California Sea Grant College Program

(1) The abundance, species richness, and community composition of nudibranchs in central California has changed significantly in the past 30 to 70 years.(2) The observed changes are consistent with those predicted by climate variability on interannual and decadal time scales. Specifically, during warm phases: (a) the proportional abundance and speciesrichness of southern species should increase, as northern species decline, and (b) geographic ranges should shift poleward. The reverse of these should occur during periods of cooling. Further, these changes should be apparent at multiple, widespread sites (otherwise, more localized environmentalfactors, and not broad scale climate factors, would be implicated as the cause).(3) Long-term, anthropogenic climate warming is superimposed on natural climate variation, and its biological signal (similar to that described above for natural periods of warming) should be apparent bycomparing the abundance, species richness, and community composition across (1) full cycles of natural decadal variation, preferably at multiple sites, and (2) across entire single phases of decadal variation.(4) While climate change may be the ultimate factor behind community changes we document, especially those related to shifts in geographic ranges, climatic impacts on ecologically important species could result in a cascade of indirect effects on other species. In 1977, the large aeolidnudibranch, Phidiana hiltoni, began spreading northward from Monterey, California. By 1992 it had reached Duxbury Reef, 100 km to the north, where other nudibranchs subsequently appeared todecline. We will also test the hypothesis that P. hiltoni attacks and consumes other species of nudibranchs and has caused these apparent declines.