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Systematics |
Research and Collections Center, Illinois State Museum, 1011 East Ash Street, Springfield, Illinois 62703 USA
Received for publication February 21, 2002. Accepted for publication May 21, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: biogeography • East Asia • North America • seed plants • species diversity
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Because of the convenient similarities in land area, latitude range, and habitat diversity in East Asia and North America, and because of the striking floristic similarities between the eastern temperate parts of the two continental regions, East Asia and North America provide a unique opportunity to study the large-scale processes and patterns of biodiversity and biogeography on different continents.
Early interests focused on the floristic similarities of the temperate regions of East Asia and North America (e.g., Gray, 1840
, 1846
, 1859
, 1878
; Hu, 1935
; Li, 1952
). As regional floras and checklists were developed, the close floristic similarities between eastern Asia and eastern North America become increasingly apparent (Qian, 2002
). In recent decades, botanists and biogeographers have become interested in comparisons of species diversity between East Asia and North America at a variety of spatial scales. For example, at meso-scales ranging from 10 to 104 km2, Latham and Ricklefs (1993a)
compared temperate-zone tree species diversity within and between continents and found that East Asian temperate forests support significantly more diverse tree floras than forests in climatically similar areas of North America. At a semi-continental scale, Qian and Ricklefs (1999)
compared species richness of vascular plants between China and the United States. Their data showed that the flora of China is significantly more diverse than that of the USA at both genus and species levels. At a continental scale, Li and Adair (1994
, 1997
; also see Guo, Ricklefs, and Cody, 1998
for statistical analyses based on the same data) demonstrated that the taxonomic richness of vascular plants in East Asia substantially exceeds that of North America.
However, the previously reported patterns on large-scale diversity between East Asia and North America may be difficult to interpret because geographic parameters, particularly area and latitude, which are among the major factors that determine species diversity (Rosenzweig, 1995
), were not always well matched between the two continental regions. For example, in Qian and Ricklefs's (1999)
comparison, the land areas of China and the USA were more or less comparable (9.6 x 106 km2 in China and 9.4 x 106 km2 in the USA), but China had a larger proportion of subtropical and tropical areas than did the USA as they discussed. In Li and Adair's comparisons, they also included a larger portion of subtropical and tropical area in East Asia than in North America, as Guo, Ricklefs, and Cody (1998)
pointed out. In addition, the geographic extent of East Asia was poorly matched with those of North America. For instance, Li and Adair included only forested areas in East Asia but included both nonforested and forested areas in North America. A large part of East Asia (i.e., the majority of western China and approximately the western half of Siberia) was ignored in their comparisons. As a result, both longitude range and land area of East Asia in their comparisons were only about half of those of North America (e.g., less than 10 x 106 km2 in East Asia but more than 18 x 106 km2 in North America). Li and Adair (1994
, 1997)
also compared the floras in temperate and boreal zones between East Asia and North America. They demonstrated that the number of species in western North America exceeded that in East Asia and eastern North America in both temperate and boreal zones. However, because the land area in their comparisons was poorly matched (e.g., western North America included a larger geographic area in the temperate zone but a much smaller one in the boreal zone than East Asia; see Fig. 3 in Li and Adair, 1994
), their conclusions are difficult to interpret.
It has long been believed that East Asia is much richer in species diversity of vascular plants than North America. A number of hypotheses have been postulated to explain how the differences in species diversity between East Asia and North America arose (Latham and Ricklefs, 1993a
; Li and Adair, 1994
, 1997
; Guo, Ricklefs, and Cody, 1998
; Qian and Ricklefs, 1999
, 2000
). However, it has not been adequately tested whether the species diversity bias in favor of East Asia still holds when East Asia and North America are compared at a similar level of geographic extent with similar climate conditions and how the differences in species diversity between the two regions observed at a larger scale (e.g., continental) can be translated to smaller scale regions (e.g., semi-continents) or different latitudinal zones.
