NAT. NAT. HIST. BULL. SIAM Soc. 51( 1): 97-108 ， 2003

THE LOCATION OF THE INDOCHINESE-SUNDAIC BIOGEOGRAPHIC TRANSITION IN PLANTS AND

David S. Woodrufl

ABSTRA Cf

Th e biogeographic transition between the In dochinese 組 d Sundaic (Indom a1 ay) bio 凶 1ies 1ies today on 出e 百lai-Malay peninsu1a. A literature review suggests that there may actu a1 1y be be two m 司jor phytogeographic transitions about 500 km ap 副. Most botanists follow V AN

STEENIS STEENIS and GOOD and place 出 e transition at 血e Kang 釘 '-Pattani line near the Th ai-M a1 ay

border. border. Others p1ace mo 陀 imp 口氏 ance on the transition between seasonal evergreen and mixed moist moist deciduous forests that occurs ne 訂 the Isthmus of Kr a， between Chumpon on the east coast coast and Tavoy on the west coas t. A recent revi 巴w of species distributions shows 白at a

high1y high1y significant number of their species and subspecies r組 1ge 1imi 臼 occur just north of 血E Isthmus Isthmus of Kr a and are associated with the northern phytogeographic transition but not 白E southern southern one. 百 le transitions in other groups of anima1s are less well documented but gene ra1 1y 1ie 1ie north of Kr a in the northern peninsu1a. A review of 由e c1 irnato10gic a1 and p a1 eogeographic

factors factors a1 1eged to co 口紅'01 血 ese transitions shows 伽 t we c創 mot adequate1y account for 自由

cu 町 ent positions.τ 'h e possible ro1e of 甘ans-peninsu1ar seaways as barriers permitting the divergence divergence of 白e In dochinese and Sundaic biotas is discussed and the directions for future

research research to test various hypotheses 紅 e outlined.

Keywords: Keywords: zoogeography ，phytogeography ，pa1 eogeography ，Te 昨iary seaways ，Tha i1 and ，Ma- 1aysia ， Isthmus of Kra ， Sunda1and

INTRODUCTION

Th e biotas of two different zoogeographic subregions and phytogeographic provinces meet in 官 lailand and 血is biogeographic provincial boundary begs bo 出 historical and biological biological expl 組 ation. (1 876) ， the founder of zoogeography ， recognized a major faunal transition in Thailand but the phenomenon has received little scientific scientific attention since then. He put the boundary between the northem Indochinese Subregion and the southem In do-Malayan Subregion in 出e Tenasserim Range at the northem

end of 血， e Thai 司 Malay peninsula. Th e corresponding phytogeographic transition sep 訂 ates the the Continental SE Asiatic Fl oristic Pr ovince and the Malayan Fl oristic Province of the

Ind o- Malayan Subkingdom (GOOD ，1964). 百出 transition is typically shown ne 紅 the Thai-Malay border 900 km further south.

IEco10gy ，Behavior ，Evo1ution Section. Division of Bio10gica1 Sciences ，University of California ， San Diego ，La

Jolla Jolla CA 92093 ・0116 U.S.A. Address Address for editori a1 correspondence: Professor David S. Woodruff Division of Bio10gica1 Sci 巴nces ，University of of C a1 ifornia ， San Diego ，La Jolla CA 92093 ・0116 U.S.A. Te1ephone: 1・858-534-2375 FAX: 1-858-534 ・7108 E-ma i1: [email protected] Received Received 31 October 2002; a∞ epted 7 May 2003.

