Origins of Varied Floristic Compositions in the Western Aleutian and Northern Bering

By Monte D. Garroutte and Stefanie M. Ickert-Bond

The Aleutian Islands and the Northern Bering Sea Islands have primarily upheld their status as distinct floristic subregions of the Arctic/Subarctic in published treatments, although multiple studies have also suggested a floristic similarity between the two island groups because of their oceanic influences and relative proximity.

Abstract

The Aleutian Islands and the Northern Bering Sea Islands have primarily upheld their status as distinct floristic subregions of the Arctic/Subarctic in published treatments, although multiple studies have also suggested a floristic similarity between the two island groups because of their oceanic influences and relative proximity. Using a combination of digitized occurrence data, review of pertinent literature, and recent floristic surveys across both island groups, we examined the influence of Asia on the island groups as well as their oceanic character. We restricted our comparison to the Western Aleutian Islands and the Northern Bering Sea Islands. Species composition was coarsely compared between the island groups by the number of overlapping species, distinctive species presence, and by the species’ growth habit. We also used Sørensen’s similarity coefficient as a statistical measure of difference between the island groups. The Western Aleutians (386 species, 0.47 species/square mile) were found to be separated from the Northern Bering Sea Islands (317 species, 0.16 species/square mile) by more than half of their floristic composition, with only 136 shared species between the two island groups, and were considerably more diverse in the number of species/area. The Western Aleutian Islands and the Northern Bering Sea Islands have similar proportions of forbs, graminoids, and shrubs/subshrubs in their flora, although the Western Aleutian Islands show ferns/fern allies in larger proportion than shrubs/subshrubs. Ferns are largely absent in the Northern Bering Sea Islands. The colder midsummer temperatures and Beringian biogeographical history of the Northern Bering Sea Islands may help explain the disparate floristic composition of these two island groups.

Introduction

While most published treatments involving the Aleutian Islands and the Northern Bering Sea Islands have upheld their status as distinct floristic subregions of the Arctic/Subarctic, these studies have also suggested a floristic similarity between the two island groups because of their oceanic influences and relative proximity (Figure 1, Hultén 1937, Hultén 1960, Yurtsev 1972, Yurtsev 1994, Elvebakk et al. 1999, Pan-Arctic Flora 2011). Yurtsev (1972, 1994) has discussed the floral similarity between the two island groups several times, emphasizing that they share characteristic oceanic species such as Conioselinum pacificum (Eastern hemlockparsley, Figure 2), although he suggests that they are very different as a whole floristically, owing to a historically warmer and more persistent oceanic climate in the Aleutians Islands as compared to the Northern Bering Sea Islands.

However, both the Aleutian Islands and the Northern Bering Sea Islands share persistent high winds and precipitation, and are similarly treeless (Figure 3, Young 1971, Talbot and Meades 2011). Hultén (1937) on the other hand identified an area containing the Northern Bering Sea and the Aleutian Islands as the “center of an equiformal, progressive group of plant species”, with populations spreading west into eastern Asia and east into western North America and the Rocky Mountains. Using the Bailey and Omernik approach to define ecosystems, Nowacki et al. (2001) placed the Aleutian Islands in a distinct ecoregion as part of the “Maritime-Aleutian Meadows” bioclimatic gradient, while the Bering Sea Islands are also classified as a unique ecoregion, which is part of the “Polar-Bering Tundra” bioclimatic gradient. Using average summer temperatures Young (1971) partitioned the arctic and subarctic into four floristic zones (zones 1-4), placing the Aleutian Islands in the same zone as two of the Northern Bering Sea Islands (St. Matthew and Hall Islands, zone 4), while St. Lawrence Island and the Diomedes were segregated into a colder floristic zone (zone 3 of Young). Most other publications have excluded the Aleutian Islands from the Arctic, following Yurtsev (1994), and have treated the Northern Bering Sea Islands as a single unit (Elvebakk et al. 1999, Daniels et al. 2000). The Panarctic Flora project (2011) suggested that an Aleutian-Bering Sea insular region could be justified floristically, although it would be difficult to ignore the even greater similarities between the Bering Sea Islands and the closer mainland floras, and the number of species in the Aleutian Islands, which do not extend into the Arctic.