In this study, I compare the taxonomic richness of seed plants at both continental and semi-continental scales using a controlled approach where land area and latitude are standardized between East Asia and North America. In order to match the geographic extent of North America (north of Mexico), latitudes primarily south of 30° N latitude in East Asia are excluded. At a continental scale, I tabulated the overall taxonomic richness of seed plants in the two continental regions. At a semi-continental scale, I compare the taxonomic diversities of the southern and northern parts of East Asia with those of North America, respectively. In addition to comparing the overall taxonomic diversity, I also compare the distributions of taxonomic diversity among major phylogenetic groups at different taxonomic levels between East Asia and North America. The specific questions addressed in this article include the following: how do East Asia and North America differ in taxonomic richness when their major geographic aspects are adjusted to a comparable level? Do the differences in taxonomic richness between East Asia and North America at the continental scale parallel those at a smaller scale such as semi-continent in different latitude zones? Are the differences in taxonomic richness between the two continental regions (at either the continental or the semi-continental scale) evenly distributed among different phylogenetic groups of plants?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Data sources
Data collection began in the early 1980s. The main data sources were the literature. During the past two decades, I reviewed more than two thousand publications pertinent to the floras of EAS and NAM in developing floristic databases for the two continental regions.
North America
A master database for the North American vascular plants (NAM-PLANTS) was created. The database was initially based upon Kartesz and Kartesz (1980)
in conjunction with Soil Conservation Service (1982)
, Shetler and Skog (1978)
, Scoggan (1978)
, and Polunin (1959)
. The NAM-PLANTS database was thoroughly updated when the following sources became available: Kartesz (1994)
, USDA (1999)
, and Biota of North America Program (1999)
. A number of more recently reported species that did not appear in any of the above-mentioned sources were added to the NAM-PLANTS (e.g., Corallorrhiza bentleyi Freudenstein, Echeandia texensis Cruden, Twisselmannia california Al-Shehbaz). The NAM-PLANTS database provided a framework for documenting detailed botanical information (e.g., native/exotic status of a plant) and distributional information (e.g., presence/absence at the state/province level). Almost all floristic books (including checklists, manuals, and atlases) pertinent to the North American regional or state/province floras were used to document presence/absence and native/exotic information for each taxon in each of the North American states/provinces. In many cases, data based on floristic books for a state/province were updated a number of times when new data became available in journal articles or other reliable sources. For example, over 300 new taxa have been added to the flora of the state of South Carolina (Hill and Horn, 1997
) since the publication of Radford et al. (1968)
.
East Asia
A master floristic database for the East Asian vascular plants (EAS-PLANTS) was developed during the same period as the NAM-PLANTS. The major sources for documenting China's plants were over 200 volumes of floristic books. These include all published volumes of Anonymous (1959–1998)
and Wu and Raven (1994–2000)
for the national flora and all published volumes of regional and provincial floras such as Fu (1995)
, Huang (1994–2000)
, and Wu (1983–1987)
. The Russian floristic data were based on Czerepanov (1995)
, Charkevicz (1985–1996)
, and Krasnoborav et al. (1988–1997)
. The Mongolian floristic data was obtained from Grubov (1982)
. Korean floristic data were compiled according to Lee (1980)
, Ri and Hoang (1984)
, and (Lee, 1996
).
Both NAM-PLANTS and EAS-PLANTS databases have been continuously updated as new information becomes available. Because the compilation of pteridophyte data for East Asia north of 30° N has not been completed, this study focuses on seed plants, which account for over 92% of vascular plants in both East Asia and North America.
Standardization of botanical nomenclature
Species level
The standardization of botanical nomenclature for the North American species generally followed Kartesz (1994)
, except for a few recently published names that were not listed. Where Kartesz treated species much differently from the majority of other authors, I followed the majority authors' treatment unless Kartesz's treatment was more compelling. For example, Aphanes occidentalis (Nutt.) Rydb. was treated as conspecific with A. arvensis L. in Kartesz, but the two taxa were separated by other authors, such as Hitchcock and Cronquist (1973)
, Douglas, Straley, and Meidinger (1989–1994)
, and Hickman (1993)
, whom I followed.