97 97

BIOGEOGRAPHIC TRANSITION 99

PosmON OF THE BIOGEOGRAPHIC TRANSmON

Although Although there is general agreement that a major biogeographic transition occ 町 s on the the Th ai-Malay pen 泊sula there 釘 'e diverse opinions 出 to its location and breadth. 百le position position of biotic 甘ansitions may be associated with present or former physical and environmental environmental barriers to dispersal ，and with the effects of dispersal ， competition and invasibility ，and obligate co ・dis 凶bution (BROWN & LοMOL 刑 0 ，1998). 百lepu 中ose of 出is review review is to draw attention to 出e problems associated with 白is particular biogeographic transition transition and to 血us con 凶bute to their solution. A1 though Th ailand sits centrally 泊血e Oriental Zoogeographic Region the faunas of northem and southem 官 lailand are clearly different at 血e species leve l. Herein ，1 will use 出e terms In dochinese 組 d Sundaic for the northem and southem biotas ，respectively. 百le zoogeographic zoogeographic transition was placed at the northem end of the peninsula ， at about 14 明 by W ALLACE (1 876). One hundred years later WELLS (1 976) provid 吋血 e frrst species level analysis analysis of the transition and ， based primarily on 出e dis 位ibutions of Sundaic birds ，he concluded concluded that the transition lay just no 目h ofthe Isthmus of Kr a at 10 0 30'N. More recently ， using using bird distributional records extending 合om Chang Rai to Singapore ，my colleagues and 1 have refined 出is result (HuG 田 S EJ AL. ， 2003; ROUND EJ AL. ， 2003). Our statistical analyses analyses revealed a significantωmover in bird species assemblages between 11 0 and 13 0 N on 血e northem 百lai-Malay peninsula. We found 血at 152 species ， or half the forest- associated associated species ，have range limits in 血is area (Fig. 1). 0 町 fmding 血at the avian transition transition lies a little further no 巾 of 血e latiωde described by WELLS 泊 1976 is ， in fact ， what he would have predicted and has subsequently found (WELLS ， 1999: xxi). Comparable documentation of the magnitude and location of the zoogeographic transition transition in other phyla are st 出 lacking but ， as a broad generalization ，manlmals ， amphibians and butterflies appear to similar exhibit pattems (LEKAGUL EJ AL. ， 1970; CORBET & PENDLEBURY ， 1992; INGER ， 1999). PAUWELS EJ AL. (2002 ， 2003) have surveyed reptiles in two provinces 200 km north and south of 血， e Is 血mus of Kr a，respectively. Th ey found 108 of of 152 species may have range limits in 血e northem peninsula but estimate 白紙出eir species species lists are only 55 -6 5% complete and 血ink 血e In dochinese-Sundaic 仕組sition lies furtherno 油田紅Kanchanaburi at about 14~. 百le geographic ranges of manlmals described by LEKAGUL & M cN EELY (1 988) and CORBET & HILL (1 992) show 伽.t numerous species and subspecies boundaries lie ne 釘 the Isthmus of Kr a. C HAl MANEE (1997) has analyzed 出e extant rodent fauna and found 血at ， of 35 species of murines ， 16 species 釘 'e re 抑制ed to 出， e no 叫 1 and 5 species to the south of the Isthmus; 白， e remaining 14 species occur in bo 血In dochinese and Sundaic subregions. For 16 species of sciurines and 10 species of petaurist 泊es ，白 e counts 釘 e 2:8:6 ，and 4:2:4 ，respectively. Of these 61 species of rodents ， 61 61 % have range limits associated with the Isthmus of Kr a. Pleistocene fossils ，on the other hand ，show 血at individual species shi 負ed th 出 r加 ges 40 0- 500 km north and south of today's today's and 白紙出e early Pleistocene transition lay 500 km south of Kr a. TOUGARD (2001) reviewed reviewed the Pleistocene and recent distribution records of large mammals (1 29 extant sp 回 ies of primates ，c紅 nivores ，proboscidi 組 s，perissodactyls 佃 d artiodactyls) 泊 southeast Asia and also concluded 出at ，al 出ough today's provincial boundary lies at 出e Isthmus of Kr a，血 e transition shifted further south to northem pen 泊sul 訂 Malaysia during Pleistocene hypothermals. hypothermals. Amphibian and butterfly species also show tumover at the isthmus; 泊 amphibians amphibians more In dochinese species extend south 出an Sundaic species extend north of 100 100 DA Vl D S. W ∞ORUFF the the transition (INGER ，1966) ，and the reverse is reported in butterflies (CORBET & PENDLEBURY ， 1992). Th ere are app 釘 ently two phytogeographic transitions on 血e pen 凶sula of which the best best known lies near 血e 百凶ーMalay border ，500 km south of the Is 伽 nus of Kr a. 百lis southem 住'ansition ，between perhumid evergreen rainforest and wet seasonal evergreen r副 orest ， is widely portrayed 部出em 吋or In dochinese-Sundaic plant boundary (v AN Sτ 宙開s， 1950; GOOD ， 1964; KENG ， 1970; WHITMORE ， 1984; UDVARDY ， 1969; ASHTON ， 1992; 1992; R1 CHARDs ， 1996; MORLEY ， 2000). W natA MANAYA 阻 EJ' AL. (2002) also followed VAN S百 ENIS (1 950) in their regional conservation habitat-mapping project and put 出e boundary boundary between the In dochina Bioregion and the and Bioregion ne 紅白eτ 'h ai-Malay border. Th ere is also a second transition ，between se 部 onal evergreen rainforest rainforest and mixed moist deciduous forest ， north of 由e Isthmus of Kr a. 百lis northem 住佃sition is widely shown on 1紅 'ge-scale vegetation maps but not fonnally documented 祖 師 litera 知re.T 盛岡・AANJAN (1 986) 叩 d COLLINS EJ' AL. (1991) pla 田 d 恥 boundary between the the In do ・Chinese and Malesian floristic regions at the Kra is 由mus. As birds are typically co-distributed co-distributed with pl 制 communities (MAcARn 町 R&MAcARn 町 R，196 1) the relationship between the avian 凶 nsition and the two ph: 戸ogeographic 回 nsitions is puzzl 泊g. Th e birds

