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

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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

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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

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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

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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;

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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),

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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 for San Nicolas

Island taxa (Weissman and Rentz 1976; Rust et al. 1985; Powell 1994; Ramirez 1995;

Chatzimanolis et al. 2010). Similarly, Riley (2012) found evidence of a north to south colonization route within the plant genus Eriogonum (Polygonaceae).

The lack of corroboration for the shortest-line distance dispersal hypothesis along with the low correlation between endemism and distance to the mainland in the complete data set in Moody’s (2000) analysis suggest that, at least for some taxa, distance to the mainland alone does not determine colonization patterns. A north to south colonization route, as inferred in the above mentioned studies, suggests dispersal by the prevailing northwesterly winds. The current wind patterns have been largely stable for the past

10,000 years (Erlandson et al. 2005; Muhs et al. 2009), a period during which the

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environment warmed and dried, favoring establishment of the modern coastal sage scrub

community (Kennett et al. 2007; Anderson et al. 2009). Circulation studies demonstrate

that the Channel Islands receive strong and steady aeolian inputs from the northwest to

the southeast throughout the spring, summer, and fall (Dong et al. 2009; Fig. 1), with the

greatest wind strength occurring across the northern islands. Beginning in the fall and

peaking in the winter, seasonal Santa Ana winds originating in the Great Basin move air

from the east, out of the Los Angeles Basin, westward onto the islands closest to the

shore (Anacapa, Santa Cruz, and Santa Catalina; Muhs et al. 2008; Raphael 2003).

Therefore, the southern islands, excepting Santa Catalina, which lies in the path of the

Santa Ana winds, receive aeolian inputs primarily from the northern islands (Dong et al.

2009; Fig. 1), minimizing the importanceDraft of the straight-line distance of these islands to

the mainland. It is likely that components of the modern biota were dispersed by the

strong prevailing or periodic Santa Ana winds, depending on the location of specific

islands. In that case, the source of colonists would be an occupied area along the axis of

the prevailing or seasonal winds, rather than the nearest occupied mainland area.

In the current work, we examined native plant diversity on the Channel Islands with

respect to flowering time, isolation, and the nestedness of species compositions among

islands. We have focused solely on native plants, because the total flora includes many

exotic taxa (49-196 per island), which are largely attributable to human introduction, not

passive colonization. Our approach sheds light on the previous analyses of Moody (2000)

examining the role of island isolation on plant diversity while explicitly accounting for

seasonal aeolian inputs, which are likely to have a significant impact on seed availability,

dispersal direction, and subsequent colonization.

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Methods

Plant diversity data for each island was obtained from Ratay et al. (2014; Table 1), which is the most current list of Channel Island plant species diversity. Although species diversity has likely been impacted by human activities on the islands (Moody 2000), no estimates of species diversity prior to human usage of the islands exist. Phenology data was obtained from the Jepson eFlora (Jepson Flora Project, 2013) for all native angiosperms. Flowering time was grouped into the following seasons: Winter (December,

January, February), Spring (March, April, May), Summer (June, July, August), and Fall

(September, October, November). Species with widespread flowering times were counted as flowering in multiple seasons. Species were classified as primarily flowering in either the Wet (>30 mm of precipitation perDraft month; November-May) or Dry (<30 mm of precipitation per month; June-October) seasons, based on 89 years (1904-1993) of rainfall data from Stanton Ranch, Santa Cruz Island (Junak et al. 1995). Because dry flowering taxa are more likely to produce seeds at the appropriate time for aeolian dispersal, all species were classified as flowering exclusively in Wet or Dry seasons based on the greatest number of months they were observed flowering. Any taxa that had an equal number of months of flowering in Wet and Dry seasons were classified as Wet season. Island Area and distance to the Mainland (DMnl ) were obtained from Moody

(2000). Distance to the nearest source (D source , Table 1) based on the autumn prevailing winds (Dong et al. 2009, Muhs et al. 2008, Raphael 2003; Fig. 1), was calculated as the straight-line distance in Google Earth (Google, Mountain View, CA). For three islands,

San Nicolas, Santa Barbara, and San Clemente, the northern islands were treated as the nearest source. All regression analysis were performed with StatPlus:mac v. 5

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(AnalystSoft Inc., Alexandria, VA). Regression analyses based on the current native plant

diversity were calculated for D Mnl and Dsource with and without San Nicolas Island to

facilitate comparison to Moody’s (2000) results. Our regression analyses have utilized

percent endemic taxa on each island to normalize the impact of island size on total

species diversity.