Differences in the botanical nomenclature among China, Russia, Mongolia, and Korea are noticeable. In general, Russian botanists tended to use a narrower species concept than those of other East Asian countries. For example, many taxa generally considered as subspecies or varieties were recognized as different species in the Russian literature. In addition, many taxa recognized as different species by Russian botanists were considered as the same species by North American botanists or botanists in other Asian countries. For example, Erophila praecox, E. spathulata, E. verna, Trisetum alaskanum, T. molle, and T. spicatum were recognized as six different species in Czeropanov (1995) but recognized as only two species (Draba verna and Trisetum spicatum) in Kartesz (1994)
. The species concept for vascular plants is generally comparable between China and North America. Qian and Ricklefs (1999)
compared 352 native genera published in two volumes of the Flora of China (Wu and Raven, 1994–2000
; vols. 16 and 17), which was compiled by a joint team of Chinese botanists and international (mainly the United States) botanists, with the same genera in the Flora Republicae Popularis Sinicae (Anonymous, 1959–1998
) compiled solely by Chinese botanists. They found that the total number of species for the genera in the two publications was more or less comparable, suggesting that there is no evidence for discrepancies in species circumscription between Chinese and North American botanists. In principle, I followed the broad species concept as in Kartesz (1994)
and China's floras in standardizing the botanical nomenclature of the taxa in Russia, Mongolia, and Korea. The Flora Europaea (Tutin et al., 1964–1980
), whose botanical nomenclature practice is more or less comparable to Kartesz's, was frequently used in standardizing the nomenclature for the East Asian (particularly Siberian) plants.
Genus level
Standardization of generic nomenclature followed Brummitt (1992)
, Greuter et al. (1993)
, Wielgorskaya (1995)
, and Mabberley (1997)
. In general, a generic name was accepted if all these works adopted it. Generic names in the literature on the floras of East Asia and North America that were absent from the above-mentioned works were treated carefully by consulting available taxonomic monographs and continental, national, or regional floras.
Data analysis
Each genus was placed in a family and an order. The placement of genera in families followed Wielgorskaya (1995)
for gymnosperms and Takhtajan (1997)
for angiosperms. Designations of orders followed Takhtajan (1986)
for gymnosperms and Takhtajan (1997)
for angiosperms. Orders were grouped into four major plant groups: gymnosperms, magnoliids, monocots, and eudicots. Three analyses of variance (ANOVAs) were conducted to assess regional differences in taxonomic richness between (1) EAS and NAM, (2) EASs and NAMs, and (3) EASn and NAMn. In each ANOVA, the dependent variable was the log10-transformed number of taxa and the effects were taxon (gymnosperms, magnoliids, monocots, and eudicots), taxonomic level (order, family, genus, species), and region (counterparts of EAS and NAM).
In more detailed comparisons in taxonomic richness between EAS and NAM, monocots were divided into four groups, alismatids, Liliidae, Arecidae, and Commelinidae, and eudicots were divided into five groups, ranunculids, Caryophyllidae, rosids, Lamiidae (euasterids I), and euasterids II, following Qian and Ricklefs (1999)
. These nine groups together with gymnosperms and magnoliids were called phylogenetic groups and were subjected to a replicated goodness of fit test (G statistic; Sokal and Rohlf, 1981
) to test the hypothesis that the proportions of the numbers of taxa in a pair of floras (one from East Asia and the other from North America) for a phylogenetic group are equal to the proportions of the two floras with all phylogenetic groups pooled. For each of the three pairs of the floras in East Asia and North America, four G-statistic tests were conducted, each testing one of the four taxonomic levels of order, family, genus, and species.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Latham and Ricklefs, 1993a
; Cornell and Karlson, 1996
; Caley and Schluter, 1997
; Guo, Ricklefs, and Cody, 1998
) have demonstrated that comparisons of species richness between intercontinental regions can provide insights into the processes and mechanisms of global diversification. Due to the general comparability in geography and climate between East Asia and North America, they certainly provide a unique opportunity for comparative studies on intercontinental patterns of species diversity at a variety of spatial scales (Guo, 1999
). This study provides the first comparison of seed plant diversity in East Asia and North America with land area and latitude (and to some extent climate) adjusted to comparable levels. The results from this study support a widely held notion that East Asia has a higher level of species diversity than North America. As shown in this study, the flora of East Asia has significantly more species than that of North America at both continental and semi-continental scales. At smaller scales, the diversity anomaly in favor of East Asia still holds. For example, R. E. Ricklefs, H. Qian, and P. S. White (unpublished data) showed that the species richness of angiosperms in floras ranging from 10 to nearly 107 km2 in area is greater in temperate eastern Asia than in temperate eastern North America, by a factor of about two. Thus, by combining the results of this and other studies, the diversity difference between East Asia and North America appears to hold at all scales from a whole continent down to at least 10 km2 in area. It is worth mentioning that all of the coastal vegetation of temperate North America was included in this study, and there is no corresponding western coastal vegetation for East Asia. The diversity bias in favor of East Asia might have been even greater if the two regions both included a western coastline.