appe 紅 to ignore the southem transition and their relationship wi 白血e northem transition is is recognized but not yet well documented (HuG 田 S ET AL. ， 2003; ROUND EJ' AL. ， 2003). Our understanding of the position of the southem 住ansition has been modified several times times during the last cen 飽ry. R1D LEY (1 911a ， b) was among 也 e frrst to note 白紙， on 也e west west coast ， the flora of what was then called Lower Siam was quite different 仕om that of Malaya. Malaya. He con 回 sted the heath floras of Satun (百則 land) and nearby Pahang (Malaysia) and listed many Malay genera 出at are not found north of Al or Se 町 (Malaysia). Al or Setar is is a town 16 km SSE of Kangar (see Figure 1). He placed the floral boundary between the northem sandstone and limestone hills of Perlis and the granite hills 白at begin south of Alor Alor Setar. On a foldout map he drew a straight line separat 泊g 血e two floras f旨'om just

south south of Alor Setar in 血e west to just south of Sai B 凶行'h ailand) on the 伊 lf co 部 t， 20 km ESE of Pattani. KLoss (1922) changed the orientation of the line from SW-NE to N-S and mapped 血 e transition to a line between Al or Setar and Songkhla. V AN STEENIS (1950: (1950: lxxi-lxxii) affmned this location and provided more details on 由 is transition based on dis 凶 bution maps he prepared for 1，200 genera of plants. He found 血at 375 genera of Sundaic Sundaic plants reach their northem 1加lits ， and 200 genera of In dochinese plants reach their southem limits ，at 血is line. WY TO. π・S阿国(1 964)andK 聞 G (1 970) suppo 巾 d 血is placement of of the transition. Th en ， as a result of additional fieldwork on species of dipterocarps WHITMO 阻 (1984) rotated 由is boundary to a line like Ridley' s，ruru 由， g west-e 槌 t between Kangar (Malaysia) and Pattani (官 lai1 and) at about 7明 .BA 阻 R EJ' AL. (1 998) also put 血e boundary boundary of 出e Malesian flora on 出， e Th ai-Malay border and presented new data on 出e ranges ranges of selected palms (including rattans) ，bamboos ， euphorbs (understorey s胎ubs) ， ela 閃 carps 伸明s) ，組da genus of fems. 百首 s Kang 紅一.Pattani line remains the widely accepted accepted position of the In dochinese-Sundaic bound 釘 y. WHITMO 阻(1 984) reviewed the many changes 白紙 occur across 出， e Kang 紅 :....p 組制i line line in the composition and function of the forests. To the north ， there is a massive drop 泊 S戸cies richness ，associated with the disappearance of the remarkable series of ecologically sympa 凶 c congeneric species so characteristic of great westem Malesian ra 泊forests. Also to 白e nor 血 there is more 企'equent aOIl Ual flowering of c佃 opy trees and a dampening or BIOGEOGRAPHIC TRANSITION 101

