Understanding the effects of nonnative species is an import ant step in preventing detrimental introduction in the future.