However, the difference in species diversity between East Asia and North America substantially decreased latitudinally from south to north. For example, southern East Asia has a 1.4-fold higher diversity than southern North America, while northern East Asia, accounting for 63% of East Asia as a whole, has 1.1 times as many species as northern North America. This pattern indicates that the greater taxonomic diversity of East Asia at the continental scale stemmed primarily from the southern part of the region, south of approximately 40° N latitude. About 44.3% of species of northern East Asia do not occur in southern East Asia while only 11.5% of species of northern North America do not occur in southern North America, suggesting that the flora of northern North America is primarily a subset of the southern North American flora. This also indicates that East Asia has a much higher rate of species turnover from south to north than North America.
The question remains why the flora of southern East Asia is so much richer than its counterpart in North America, or in other words, what factors may have created this diversity anomaly. Many previous authors (e.g., Raven and Axelrod, 1974
; Latham and Ricklefs, 1993b
; Axelrod, Al-Shehbaz, and Raven, 1996
; Guo, Ricklefs, and Cody, 1998
; Guo, 1999
; Qian and Ricklefs, 1999
, 2000
) proposed a variety of scenarios to explain this diversity anomaly. The proposed scenarios include (1) East Asia was part of or is closer to, compared to North America, the centers of origin and diversification of flowering plants; (2) East Asia was less influenced by Quaternary glaciations and had a lower extinction rate during those times; (3) East Asia is geomorphologically older and more complex than North America; (4) East Asia was formed from several tectonic plates that came from both Laurasia and Gondwanaland while North America was only part of Laurasia; (5) East Asia received more species from the surrounding areas than North America due to the continuity with central and western Asia, Europe, and Africa; (6) East Asia is influenced by subtropical and tropical floras more than North America; and (7) East Asia has a greater vegetational continuity between tropical, subtropical, temperate, and boreal forests. All of these hypotheses may explain part of the diversity bias in favor of East Asia. These hypotheses have been addressed at length by various authors (e.g., Raven and Axelrod, 1974
; Axelrod, Al-Shehbaz, and Raven, 1996
; Guo, Ricklefs, and Cody, 1998
; Qian 1999a
, b
, 2001
; Qian and Ricklefs, 1999
, 2000
; Tiffney and Manchester, 2001
), and they are not the focus of this discussion. The following discussion focuses on the effect of the collision of the Indian subcontinent with the Asian continent on the species diversity of East Asia. This less discussed hypothesis may explain much of why East Asia is more diverse in plant species than North America.