absence absence 'O f synchr 'O n'O us mass (s 'O metimes ca1 1ed mast) fruiting. He attributed the p'O siti 'O n 'O f the f 'O rest transiti 'O nt 'O the increasing seas 'O na1i ty and n'O ted 白紙n'O rth 'O f 出is line nearly a1 1 seedlings 'O f climax species die du 出19 their frrst 命y seas 'O n， within 12 m 'O nths 'O f germinati 'O n in m 'O st years. ASHTON (1 992 ， 1995) agreed with these gener a1 izati 'O ns and c'O ncluded 白紙a1 th 'O ugh we d 'O n'O t have statistics t'O dem 'O nstrate it ， there is a massive decrease 泊 species n'O rth 'O f the K 阻 g紅一.Pattani line. M 'O re 由an twice 出 many plant genera reach reach their n 'O rthern range limits here as th 'O se 白紙 reach their s'O uthern ，and there is n 'O d'O ubt 白紙偽is difference is greater still at the species leve l. In In additi 'O nt 'O the need f'O rf 'O rm a1 mapp 泊g there are tw 'O 'O ther aspects 'O f 出e Kangar- Pattani Pattani transiti 'O n 白紙sh 'O uld be n'O ted here. First ，climat 'O l'O gic a1 rec 'O rds d 'O n'O t supp 'O rt the alleged alleged ass 'O ciati 'O n 'O f the transiti 'O n between f 'O rest types 加 d 血es 'O uthernm 'O st p'O siti 'O n 'O n the the pen 泊sula where a 命y seas 'O n 'O f 'O ne m 'O nth's durati 'O n 'O ccurs. WEL 凶(1 999: xix ，Map 2) 2) sh 'O ws that 血.e east side 'O f the centr a1 peninsula d'O es n'O t experience a dependable m 'O nth's dr 'O ught bel 'O w the latitude 'O f Surat 官 lan i.百lU S，'O n 出ee 部 t side 'O f 血e peninsula 'O ne w 'O uld expect the 住ansiti 'O nt 'O be 300 km 白地er n'O rth if seas 'O na1 dr 'O ught was 血e c'O ntr 'O lling fact 'O r. Sec 'O nd ，s 'O me writers have described the ge 'O graphic l'O cati 'O n 'O f 由is transiti 'O n inc 'O rrectly. Th e transiti 'O n at the K 佃 .gar-Patt 創 U line lies ab 'O ut 500 km s'O uth 'O f 白 e Isthmus 'O f Kr a but despi 総 their ge 'O graphic separati 'O nb 'O白 WH 汀MO 阻(1 984: 201) and 胞団ARDS (1 996: 403 -4) have described 血is s'O uthern 釘ansiti 'O ns 'O mewhat ambigu 'O usly as 血e “Kr a ec 'O t'O ne". SOEPADMO (1995) ，discussing plan 臥 explicitly equates 出e Isthmus 'O f Kr a with V AN Sτ 芭.E NIS' 知 rn 'O ver 'O f plant genera at 血eτ 'h ai-M a1 ay b 'O rder. HlRAI E/' AL. (2002) (2002) entitled their paper 'O n tree shrews “f'O und in 血es 'O uthern p紅 t 'O f the Is 伽 nus 'O f Kra" Kra" even 白 ough their study site w 部 4∞km further s 'O uth. Similarly ， 'O thers (e.g.: TOSI E/' AL ， 2002) have claimed evidence f'O ra bi 'O ge 'O graphic barrier at 血e Isthmus 'O f Kr a when their their 'O nly evidence has been the 'O bservati 'O n 'O f di 佐erent anim a1 s 仕'O m sites hundreds 'O f km n'O rth and s'O uth ，respectively ，'O f Kr a. 百 e term Isthmus 01 Kra must be used with greater greater pr 回l isi 'O n and 血e term Kra sh 'O uld be reserved f'O r the n'O rthern transiti 'O n， which w i1l n'O w be described. 百le sec 'O nd ，'O rn 'O rthern ，transiti 'O nωcurs ，'O n 血e east side 'O f the penins u1 a， immediately n'O rth 'O f the Isthmus 'O f Kr a ne 紅白et 'O wn 'O f Chumph 'O n at 10 0 3'N. Fl'O ristic a1 1y ，出 is transiti 'O n is between wet seas 'O na1 evergreen dipter 'O carp rainf 'O rest and mixed m 'O ist decidu 'O us f'O rest. 百le mixed m 'O ist decidu 'O us f 'O rest differs pr 'O f'O undly 企'O m the rainf 'O rest 泊 stature ， absence 'O f emergents ，decidu 'O usness (nearly 100%) ，s 'O il surface envir 'O nment ， and generic c'O mp 'O siti 'O n (ASHTON ， in litt. ，1999). 官邸n'O rthern phyt 'O ge 'O graphic tr 叩 siti 'O n is is n'O t well documented in the literature. RICHARDS (1 996: 162) describes it as a transiti 'O n between semi-evergreen rainf 'O rest and m 'O ns 'O'O nf 'O rest. Acc 'O rdingωW 町 rMO 阻 (in litt. ， 1995) 1995) evergreen rainf 'O rest is 問 placed by decidu 'O us “m 'O ns 'O'O n" f 'O rest at Chump 'O n. Th ere is is at 'Ota1 fl 'O ristic change inv 'O lving the wh 'O le fl 'O ra including many genera. In c 'O ntrast t 'O 血e impressi 'O n given by V AN STE 町 IS (1 950) ，WHITMO 阻 views 出eKang 釘 '-Pattar 首位制lsiti 'O n as as inv 'O lv 泊g species but n'O t genera and the Kra ec 'O t'O ne as inv 'O lving am 'O re sig 凶自 cant 臥'O n'O mic turn 'O ver. As 町'O N (in 1批， 1999) c'O ncurs ，and n'O tes that despite what V AN S1 芭ENIS wrote 泊 1950 he re a1 1y envisi 'O ned a br 'O ad 民'O t'O ne extending 合'O m 血e Kangar- Pattani Pattani line n'O r血 f'O r 500 km 'O n 血ee 出 tc 'O ast and f'O r 80 0- 1000 km 'O n 白 .e west side 'O f

