REPLACEMENT OF CAKILE-EDENTULA BY C-MARITIMA IN THE STRAND HABITAT OF CALIFORNIA

We conducted field experiments on two naturalized species of searockets (Cakile: Brassicaceae) to discover why C. maritima has replaced C. edentula throughout most of coastal California but not in Oregon and Washington. Both species had a seed bank which persisted at least 2 yr in foredune sand. No significant interference between species was detected in foredune or open beach habitats. In the foredune, a manipulative experiment showed no influence of insect herbivory on survivorship, whereas rodent herbivory did have a significant impact (but this impact was similar for both Cakile species). Both species grew best in the open beach habitat, where reproductive output per plant of the two was similar. In the foredune, C. maritima had higher survivorship than C. edentula, with many more individuals surviving into a 2nd reproductive season resulting in at least an 18-fold difference in seed output in that habitat. We concluded that the advantage of C. maritima over C. edentula lay in its ability to better-tolerate foredune environmental conditions and survive into a 2nd or 3rd reproductive season. Species coexistence at more northerly latitudes is probably facilitated by a strictly annual habit forced upon both species by more severe winter conditions.