The repeated mention of similarity between these Western Aleutian Islands and the Northern Bering Sea Islands merits a direct comparison of their floral assemblages. To this end, recent digitization efforts of herbarium collections at the University of Alaska Fairbanks available in ARCTOS (http://arctos.database.museum) with associated metadata as well as increased digitization and review of historical literature records can be used to make syntheses of the collection history across large geographic areas for comparison and analysis. In addition we have been involved in recent survey activities on the Aleutian Islands, including vascular plant surveys on Buldir Island (2010-2011; Figure 4), Rat Island (2010), Attu Island (2011), Kiska Island (2011), St. Matthew Island (2012; Figure 5) and Hall Island (2012). We provide details of the island groups below, created species lists of the vascular plants, and then compared the species composition statistically to find a measure of beta diversity between the islands groups.

The Aleutian Islands are a long chain of mostly volcanic islands which run west of the Alaska Peninsula towards the Russian Kamchatka Peninsula, arcing between 51° and 56° N latitude (Figure 1). This treeless island chain has often been placed in the Arctic floristic zone (Hultén 1960, Young 1971), although more recent definitions of the Arctic floristic zone have alternatively placed the Aleutian Islands in the hypoarctic or “treeless boreal” floristic zone, due to the lack of permafrost and relatively high average temperatures (Tatewaki and Kobayashi 1934, Yurtsev 1994, Elvebakk et al. 1999). The numerous active volcanoes in the chain contribute to a rugged topography; the highest point in the chain is the volcanic Mount Shishaldin (9,373 ft., 2,857 m) on Unimak Island.

The Northern Bering Sea islands, in contrast, lie on the continental shelf off the western coast of Alaska, sparsely arranged North-South from 60° N latitude towards the Bering Strait at 66° N latitude (Figure 1). Even though these islands lie several hundred miles south of the Arctic Circle, the environment in the Bering Sea Islands is truly arctic, with a polar maritime climate and a persistent pack ice in the winter and spring (Young 1971). Permafrost can be found on these islands as well, often up to several feet thick (Young 1971). While the Northern Bering Sea islands cover a large area, they also have a relative flat relief, with the highest point in the group, Atuk Mountain on St. Lawrence Island, reaching 2,070 ft. (631 m) (USDA NRCS 2006).

The greatest difference between the two island groups may, however, be their recent geologic history. The Northern Bering Sea Islands were connected as part of the Bering Land Bridge which stretched between Alaska and Russia when sea levels were lower during the last glacial maximum (60-25,000 years ago, Hopkins 1967, Hoffecker et al. 1993). The Bering Land Bridge was a major crossroad for plants from Northeast Asia into North America, though for some plant species the land bridge acted as a filter or complete barrier for plant dispersal between continents (Hultén 1968, Weber 2003, DeChaine 2008, Ickert-Bond et al. 2009). During these periods, the Northern Bering Sea Islands would have been mountain ranges or isolated highlands, and may have served as refugia when sea levels rose between glacial advances (Hultén 1968, Young 1971).

In contrast, the Aleutian Islands were not connected by land to Asia via the Russian Kamchatka Peninsula or to North America during the last glacial maximum (Hopkins 1967, Hultén 1968). However, lowered sea levels would have connected all of the smaller Aleutian Island groups by land, shortening the gap between the Aleutian Islands and mainland North America, and facilitating the distribution of North American and Asian species across the islands (Lindroth 1961). Researchers have concluded that the islands have been an area of floral exchange between Asia and North America (Hultén 1960, Tatewaki 1963, Talbot et al. 2010), nevertheless some species have restricted distributions across the Aleutian chain (Talbot et al. 2010). Lindroth (1961) was the first to recognize that Asian endemic species were found in larger numbers in the Western Aleutians, while North American endemic species represented an increasing proportion of the flora with closer proximity to the Alaskan mainland.

To direct our comparison of the Northern Bering Sea Islands and the Aleutian Islands on their similarities of Asian influence and oceanic character, we will base our Aleutian Island assemblage on that of the Western Aleutian Islands, which we here define to include the Rat Island group, Buldir Island, and the Near Island group (Figure 1).