出epe 凶nsula ， as far n'O rth 儲 the wet seas 'O na1 evergreen f 'O rest extends 'O n the Burmese side ，面白e Dawna Range at 17 0 30 'N where such rainf 'O rest genera as Durio fmally peter 'O ut. On 由e Th ai side a1'O ne ，a big change 雌 es place at quite a sh 岬 b'O undary just n'O rth 102 102 DAVID S. W ∞>D RUFF

of of Chumpon. In a comp 佃 ion paper ，ROUND El' AL. (2003: Fig. 1) show a further northward distribution distribution of the Sundaic birds on 出e moister (Burmese) 也anon 也e drier 官 1ai side. 百lI s supports supports W 乱凶， (1 999) 訂 'gument 白紙 vege 旬凶 on type is the major constraint on bird dis 位ibution. Both Wh itmore and Ashton reviewed a preprint of HUGHES El' AL'S (2003) repo 武 showing showing the avifaun a1住佃 sition to be coincident wi 由也.e northem phytogωgraphic transition and neither were su 中市ed 出at no omithologic a1 boundary 0∞ urs at 血e southem 回iIl sition as ，泊 their opinions ， the forest there shows on1 y minor change in s佐uc 旬 re and appearance. Unfortunately ， the distribution a1 records of the M a1 esian and In dochinese pl 血 t genera and species species upon which their insights are based have never been published and the di 町erences may be more 由加 m 泊or. Th eir knowledge rests on extensive person a1 fieldwork and 出e sca 伽 red herbarium 油田ts known to 0叫ya few bo ぬni 蜘. Unt i1 the r加 .ge limits of plant species species and genera are collected into one database (see P組問LL El' AL. ， 2003) and plotted out out against 出e geography of the peninsula it is difficult to compare 出e avifuan a1 and phytogeographic 位'ansitions. 町出e pl 佃 t generic limits and the bulk of the species limits actu a1 1y lie between 11 0 and 13 0 N ，and not at 7明， as depicted by VAN S百 ENIS (1950) 佃 d those those who copied his figure ， then the plant and bird pattems may be coincident and caus a1 1y related. Further ， the width of the avian 回 nsition may be related to the differences 泊血e plant community dis 凶butions on either side of the pen 泊sula. Th e wet season a1 evergreen rainforest ， which historic a1 1y covered both lowland and hi11 s of the cen 住a1 and northem peninsula ， has been largely repla 田 d 泊 the lowlands 泊 the last last hundred years by farms and fores 町 plantations. Remaining deciduous forest usu a1 1y predomina 蜘 in the lowlands ， and evergreen rainforest is re 柑 icted to the hi11 s，especi a1 1y in 血.e northem penins 叫a. 官 1e apparently sh 紅 p east side avifaun a1凶iIl sition may reflect 泊terspecific competition 加nong c1 0se congeners ， with the Sundaic 旬xabe 泊g at a competitive advantage advantage in moister hi11 side vegetation and the In dochinese 旬xa in drier lowland vege 旬.tion. 官邸is what happens 泊出e 仕組 sition zone between ， for example ， the Sundaic red-bearded bee-eater ， Nyctyornis amictus ，and 也e In dochinese blue-bearded bee-eater ， N. athertoni ， with with the former occurring 泊出e moister evergreen forest of the middle hi11 slopes and 出e latter 加恥耐er foo 削 Is habitat 仏E臥 GUL & ROUND ， 1991). Other cases of apparent competitive competitive displa 回 ment prob 油 ly exis t. 出 NGE 四 LD (1 990) ，ERDE 回 N (2001) ，Tu RNER El' AL. (2001) and WELZEN El' AL. (2 ∞ 3) discuss discuss the problems 白紙釘ise from defming biogeographic boundaries b邸 ed on different data data sets. In出 is case ，we must recognize bo 出 variab i1i ty in 血e qu a1 ity of 血e unde r1 ying dis 住ibution a1 records and re a1 biologic a1 variation in 血er 加 ge limits of di 旺erent groups of of plants and anim a1 s. 百1e latter phenomenon characterizes the better documented faun a1 transition transition in C 印刷1 Am erica where mammals show a clear species tumover 佃 da strong southe r1 y transgression ， while birds ，rept i1 es and 創 nphibians provide little evidence for a faun a1仕組 sition and the opposite dispers a1 bias (STEHLI & WEBB ， 1985; WEBB ，199 1). Further Further an a1 ysis of 血e biogeographic 回iIl sition between In dochinese and Sundaic taxa should should in c1 ude the form a1 documentation of the dis 凶bution pa 恥 ms of individu a1 species in in different phyla. O n1 y then can we begin to 泊vestigate the ecologic a1 determin 組 ts of their their range limits and seek to account for the cu 町 ent position of 出e 回iIl sition. Unt i1 comp 釘 able 加 alyses of the roles of loc a1 physiologic a1 adaptation ，behavio ra1 ecology as it it affects competition 創 nong congeners ， and community invasib i1i ty 釘 'e undertaken ， the determinants determinants of the position of each species boundary c鑑 mot be established. 官lI s is ， of BIOGEOGRAPHIC TRANSITION 103

course ， not a problem peculiar to ; there 紅 'e remarkably few studies of 白e determinants determinants of species ranges in birds (TERBORGH ， 1977) or other organisms (HOFFMAN & BLOWS ， 1994). Th at such investigations 紅 'e worth pursuing is well illustrated by a recent report report comparing tenebrionid beetle ，lizard and rodent communities along an aridity gradient that that determines the position of a phytogeographic province boundary in the Negev desert (KRASNOV & S国間ROT ， 1998).