East Asia may have gained a substantial portion of its species diversity from the collision of the Indian subcontinent with the Asian continent during the Eocene (55–45 x 106 yr ago) (
engör and Natal'in, 1996
). This collision has resulted in the enormous modification of geographic features in southwestern China (an area with Yunnan, Guizhou, and southern Sichuan combined), which was favorable to maintaining previous diversity as well as creating new species. The process of the collision has probably played a significant role in creating the diversity anomaly between East Asia and North America. The northward thrusting of the Indian subcontinent under the southern edge of the Asian continent increased heterogeneity of the Asian landmass through the formation of the Earth's highest mountain ranges, the Himalayas and the Kunlun Mountains, and the generation of the Earth's most extensive continental high land, the Qinghai-Xizang (Tibetan) Plateau, with 2.5 x 106 km2 in area and 4500 m mean elevation above sea level (Axelrod, Al-Shehbaz, and Raven, 1996
). The Himalayas were uplifted several thousand meters over the past 30 x 106 yr, with about 2300–3000 m increase since the middle Miocene (15 x 106 yr ago) (Sharma, 1984
; Axelrod, Al-Shehbaz, and Raven, 1996
). Significant uplift has continued even during the past several million years (Noble and Searl, 1995
). In addition to the major Himalaya Mountain range, the collision also resulted in the generation of many separate high mountain ranges generally oriented north-south in southwestern China and produced many geologically young habitats in which a great biological evolution has proceeded (Axelrod, Al-Shehbaz, and Raven, 1996
). The collision may have profoundly affected the species diversity in East Asia, particularly in southwestern China, in several ways. First, the resulting high (usually >6000 m), fairly rugged mountain ranges such as the north-south Hengduan Mountains and Gaoligong Mountains and large river systems such as Nujiang (Salween) River, Lancangjiang (Mekong) River, and Dulong River became natural barriers preventing species from spreading (Li et al., 1999
). A significant number of species became vicariants on different mountains during the orogenic processes, which would favor allopatric speciation. The newly created habitats during the quick and continued uplift of the Himalayas plus the existing habitats along a wide altitudinal gradient provided a wide array of habitat types, in which both relict species and newly evolved species could survive (Wu and Wu, 1996
). In other words, the extinction caused by the uplift of the Himalayas may have been offset by a higher rate of speciation in southwestern China. Second, the collision of the Indian subcontinent with the Asian continent resulted in horizontal compression in southwestern China and adjacent areas (Press and Siever, 1986
). This compression process certainly reduced the total area of the region, although some of the reduced horizontal area was transformed into mountain slopes through vertical expansion. The low rates of extinction and abundant opportunities for speciation, as discussed above, likely resulted in the increased species density in southwestern China.
Southwestern China, because of its extremely rich flora (over 17 000 species of seed plants), has been considered the cradle of the East Asian biota (Li et al., 1999
). According to Wu and Wu (1996)
, the Yunnan Plateau and part of Hengduan Mountains, which only accounts for a small proportion of the land area of southwestern China, have over 12 000 species of vascular plants. Many of the widely distributed genera are extremely diverse in southwestern China, and their species richness often drastically decreased eastward to southeastern China, even though both southwestern and southeastern China are located in the same climate zone. One of such examples is Rhododendron. Of its about 800 species worldwide, 330 occur in southwestern China, and no less than 277 occur in Yunnan (Moore, 1991
), a province with many highly rugged mountains (a single mountain may support 25 species of Rhododendron). But this genus has only 142 species in southeastern China, an area even larger than southwestern China. The species of Rhododendron in southwestern China vary widely in size and habit. Some are trees that can grow to a height of 24 m, and others are tiny, mat-like forms (Moore, 1991
). Different characteristics of Rhododendron reflect their adaptation to their widely varying habitats. Among many of other examples showing this markedly decreasing trend in species diversity from southwestern to southeastern China are Gentiana, Pedicularis, and Primula. They have respectively 176, 235, and 187 species in southwestern China, and have only 27, 19, and 25 in southeastern China. The great topographic diversity largely resulting from the collision of the Indian subcontinent with Asia has been invoked as stimulating allopatric speciation. Newly evolved species in southwestern China may have penetrated to subtropical and warm temperate regions to the east and penetrated to temperate and boreal regions in the north. Testing this hypothesis will be a large, complicated research program, requiring the addition of new data (e.g., fossils and DNA sequencing of plants).
The flora of East Asia may also have been enriched by the addition of the Gondwanaland element that the Indian subcontinent brought with it when it broke away from Gondwanaland. However, this addition may not be significant because the flora of the Indian subcontinent experienced a dramatic change during the approximately 100 x 106 yr of its drift before it made subaerial contact with Asia (Briggs, 1987
), in response to the subcontinent new, largely tropical position and to changes in topography (Moore, 1991
). According to Mani (1974)
, the richness of the Indian flora is attributable to the immigration and colonization of plant species from widely different bordering areas. The Indian subcontinent is dominated by families that are found in Southeast Asia. There is a striking poverty of endemic genera. Although there is a large African element, the Malayan floristic element is dominant on the Indian subcontinent (Moore, 1991
).
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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