Morphology and spatial distribution of alien sea-rockets (Cakile spp.) on South Australian and Western Canadian beaches

Abstract

Sea-rockets (Cakile spp., Brassicaceae) are annual plants of sandy beaches. Cakile edentula (Bigel.) Hook. is native to the eastern coast of North America, C. maritima Scop. to western Europe and the Mediterranean basin. The two species differ in several morphological features, including leaf form, fruits and petal size. Both are long-established aliens on beaches in western Canada and southern Australia, at sites where we examined their morphological and distributional attributes.

The two Cakile species co-occur at Pachina Beach, British Columbia, Canada, with C. edentula more common and widely distributed over broader range of beach elevations and C. maritima restricted to the upper beach. Although a few putative hybrids occur, the species are morphologically quite distinct. In contrast, on Westlake Shores beach, South Australia, Cakile is at least in part perennial, with widely variable morphologies, and the taxon is not separable into two morphologically distinct entities. Species boundaries have blurred apparently because of introgression. Factors that may have lead to this contrasting situation in South Australia are discussed.