O 悶 GIN OF THE BIOGEOGRAPHICτRANSITION

AI 血ough the locations of today's biogeographic transitions may be controlled by climatic climatic factors they cannot explain the origination of the different In dochinese 組 d Sundaic biotas. biotas. At some time in the past these two biotas must have been ， or were most probably ， separated separated for long enough for their differences to evolve. Recent studies of speciation in diverse diverse taxa suggest 出at divergence of this magnitude probably required one or more periods periods of a1 lopatry of on the order of 10 5 to 10 6 ye 釘 s (A VISE ， 2000). Unlike the major biogeographic biogeographic transition associated with W a1 lace's Line and the eastem edge ofthe Sundaic biota biota (W 田 TMO 阻， 1987; METCALFE ET AL. ，2001) ，no simple physical barriers are clearly associated associated with the transitions on the Th ai-Malay peninsula. The peninsula has existed in approximately approximately its present form for a1 1 of the Tertiary (HALL ，2002) ，f，訂 longer th 組曲e species 出 at characterize the two provinci a1 biotas. Similarly ，dis 住ibution a1 changes associated with with multiple Pleistocene glacial phases and isolation in glacial forest refugia (HEANEY ， 1991; 1991; MORLEY ， 2000; VORIS ， 2000; GATHORNE-HARDY ET AL. ，2002) ぽ e unlikely to have resulted resulted in this level of biotic divergence. What circumstances ，then ， have promoted the observed observed development of provincialism? In a companion paper (WOODRUFF ，2003) ，I hypothesize hypothesize that previously unappreciated marine transgressions during the Mi ocene and the the Pliocene breached the cen 甘al peninsula and permitted the evolutionary di 妊"e rentiation of of some congeners north and south of the seaways. Th e older of these transgressions apparently apparently flooded the isthmus for about 10 million ye 訂 s from 24 to 13 million years ago (middle (middle Miocene). More recently ，sh a1 low marine straits probably formed again for about 1 million ye 釘 s during the early Pliocene from 5.5 to 4.5 million years ago. Today a sea level level at +100 m would flood the peninsula in two places (WOODRUFF ， 2003); in the north strait a strait would open between Surat Thani and Kr abi and ， in the south ， a strait would open between between Songkhla and Kangar (Fig. 1). Transgressions in the Miocene and Pliocene were probably probably in the range of + 140 to + 150 m above today' s sea level ， although HUTCHISON (1989) (1989) estimated the Miocene 仕組sgression was +220 m. The northem and southem straits were were probably 30 ー100 km wide and 40- 50 km wide ，respectively. Both were oriented roughly roughly north-south and contained a number of prominent islands. Between these two straits straits 80% of southem Th ailand would be submerged and forest habitat would remain only on the emergent Nakhon Si Th ammarat Range (B 叩 thad Range) and westem hills. Th ese hypothetic a1 seaways are the only barriers proposed so far to account for the origin origin and differentiation of the Indochinese and Sundaic biotas. Although such seaways have have not featured in most recent biogeographic reconstructions ，some earlier workers were aware aware of their possible existence (e.g. PARNELL ， 2000). GERINI (1909) cited geological and historic a1 evidence to show that the land between Kedah and Songkhla is an old seabed and argued argued that boats crossed the peninsula here until a thousand years ago. RIDLEY (1 911b: 59) 59) was explicit: ‘One can gather from the flora that at no great length of time ago the 104 104 DAVID S. W ∞DRUFF

Malay Peninsula was cut '0百台'O m Bunnah s'O uth 'O fKedah ，by the sea'. CORBET (1 941:116) accepted accepted Ridley' s 紅 gument f'O ra sea channel but 釘 gued 血at it lay further n'O吋 1: ‘也 at while while the present Malay Peninsula has und 'O ubtedly been separated 合''O m the Asiatic mainland sinc 泡 the advent 'O f the present species 'O f bu 恥 rflies ， this separati 'O n 'O ccuπ'ed n'O rth 'O f Kedah and the s'O uthward spread 'O f insects and plants has been 'O bstructed by a barrier which is 1紅 'gely climatic'. An y relati 'O nship between 出 is hist 'O rical seaway and an earlier prehist 'O ric fl 'O'O d回g ass 'O ciated with the +5 m hypsithennal high stand (7 ，000 yr BP) and the the current bi 'O ge 'O graphy has yet t 'O be established. Clearly ， the physical 'O r envir 'O nmental barriers barriers 白紙 perr 凶伽d the ev 'O luti 'O nary divergence 'O f the In d'O chinese and Sundaic bi 'O tas need further investigati 'O n.τ 'h ere is currently n 'O evidence f'O ra seaway at 由e Is 位lffiUS 'O f