The level of nestedness in the island specific species assemblages was calculated

using BINMATNEST (Rodríguez-Gironés & Santamaría, 2006), which orders the rows

and columns of presence/absence data to minimize the nestedness temperature and

compares this maximally packed matrix to three alternative null models. We tested the

Species-by-Island assemblages against 1000 matrices randomly generated under the most

conservative null model (Model 3), Draftusing the recommended default parameters.

Nestedness analyses examined either all native taxa or native taxa flowering during the

dry season. Because a high proportion of endemic taxa has been shown to inflate

measures of nestedness (Greve and Chown 2006), both analyses were performed with

and without species limited to single islands. The pattern of nestedness was compared to

that of island area and distance to the nearest source by calculating Spearman’s rank

correlation in StatPlus:mac v. 5.

Results

Plant species diversity by island, and island characteristics are given in Table 1. The

previous analyses of Channel Island plant diversity by Moody (2000), utilized species

counts from Junak et al. (1995), which on average had fewer total taxa on each island

than the Ratay et al. (2014) species counts. The mean percentage of native angiosperms

flowering was lowest in winter and highest in spring, with 24%, 88%, 80%, and 34% of

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taxa flowering in winter, spring, summer, and fall, respectively (Fig. 2). More taxa where found to primarily flower in the dry season (mean = 52%) than the wet season (mean =

48%).

Number of native taxa per island is positively correlated with island area (r2 =

0.912, p < 0.001; data not shown) and the species-area residuals are positively correlated with distance to the mainland ( r2 = 0.522, p = 0.043; data not shown), confirming the previous results of Moody (2000) with the current species counts. The percent of endemic taxa in the native flora was positively correlated with modified distance to a seed source

2 (D source ; r = 0.482, p = 0.056; Fig 3A) but not straight-line distance to the mainland

2 (D mnl ; r = 0.0986, p = 0.449; data not shown). With removal of San Nicolas Islands, following the approach of Moody (2000),Draft the percent of endemic taxa in the native flora

2 2 was positively correlated with D source (r = 0.583, p = 0.046; data not shown) and Dmnl (r

= 0.492, p = 0.079; Fig 3A). An isolation effect was also observed with regard to flowering season, with the percent dry season flowering endemic taxa in the native flora

2 2 being positively correlated with D source (r = 0.665, p = 0.014; Fig 3B) but not D mnl (r =

0.155, p = 0.334; data not shown). However, there was no effect of isolation on the

2 percent of wet season flowering endemics on D source (r = 0.081, p = 0.495; data not

2 shown) or D mnl (r = 0.038, p = 0.646; data not shown).

Nestedness temperatures are calculated from maximally packed matrices, in which columns are sorted left to right, so that species on many islands are to the left, and rows are sorted top to bottom, so that islands with many species are toward the top. A perfectly nested matrix will have no empty cells in the upper left, which indicates lack of common species on a species-rich island, and no occupied cells in the lower right, which

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indicates presence of a rare species on a species-poor island. The native species and dry-

season flowering species assemblages are highly nested (p < 0.0001; Fig. 4). Nestedness

is characterized by matrix temperature, a measure of the signal to noise ratio that varies

from 0 (perfectly nested) to 100 (randomly distributed; Atmar and Patterson 1993).

Matrices that included species with single island distributions have calculated

temperatures more than 30 degrees below the expected temperatures (T obs = 16.64 and

Tobs = 16.49, for all native and dry-season flowering native species, respectively), while

the matrices excluding species with single island distributions have calculated

temperatures more than 20 degrees below the expected temperature (Fig. 4). The pattern

of nestedness is positively correlated with island size (rho = 0.91, p < 0.01), but not with

distance to the mainland or the nearestDraft source (data not shown).