Methods

Acquiring Regional Species Data – The floristic composition of the Aleutian Islands and the Northern Bering Sea Islands were primarily obtained from the University of Alaska Museum database ARCTOS (http://arctos.database.museum). Searches in ARCTOS were made using the “Locality” search option with island names specified (accessed October 10, 2012). Vascular plant species names and locality information were downloaded from ARCTOS. Published and unpublished island floras (Hultén 1960, Shacklette et al. 1969, Young 1971, Byrd 1984, Talbot and Talbot 1994, Freeman and Garroutte unpubl., Jones unpubl., Kenney and Kahler unpubl.) which are based on museum voucher specimens were also used as supplementary sources for assembling regional species lists. Furthermore, vouchered and identified collections from the first author’s visits to several Aleutian and Northern Bering Sea islands were added to the species lists (Buldir Island: August 3-26, 2010, August 5-27, 2011; Rat Island: August 28, 2010; Attu Island: August 2-3, 2011; Kiska Island: August 29, 2011; St. Matthew Island, August 1-6, 2012; Hall Island: August 5, 2012). The USDA Plants Database (USDA and NRCS 2012) was used to find information on the growth habit of plant species.

Comparing Floral Assemblages

We made comparisons of the regional floras using several approaches: (1) We compared overlapping and regionally exclusive species numbers, overall species number by area, and the proportion of species by growth habit. (2) We highlighted the presence of Asian disjuncts and endemic species in each of the island groups using species distribution maps (ARCTOS, Tatewaki 1963, Hultén 1968). (3) Sørensen’s similarity coefficient (Sørensen 1948) was calculated to determine a presence-absence-based statistical value for similarity between the island groups.

Results

Coarse Species Comparison

Based on the data assembled from ARCTOS and individual island floras (Hultén 1960, Shacklette et al. 1969, Young 1971, Byrd 1984, Talbot and Talbot 1994, Freeman and Garroutte unpubl., Jones unpubl., Kenney and Kahler unpubl.) the Western Aleutian Islands are represented by 386 plant species over ~820 square miles of island area (0.47 species/square mile), while the Northern Bering Sea Islands had 317 species in an area of ~1950 square miles (0.16 species/square mile). 136 species are shared between the two island groups, while 250 species are unique to the Western Aleutian Islands (64% of the total species number), but only 181 species are unique elements of the Northern Bering Sea Islands (57% of the total, Table 1). In the Western Aleutian Islands forbs form the largest proportion of the flora, followed by graminoids, ferns/fern allies and then shrubs/subshrubs (Figure 1). Similarly, in the Northern Bering Sea Islands forbs represent the largest proportion of the flora, followed by graminoids, but the ranking of shrubs/subshrubs and ferns/fern allies is reversed in the Northern Bering Sea Islands as compared to the Western Aleutian Islands (Figure 1).

Endemic and Asian Species

There were six endemic species present in the Western Aleutian Islands: Saxifraga aleutica (Aleutian saxifrage, Figure 6), Artemisia aleutica (Aleutian wormwood), Cerastium aleuticum (Aleutian chickweed), Phyllodoce aleutica (Aleutian mountainheath), Campanula chamissonis (Aleutian bellflower) and Draba aleutica (Aleutian draba). In contrast, in the Northern Bering Sea Islands we identified only one endemic species: Artemisia globularia var. lutea (Purple wormwood).

Several species show a unique disjunct biogeographic distribution between Asia and the Western Aleutian Islands, being present in one or several Aleutian islands and in Asia, but with a large geographical gap between the populations: Sorbus sambucifolia (Siberian mountain-ash), Trollius riederianus (Kamtschatka globeflower), Aconitum maximum ssp. maximum (Kamtschatka aconite). In contrast, there are no species in the Northern Bering Sea Islands which are not also well-established on both the Alaskan and Russian mainland.

Sørensen’s similarity coefficient

A similarity of 0.38 was calculated between the island groups using the Sørensen’s similarity coefficient (Table 2).

