The California Channel Islands are a group of eight oceanic islands located off the coast of southern California that are substantially closer to the mainland than most other well‐studied island systems. The equilibrium theory of island biogeography proposed by MacArthur and Wilson posits that species diversity on an island will be positively impacted by island area and negatively impacted by isolation, which has been confirmed for the Channel Islands. In this study, we have extended MacArthur and Wilson's theory to examine how levels of genetic diversity relate to four island characteristics (island area, distance to the mainland, distance to the nearest island, plant diversity) in the endemic perennial taxa of Acmispon (Fabaceae) on the Channel Islands. We sampled two island species of Acmispon, A. argophyllus and A. dendroideus, from all islands, and mainland sister taxa for nuclear microsatellites, low‐copy nuclear sequence and plastid sequence data. We found that only one measure of diversity from one genetic region (low‐copy nuclear) was correlated with island area, that there was no support for a relationship between genetic diversity and distance to the mainland and that distance to the nearest island was a predictor of low‐copy nuclear genetic diversity. Plant diversity was a significant predictor of plastid genetic diversity when considering all samples. We conclude that the equilibrium theory of island biogeography does not hold for measures of genetic diversity in the Channel Island endemic Acmispon based on island area and distance to the mainland. The short distance between individual islands and the mainland probably facilitates a moderate rate of mainland to island dispersal, preventing the islands from functioning as isolated biogeographic units. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174, 289–304.

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This study revisits the hypothesis that dispersal to California’s Channel Islands follows a stepping-stone pattern from mainland California, based on earlier work indicating that the floras conform to classic island-biogeographic expectations. A re-examination of data incorporating the directions of prevailing and seasonal Santa Ana winds greatly strengthens the power of the model to explain levels of endemism in the Channel Island floras, and suggests the importance of aoelian processes for island colonization. Regression analysis of percent endemism in the native flora against distances measured along the axis of winds improves the r2 from 0.099 to 0.482. The endemic species that flower in the dry season as a percent of the native flora of the islands is also strongly related to these revised source distances (r2 = 0.665). Furthermore, the native floras of the southern islands are nested subsets of the floras of the northern islands, and angiosperm flowering peaks during the dry season, providing seed for seasonally based dispersal. These results suggest that the northern islands may have served as a source of colonists for the southern islands, and that the pattern of aeolian inputs into an island system should be considered in other plant biogeographic studies.

Galium catalinense (Rubiaceae) is a perennial shrub consisting of two subspecies endemic to California's Channel Islands: Galium catalinense subsp. catalinense on Santa Catalina Island, and G. catalinense subsp. acrispum, a state-endangered taxon on San Clemente Island. A long history of overgrazing by introduced herbivores has contributed to population declines in G. catalinense subsp. acrispum. We surveyed 12 populations throughout the taxon's range for genetic variation using eight polymorphic microsatellite loci to determine the genetic impact of this demographic bottleneck. At the taxon level, 65 alleles were identified with an average of 8.1 alleles per locus, although many alleles were rare; the effective number of alleles per locus averaged 2.6. Expected heterozygosity was 0.550. Individual populations had between six and eight polymorphic loci, with expected heterozygosities ranging from 0.36 to 0.60, and effective numbers of alleles ranging from 1.8 to 3.5 per locus. Populations fell into three or four genetic clusters, depending on type of analysis, which may represent refugia where the populations persisted during intense herbivory. There is little evidence of genetic bottlenecks or substantial inbreeding within populations. These findings, coupled with indications of recent migration between populations, suggest that G. catalinense subsp. acrispum is currently unlikely to be endangered by genetic factors, but small population sizes make the taxon vulnerable to future loss of genetic diversity. Management strategies based on these genetic data, population sizes, and the spatial distribution of populations are discussed.

Galium catalinense subspecies acrispum (Rubiaceae) is a state-endangered perennial shrub endemic to San Clemente Island. Eight polymorphic microsatellite loci were isolated from G. catalinense ssp. acrispum. These loci show high levels of variability, averaging 6.5 alleles per locus and an expected heterozygosity of 0.550. One locus exhibited significant deviations from Hardy-Weinberg equilibrium (P < 0.01) and one pair of loci exhibited significant linkage disequilibrium.