Deviations from the rank order between nestedness and area are Santa Catalina,

which is higher in the maximally packed matrix for all native, but not dry-season

flowering taxa, the small northern islands of San Miguel and Anacapa, which are higher

in both matrices, and the small southern island of San Nicolas, which is lower in both

matrices. Idiosyncratically distributed species, which appear as gaps or disjunct blocks in

the matrix, may highlight species that are more strongly influenced by features other than

island area, such as dispersal corridors, competitive exclusion, or unique habitat

requirements (Atmar and Patterson 1993). Islands with notable idiosyncratic

distributions in our data include Santa Barbara, which lacks many common species and

has a relatively high proportion of more narrowly distributed species, and the large

southern islands (Santa Catalina and San Clemente), which both lack species found on

smaller islands and share species not found on larger islands (Fig. 4). Other disjunct

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blocks in the matrices are species common to the northern islands and found on isolated southern islands, like San Clemente or San Nicholas, but not on Santa Catalina (Fig. 4).

Overall, the smaller northern islands share 83.4 – 96.1% of their floras with the most species rich island, Santa Cruz. The smaller southern islands also share the majority

(70.1–72.3%), of their floras with Santa Cruz, but have floras that contain a larger proportion (14.3 – 16.3%) of species found on the large southern island of Santa Catalina, but not Santa Cruz.

Discussion

Our re-examination of the influence of isolation on floral endemism in the California

Channel Islands shows that incorporating prevailing winds improves the fit of the model

(Fig. 3A), relative to Moody (2000).Draft These analyses demonstrate that incorporating knowledge of abiotic factors, prevailing winds and flowering time in our system, can enhance understanding of the ETIB. Although the native island floras are most strongly influenced by area, as evidenced in Moody’s (2000) and our analyses, deviations from perfect nestedness and the patterns of endemic species richness indicate that other features are also important drivers of island species diversity.

In Moody’s (2000) analysis, San Clemente Island had fewer, and San Nicolas

Island had more, species than expected from the intersection of species-area and isolation curves. Furthermore, San Clemente had a higher, and San Nicolas had a lower, proportion of endemic taxa than predicted by distance from the mainland. Moody (2000) determined that if San Nicolas Island, which is the greatest outlier among the data with relatively low endemism (10.1%), was excluded, isolation explained much more of the pattern of endemism. However, when distance along the path of the dominant wind

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currents is used rather than shortest distance to the mainland, San Clemente Island is

further from, and San Nicolas Island is closer to, potential colonists than previously

calculated (Table 1; Fig. 2). Using this modified measure, distance explains much of the

variability in native floral endemism (r2 = 0. 482; Fig. 3A), without the need to remove

any observations. When considering only the endemics that flower in the dry season,

which would be more likely to be wind dispersed, the correlation is stronger (r2 = 0. 68;

Fig. 3B)

The relative importance of seasonal dispersal is also supported by flowering data

and nestedness within the flora. Classification of angiosperms into predominantly wet

and dry flowering groups indicates that the flora skews slightly towards more dry season

flowering (Fig. 2). The abundance ofDraft dry flowering angiosperms translates into dry

season seed production and the potential for dispersal events when seasonal winds are at

their strongest (Dong et al. 2009, Raphael 2003). Strong winds such as these have been

shown to be effective long distance dispersal agents, even for taxa without specific

adaptations to facilitate aeolian dispersal (Nathan 2008). Analyses of nestedness also

support seasonal dispersal because the dry-season flowering species of the southern

islands are nested subsets of the dry-season flowering species of the large northern

islands, Santa Cruz and Santa Rosa.

Although the close proximity of the Channel Islands to mainland California has

likely impacted colonization patterns among organisms with varied dispersal

mechanisms, a seasonal dispersal pattern may be common among aeolian influenced

dispersers, especially for the isolated southern islands. Future studies should test the

importance of the periodic Santa Ana winds blowing west off the mainland coupled with

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the prevailing northwesterly winds in driving colonization patterns. Our findings are limited by the small number of islands in the system, and lack of understanding of the evolutionary history of taxa and populations. Nevertheless, the results of this study demonstrate that abiotic processes, such as aeolian inputs, should be considered along with potential biotic (e.g. migration patterns) processes when assessing biogeographic patterns.

Acknowledgements

We are grateful to Sarah Ratay for sharing her species lists and to two anonymous reviewers for helpful comments on a previous version of the manuscript. Draft

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Table 1. California Channel Island names, codes, and characteristics.