Discussion

A comparison of species number between the Western Aleutian Islands and the Bering Sea Islands showed them to differ considerable in their floristic composition. The Western Aleutians were separated from the Northern Bering Sea Islands by more than half of their floristic composition in a coarse comparison of shared and exclusive species (Table 1). In addition, the Northern Bering Sea Islands were found to be less species-diverse in comparison with the Western Aleutian Islands (Table 1). Young (1971) remarked on the relatively depauperate plant species number on St. Lawrence Island, noting that the approximately 200 species on that island were less than half of the number of species found on adjacent areas of mainland Alaska. The colder midsummer temperatures on St. Lawrence, due to the spring presence of the polar ice pack and the consistently overcast weather, were stated by Young (1971) to explain this pattern of species diversity. Summer warmth during the critical phase of reproductive growth was found to be strongly correlated with species presence across the Arctic (Young 1971). St. Lawrence Island had average summer temperatures several degrees lower than those observed on the Alaska Bering Sea coast, closer to those observed at the Alaska arctic coast (Young 1971). However, these colder summer temperatures are not present to the same extent on all of the Northern Bering Sea Islands. Young’s (1971) partition of the arctic and subarctic into four floristic zones (zones 1-4) split the Northern Bering Sea Island group: The Aleutian Islands were placed in the same zone as St. Matthew Island and Hall Island of the Northern Bering Sea Islands (zone 4), while the more northern St. Lawrence Island and the Diomedes were segregated into the next colder floristic zone (zone 3 of Young). However, most authors have not included the Aleutians in their consideration of the Arctic, and so have not made this comparison (Yurtsev 1994, Elvebakk et al. 1999, Daniels et al. 2000, Pan-Arctic Flora 2011).

The two island groups were most similar when the growth habit (shrubs, graminoids, forbs) of the vascular plants is compared (Figure 1), although the Western Aleutian Islands were found to have a slightly larger percentage of ferns and fern allies (Figure 1). This was expected, as ferns are abundant in the Western Aleutian Islands, both in the lower-elevation vegetation complexes on islands fertilized by seabirds (Figure 8, Byrd 1984) and in the coastal vegetation (Talbot and Talbot 1994). Only two fern species are found in the Northern Bering Sea Islands (Cystopteris fragilis and Dryopteris fragrans), neither of which are associated with seabird colonies or the coastal vegetation. The other members of this growth habit in the island group are fern allies.

As expected several characteristic oceanic species were found in both island groups. Amphiberingian coastal species made up a portion of the shared taxa in each island group, but several distinctive species, primarily endemic to Asia, were found to extend their range into the Western Aleutian Islands, which helped to distinguish the Western Aleutian Islands from the Northern Bering Sea Islands easily as those lack the Asian disjunct species. The Western Aleutians Islands also had six endemic species, while the Northern Bering Sea Islands only had one endemic species. The abundant Bering Sea Island legume genera, Oxytropis and Astralagus, are absent from the Western Aleutian Islands, previously cited as an indicator of the large difference between the two island groups in a coarse comparison by Yurtsev (1972).

Calculating the Sørensen similarity coefficient between the Aleutian Islands and the Northern Bering Sea Islands could reveal statistically if any similarity between the entire floral compositions of the island groups exists. A Sørensen coefficient of 1 would indicate that the two island groups had complete species overlap (i.e., their species compositions are the same), while a coefficient of 0 would indicate no species overlap (i.e., their species compositions is completely different). With a calculated value of 0.38, the floras of the two island groups were quite dissimilar statistically, even with similar species groups and very similar proportions of species by growth habit. Although the Sørensen similarity coefficient is known to underperform when rare species are present or species richness is large (Chao et al. 2006), the large area and relatively low diversity in the samples should have overcome these limitations in our current study. Species abundance data would greatly improve this metric.

Both island groups are likely affected by oceanic influence, yet this effect appears to differ by geographic location, as the oceanic icepack, which approaches the Northern Bering Sea Islands and not the Aleutians, apparently subjects the islands to lower summer temperatures, which results in a particular composition of vascular plants on each of the island groups (Young 1971). Furthermore, the lasting effects of differing geological histories between the Western Aleutian Islands and the Northern Bering Sea Islands have shaped their floral assemblages to reflect a greater difference than what is perhaps expected of their relative geographical proximity. Although the Aleutian Islands and the Bering Sea Islands have previously been differentiated by authors examining species ranges and characteristic species presence (Hultén 1968, Yurtsev 1972, 1994), our study is the first comparison between the two island groups based on new collecting efforts in the region and distributional metadata available at the University of Alaska Museum online database (ARCTOS) on the entire floral assemblages of these two enigmatic island groups.