Endemism in Native California Channel Island Floras Correlated with Seasonal Patterns of Aeolian Processes

Botany Endemism in native California Channel Island floras correlated with seasonal patterns of aeolian processes. Journal: Botany Manuscript ID cjb-2015-0143.R1 Manuscript Type: Note Date Submitted by the Author: 02-Nov-2015 Complete List of Authors: Riley, Lynn; University of South Dakota, Biology McGlaughlin, Mitchell; University of Northern Colorado, Biological Sciences Keyword: aeolian, CaliforniaDraft Channel Islands, dispersal, endemism, plant diversity https://mc06.manuscriptcentral.com/botany-pubs Page 1 of 27 Botany Endemism in native California Channel Island floras correlated with seasonal patterns of aeolian processes. Lynn Riley 1 and Mitchell E. McGlaughlin 2 1 – Department of Biology, University of South Dakota, Vermillion, SD 57069; [email protected] 2 – School of Biological Sciences, University of Northern Colorado, Greeley, CO, 80639; [email protected] Draft Corresponding Author: Lynn Riley, Department of Biology, University of South Dakota, 414 East Clark St., Vermillion, SD 57069, [email protected], Fax: 605-677-6557 1 https://mc06.manuscriptcentral.com/botany-pubs Botany Page 2 of 27 ABSTRACT This study revisits the hypothesis that dispersal to the California Channel Islands follows a stepping-stone pattern from mainland California, based on earlier work indicating the floras conform to classic island-biogeographic expectations. A re-examination of data incorporating directions of prevailing and seasonal Santa Ana winds greatly strengthens the power of the model to explain levels of endemism in 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).Draft 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. Key words: aeolian, California Channel Islands, dispersal, endemism, island biogeography, plant diversity 2 https://mc06.manuscriptcentral.com/botany-pubs Page 3 of 27 Botany Introduction MacArthur and Wilson’s (1967) equilibrium theory of island biogeography (ETIB), originally an influential model to explain species richness on islands, remains the accepted null model for island biogeographic studies (reviewed in Losos et al. 2010). Just two variables, island area and distance from a source of colonists, predicts the number of species in island biotas all over the world. Given that many different biological phenomena, not just dispersal but also successful colonization and perhaps speciation, are subsumed into these two variables, it is impressive that the model has been so widely and successfully applied. The ETIB has also been effectively extended to explain species richness on metaphorical islands like metapopulations and islands of habitat (reviewed in Hanski 2010). DraftThis theory is especially important in the field of conservation biogeography (Whittaker et al. 2005; Richardson and Whittaker 2010), because protected areas are, or may ultimately be, very small areas of wild habitat immersed in a hostile matrix, like literal islands in water. Using the ETIB as a null model, many studies have addressed how specific factors may impact diversity on various island systems. Inevitably, the theory does not explain all the variation in a data set. Deviations from ETIB expectations have allowed researchers to identify other important factors that influence the composition of island biotas, such as nested subsets (Wright et al. 1998), order of colonization (Ricklefs and Bermingham 2001; Gardner and Engelhardt 2008), or competition (Gardner and Engelhardt 2008). In some studies, only one or two islands in a system deviate from expectations based on ETIB; for example, a particular island may have more or fewer species than expected on the basis of the model (Moody 2000). These deviations might 3 https://mc06.manuscriptcentral.com/botany-pubs Botany Page 4 of 27 be caused by something specific about a particular island that merits further investigation, such as geology, island age, the presence of predators, or human mediated disturbance. However, deviations might also be explained by more general biotic and abiotic factors. For example, distance from a continental source is an important component of the original model because the probability of dispersal is likely to be correlated with distance (MacArthur and Wilson 1967). However, straight-line distance from a source of colonists, with no regard for details of geography, habitats, and dispersal vectors, may not be the best estimator for probability of successful dispersal. The California Channel Islands, a group of eight oceanic islands located off the coast of southern California (Fig. 1), have long provided a natural laboratory for investigating the biogeography of variedDraft taxa on near-shore oceanic islands (Lyon 1886 a, 1886 b). Although the islands have been the focus of many biogeographic studies, no consistent pattern of colonization or diversification akin to the Hawaiian progression rule (Funk and Wagner 1995) has been proposed for the archipelago. Early biogeographers, working under the assumption that the islands were continental in origin, relied on inferred ancient land bridges to explain observed patterns (Fig. 1; Garth 1967; von Bloeker 1967; Rentz and Weismann 1973; Philbrick 1980; but see Savage 1967; Wenner and Johnson 1980). Even the more distant and isolated southern islands were assumed to have had land bridges or to be within easy and regular colonization distance from the continent and, therefore, within the routine dispersal distances of mainland taxa. Consequently, many endemics were thought to be the relicts of former panmictic island- continental assemblages that were disrupted when the mainland populations withdrew, predominantly to the north, after the last glacial maximum (Parish 1903; Axelrod 1967; 4 https://mc06.manuscriptcentral.com/botany-pubs Page 5 of 27 Botany Muller 1967; Raven 1967; Thorne 1969; Oberbauer 2002). More recent studies (Vedder and Howell, 1980) have clearly demonstrated that all of the Channel Islands are oceanic in nature, having never been connected the mainland, although, during the last glacial maximum the northern islands were only separated from the mainland by a narrow (ca. 4 km) deep-water channel (Johnson 1983; Kinlan et al. 2005; Fisher et al. 2009). Among taxa for which land bridge colonization or relictual endemism were not inferred, no general biogeographic patterns emerged. Several different biogeographic predictions are suggested depending on dispersal methods and island geography. For example, little avian differentiation might be expected because the California coastline curves around the islands creating the Southern California Bight such that the entire archipelago is within the flyway of Draftmany birds (Diamond 1969). Alternately, organisms transported primarily by ocean currents would be expected to have colonization routes mirroring the counter-clockwise path of the California Eddy (Moody 2000), while those moved by catastrophic floods would be expected to colonize opposite the outflow of large mainland rivers (Schoenherr et al. 2003). Despite the complex and varied patterns that might be expected given the interplay of these biotic and abiotic features, Moody (2000) found that Channel Island plants conformed to the McArthur and Wilson (1967) expectations of richness and endemism. The northern islands (San Miguel, Santa Rosa, Santa Cruz, and Anacapa), which are relatively close (20 – 44 km) to the mainland, support rich plant biotas (191-495 native species) with moderate endemism (mean of native flora = 10.78%; Table 1). The generally more distant southern islands (San Nicolas, Santa Barbara, Santa Catalina, and San Clemente; 32 – 98 km) harbor more depauperate plant biotas (87-437 native species), 5 https://mc06.manuscriptcentral.com/botany-pubs Botany Page 6 of 27 often with higher endemism (mean of native flora = 12.38%). Moody (2000) found that Channel Island plant diversity is strongly impacted by island size and that endemic plant diversity is secondarily impacted by island isolation. However, in the analyses examining isolation, distance to the mainland explained little of the variability in endemism among all islands ( r2 = 0.17), but it explained the majority of the variability in the seven nearest islands ( r2 = 0.78, excluding San Nicolas) This strong correlation is consistent with both independent colonization of each island from mainland sources and stepping-stone colonization from adjacent islands (Fig. 1). Few studies have addressed Channel Island colonization histories. However, studies that have investigated the source populations for southern island taxa have failed to find evidence for colonization fromDraft Santa Catalina, the largest and closest to the mainland of the southern islands. Instead, several biogeographers have inferred a northern island source for southern island invertebrates, particularly

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