The Geography of Phytochemical Races

178 4 Intercontinental Disjunctions
Mediterranean was the source of propagules of this species responsible for colonization
of southern Australia. However, plants collected from the coast of South Australia
exhibited a somewhat different glucosinolate profi le, suggesting that a second colonization
(possibly more?) may have occurred at some point. Morphological heterogeneity
in the southern Australian specimens further suggested that hybridization between
individuals having different chemical profi les might have occurred.
Reference to the colonization of western North American beaches by Cakile
offers an opportunity to look at rates of dispersal of newly introduced organisms
(Sims, 1968; Barbour, 1970; Barbour and Rodman, 1970). Strand plants provide
a particularly convenient vehicle since their movement is restricted to two directions,
essentially north and south along the coasts in North America and along the
southern coast of Australia. Cakile maritima was fi rst collected in North America in
May, 1935, on Stinson Beach, Marin County, California (just north of San Francisco
Bay), growing with the naturalized C. edentula (Bigel.) Hook. subsp. californica
(Heller) Hult. (Cakile edentula is native to eastern North America.) Two years later,
it was found growing on Salmon Beach, about 40 miles north of Stinson. It had
reached Samoa Beach, Humboldt County, California, by 1940 and Sunset Bay, Coos
County, Oregon, by 1942. By 1957, it had become established on the beaches in the
Queen Charlotte Islands, British Columbia. Thus, in about 30 years, C. maritima had
advanced its range northward by approximately 1000 miles, which amounts to a rate
of about 33 miles per year. Its movement south appears to have resulted in a population
observed on Cedros Island, Mexico, in 1963 (ca. 28°30′N latitude). A similar tale
can be told about the appearance of C. maritima in the Southern Hemisphere. The
species was fi rst noticed in Australia in 1897, in the vicinity of the port of Fremantle
(Perth), Western Australia. It had spread northward along the coast as far north as
31°S by 1963 (Sauer, 1988). After introduction in Western Australia, C. maritima
moved steadily eastward, reaching the coast of Victoria by 1922 (Fig. 4.4).
Cakile edentula (recognized on the west coast of North America as subsp.
californica, but not native to that area) fi rst appeared in the fl oral records of western
North America in the form of a specimen collected in 1882 on the shore near
Berkeley, California (eastern shore of San Francisco Bay). It is thought to have
traveled from its eastern North American home in wet ballast. It wasted little time
in expanding its range, advancing southward to the beaches near San Diego, where
it was sighted in 1906 (warmer water presumably prevents it from going further
south). It is its northward journey that is most striking, however. Without detailing
its intermediate stops, C. edentula reached Kodiak Island, Alaska, by 1931, which
amounts to a rate of advance of about 50 miles per year. It is not likely that either
C. edentula or C. maritima moved steadily northward (or southward), but rather
reached new habitats through jumps of intermediate lengths. The involvement of
the tides and ocean currents is very likely as indicated by observations on propagule
viability made by Rodman (1974), who observed that seeds of C. edentula retained
a signifi cant degree of viability after emersion in seawater for up to 10 weeks, and
that upper joints of fruits could remain afl oat and viable after 11 days in seawater.
This level of propagule viability in combination with tides and ocean currents
created a potent force for migration.