Kr a 泊出e past 25 milli 'O n ye 紅 s. 百1ese 'O bservati 'O ns raise the questi 'O n 'O f whether the palyn 'O l 'O gical rec 'O rd (f 'O ssil p 'O llen grains) grains) all 'O ws US t 'O r，民'O nstruct the hist 'O ry 'O f this transiti 'O n. MORLEY (2000) reviews the still still limited rec 'O rd and provides the m 'O st c'O mplete s卵白esis available 'O n the hist 'O ry 'O f rainf 'O rests in s 'O utheast Asia since the Cretace 'O us. T 'O day's f'O rests 住'ace their 'O rigins back t'O出 ee 釘 Iy Tertiary when the s 'O utheast Asian peninsula was vegetated by c 'O nifers and temperate temperate angi 'O spenns 'O f La urasian 'O rig 泊. Since 白 en the 'O riginal fl 'O ra has been invaded by G 'O ndwanan species frrst 合om In dia and later fr 'O m Aus 釘alia. T 'O day's very diverse fl 'O ra (perhaps (perhaps 8000 species and 1400 genera 'O n 血e 官1ai-Malay peninsula) is a product 'O f 出 is d'O uble invasi 'O n hist 'O ry. In dia c 'O llided int 'O Asia ab 'O ut 45 Ma and intr 'O duced such G 'O ndwanan elements as the dipter 'O carps t 'O s 'O utheast Asia. c 'O llided with the Asian Asian plate ar 'O und 22 Ma 叩 d introduced varl 'O us elements 'O f the current fl 'O ra including 出e Myrtaceae (1 7 Ma) ， the fern Stenochleaena (9.5 Ma) ，and gymn 'O spenn Podocarpus (3.5 (3.5 Ma). Thr'O ugh 'O ut the E 'O cene the vegetati 'O nw 'O uld be described as tropical evergreen f 'O rest but 泊出e Olig 'O cene and early Mi 'O cene the l'O wland rainf 'O rest w 邸 replaced in many 釘 'eas by m 'O ns 'O'O n (seas 'O nal) f 'O rest ass 'O ciati 'O ns. 百1e dipter 'O carps diversified du 巾19 白 e Mi 'O cene and have d'O minated the Sundaic f 'O rests since 出 en. Acc 'O rding t 'O MORLEY 出 e vegetati 'O n thr 'O ugh 'O ut the Ne 'O gene has inv 'O lved a shifting balance between f'O ur d'O minant ass 'O ciati 'O ns: rainf 'O rest ，seas 'O nal m 'O ns 'O'O nf 'O rest ，decidu 'O us f 'O rest and sav 組問. Th e evidence suggests 出 at in the rainf 'O rest d'O ubled 泊 area in the Late Mi 'O cene but gave way t 'O savanna in the drier Pli 'O cene and c'O'O ler Pleist 'O cene. Th e savanna 'O ccupied b'O th current c'O ntinen ta1紅 'eas and the exp 'O sed c'O ntinental shelves acc 'O rding t 'O traditi 'O nal 'O pini 'O n，alth 'O ugh KERSHAW Ef AL. (2001) 佃 ds 'O me 'O thers disagree. M 'O st

b 'O tanists have held 白紙savanna with abundant gr 酪 ses extended fr 'O m c 'O ntinental Th ailand t 'O and bey 'O nd during hyp 'O thennal peri 'O ds 泊 the Pleist 'O cene. Dur 担g such c'O'O l peri 'O ds the rainf 'O rest may have c 'O ntracted int 'O refugia 加 western and 血e Mentawi islands ，加d 泊 n'O rthern B 'O rne 'O and p 'O ssibly western Java. It is unclear whether rainf 'O rest persis 旬 d 'O n the exp 'O sed c'O ntinen 凶 shelf 'O ther 白組制riverine gallery f'O rest; KERSHA W Ef AL. ， 2001 believe that rainf 'O rest remained extensive and d'O ubt the 'O ccurrence 'O f the “世 yc 'O rrid 'O r" described ab 'O ve. Alth 'O ugh MORLEY'S (2000) palyn 'O l 'O gical hist 'O ry has been useful useful in testing and discrediting such hypo 曲目白部 the durian 白 e'O ry [d 町 ians as am 'O del 'O f the m 'O st primitive rainf 'O rest trees] ，and the n'O ti 'O n 白紙s'O utheast Asian rain f 'O rests were the the w 'O rld' s 'O ldest ，it does n'O t explain the p'O siti 'O n 'O f the cu 町 ent bi 'O ge 'O graphic transiti 'O n. Th ere is n'O thing in Mo 悶 .EY'S acc 'O unt 出 at speaks t'O白 e 'O ccur 民 nce 'O fa present day transiti 'O n at either the Kangar-Pattani line 'O r at the Isthmus 'O f Kr a. He d'O es ，h 'O wever ， rec 'O gnize 出 at a transiti 'O n 'O ccurs at 血e Isthmus 'O f Kr a， between s'O uthern rainf 'O rest and what he calls n 'O rthern m 'O ns 'O'O nf 'O rest with tr 'O pical rain f 'O rest patches. --且nu弓 BIOGEOGRAPHIC TRANSITION U