2 Code Area (km ) NTot NNat NEnd DMn l (km) Source DSource (km) Santa Barbara BAR 2.6 135 87 13 61 Northern CCI 82 Anacapa ANA 2.9 262 191 24 20 Mainland CA 20 San Miguel MIG 37 291 213 19 42 Mainland CA 42 San Nicolas NIC 58 275 158 16 98 Northern CCI 76 San Clemente CLE 145 436 303 47 79 Northern CCI 145 Santa Catalina CAT 194 617 437 39 32 Mainland CA 32 Santa Rosa ROS 217 500 399 46 44 Mainland CA 44 Santa Cruz CRU 294 636 485 49 30 Mainland CA 30

Table Notes. Area and distance to mainland California (D Mnl ), from Moody (2000); number of total plant taxa (N Tot ), number of native plant taxa (N Nat ), and number of endemic plant taxa (N End ) from Ratay et al. (2014); the inferred location of the nearest large population of potential aeolian colonists (Source), as either the northern California

Channel Islands (CCI) or mainland DraftCalifornia (CA), depending on which lies closer along the direction of prevailing (Dong et al. 2009) or seasonal Santa Ana winds

(Raphael 2003, Muhs et al. 2008); and the distance to the inferred location of the nearest large population of potential aeolian colonists (D Source ) as straight-line distance to nearest source along direction of prevailing or seasonal Santa Ana winds calculated in Google

Earth (Google).

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Fig. 1. Location of the California Channel Islands in the Southern California Bight

indicating the direction of prevailing northwesterly winds (black solid arrows; Dong et al.

2009) and annual Santa Ana winds (dashed gray arrows; Muhs et al. 2008; Raphael

2003).

Fig. 2. Number of native flowering angiosperms by season and percentage of native

species that flower in Wet (>30 mm of precipitation per month; November-May) or Dry

(<30 mm of precipitation per month; June-October) seasons for each of the California

Channel Islands. Island abbreviations are given in Table 1.

Fig. 3. Regression of percent of (A) total endemic and (B) dry season flowering

endemic species in the native flora of the California Channel Islands on geographic

distance to a source of colonists. DraftBlack diamonds and black solid lines calculated as

distance along the axis of prevailing winds (D source ; Table 1). Gray diamonds and

gray dashed lined calculated shortest distance to mainland (D Mnl ; Table 1), excluding

San Nicolas Island (open gray diamond). Island abbreviations are given in Table 1.

Fig. 4. Maximally packed matrix of native (above) and dry-season flowering native

(below) plant taxa, excluding taxa reported from a single island. Shading

differentiates among species distributed evenly among both the northern island and

southern islands (medium gray) and species with predominantly southern (light

gray) or northern (dark gray) island distributions. Island abbreviations are given in

Table 1.

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Draft

Fig. 1. Location of the California Channel Islands in the Southern California Bight indicating the direction of prevailing northwesterly winds (black solid arrows; Dong et al. 2009) and annual Santa Ana winds (dashed gray arrows; Muhs et al. 2008; Raphael 2003).

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Draft

Fig. 2. Number of native flowering angiosperms by season and percentage of native species that flower in Wet (>30 mm of precipitation per month; November-May) or Dry (<30 mm of precipitation per month; June-October) seasons for each of the California Channel Islands. Island abbreviations are given in Table 1. 181x219mm (300 x 300 DPI)

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Draft

Fig. 3. Regression of percent of (A) total endemic and (B) dry season flowering endemic species in the native flora of the California Channel Islands on geographic distance to a source of colonists. Black diamonds and black solid lines calculated as distance along the axis of prevailing winds (D source ; Table 1). Gray diamonds and gray dashed lined calculated shortest distance to mainland (D Mnl ; Table 1), excluding San Nicolas Island (open gray diamond). Island abbreviations are given in Table 1. 127x168mm (300 x 300 DPI)

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Fig. 4. Maximally packed matrix of native (above) and dry-season flowering native (below) plant taxa, excluding taxa reported from a single island. Shading differentiates among species distributed evenly among both t he northern island and southern islands (medium gray) and species with predominantly southern (light gray) or northern (dark gray) island distributions. Island abbreviations are given in Table 1. 155x70mm (300 x 300 DPI) Draft

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