Herbivory and Species Replacement in the West Coast Searockets (Cakile, Brassicaceae)

Abstract

Two species of searockets (Cakile maritima and C. edentula ssp. edentula var. edentula) have been sequentially established on the West Coast of North America since the 1880s. In California, C. maritima has replaced C. edentula in the southern 1000 km of their former sympatric distributions. This research tested the hypothesis that differential herbivory contributed to the ascendance of C. maritima. Choice experiments were conducted with three herbivores (two insects and a rodent) which consume Cakile at Point Reyes National Seashore, California, where only C. maritima now occurs. Larvae of the moth Platyprepia virginalis (Arctiidae) displayed a significant preference for foliage of C. maritima in a laboratory test. No evidence of any foliage preference was found for the short-horned grasshopper Microtes occidentalis (Acrididae) in laboratory and field experiments. Field experiments with deer mice (Peromyscu's maniculatu,s) demonstrated a significant preference for C. edentula seedlings and fruits. Degree of preference by mice for seedlings was affected by the distance from the experimental location to patches of introduced beachgrass (Ammophila arenaria), which mice use for nesting sites and cover, but this distance-dependence did not occur for fruit predation. Herbivory by the two insect species was not a factor in this case of species replacement. Peromyscus preference for seedlings and fruits of C. edentula would have affected the rapidity of local species replacement at Point Reyes, but this preference does not explain the current distribution of Cakile on the Pacific Coast. I concluded that differential predation by mice was not the ultimate cause of Cakile species replacement in California.