RECOMMENDATIONS FOR FUTURE RESEARCH

In In conclusion ，it is not yet clear when or how the differences between the Indochinese and and Sundaic biotas developed. Al 出ough they have not yet been applied to the southeast Asian Asian region ，current hypotheses of diversification invoked elsewhere in the tropics include the the refugia model ， the riverine model ， the vanishing refuges model ， the disturbance model and and the gradient model (MORITZ ET AL. ，2000). 百le last mentioned c如 occur between p紅 apatric populations and without physical barriers to gene flow ， while the other models diversification of diversification all require barrier-supported speciation in allopatry. Similarly ，we cannot explain explain the cu 町'ent position(s) of the transitions between various groups of fauna and flora and and we have to allow that the position(s) have changed in the past ， as suggested by the ranges ranges of individual manlffial species during the Pleistocene (CHAIMANEE ， 1997; TOURGARD ， 2001). 2001). To test the various physical and environmental hypotheses ， a five-part multi-faceted research research program should be initiated. First ，it is essential to survey existing dis 位ibution pattems pattems and compile reliable databases of locality records. Th is has been undertaken with birds birds to good effect (HuG 田 S ET AL. ， 2003; ROUND ET AL. ，2003) ， but other phyla appear well-enough well-enough known taxonomically to permit preliminary analysis ， including mammals ， reptiles ，amphibians ，butterflies and flowering plants. Perhaps detailed studies of today's plant plant species dis 位ibutions will provide us with insights that are not in the fossil record. Second ， for those taxa represented in the fossil record ，effort should be made to recons 加 ct their their paleo-distributions. 百lI rd ，alleged climatic control of the position of the northem and southem phytogeographic transitions needs to be verified. Accurate ，cu 汀 ent meteorological maps are a prerequisite for testing the alleged associations involving duration of the dry season season and soil moisture availability on both a regional and a microgeographic scale. Fourth ，estimates of the age of the transitions can now be obtained using phylogeographic methods. methods. One recent report on the diversification of tree-squirrels (Sciuridae) in Sundaland traces traces a burst of speciation to the lowest pre-Pleistocene Cenozoic sea-level stand ，1 1. 4 to 10.5 10.5 million years ago (MERCER & ROTH ， 2003). Th e molecular genetic comparison of Indochinese Indochinese and Sundaic sister-species with appropriate molecular clock calibrations may reveal reveal the approx 加late age of the development of the transitions. Genealogical concordance across across multiple co-distributed species implies shared historical biogeographic factors shaping their their distributions. Phylogeographic comparisons may ， for example ，indicate that many species-pairs species-pairs originated at the same time or times (e.g. ，1 ，2，5， 10 or 15 million ye 訂 s ago) and and thus help focus further research. Highly informative regional phylogeographic analyses involving involving vertebrates in the southeastem U.S. A.， in lowland Amazonia ， in Queensland rainforest rainforest fragments ，and in glaciated Europe are reviewed by A VISE (2000) ，HEwm (2000) 叩 dMo 町ロET AL. (2000). Finally ， the geology ，geomo 中hology and paleoclimatology of of the Th ai-Malay peninsula needs re-examining for evidence of the type of former physical or or environmental barriers sough t. Th e ultimate goal of this report is thus to stimulate such research ， for as WALLACE (1 869: 154) pointed out ‘a knowledge of the distribution of animals animals may reveal unsuspected facts in the past history of the earth'. 106 106 DAVID S. WOODRUFF

ACKNOWLEDG 勘ffiNTS

I 血ank Peter Ashton ，Warren Brockelman ，Stuart Davies ，Jennifer Hughes ，Philip Round ，David Wells ，Peter van Welzen ，and Tim Wh itmore for guidance and stimulating comments on the manuscript ，and the U.S. National Science Foundation for support.

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