Extinction and survival in the endemic vascular flora of ascension island

81ologwa/Con.~ert'atton 17 (1980) 207- 219

EXTINCTION A N D SURVIVAL IN THE ENDEMIC VASCULAR FLORA OF ASCENSION ISLAND

Q. c . B. CP,or~K

CoiTus Christi College, Cambridge, CB2 1RH, Great Britain

ABSTRACT The available inJormation on the ten species of endemic vascular plants recordedji'om Ascension Island (including O l d e n l a n d i a adscensionis ( DC. ) Cronk andXiphopteris ascensionense (Hieron.) Cronk j o r which new combinations are proposed) is summarised. The present state of the population is given/br each species, reasonsjor extinction or survival are suggested and the need Jot conservation is discussed. When the island was discovered it was almost barren with only about 25 indigenous vascular plants. The rubiaceous shrub O l d e n l a n d i a adscensionis has become extinct and three other species are possibly extinct. The rest were observed in 1976. The main cause oj this extinction is the introduction o f alien plants which is now irreversible. It is thereJore probable that jurther extinction will /bllow.

INTRODUCTION Ascension Island (lat. 7°57'S, long. 14°22'W) is a volcanic island o f 9 4 k m 2 in the S o u t h A t l a n t i c Ocean, One o f the p e a k s o f the M i d - A t l a n t i c Ridge, it has been the scene o f persistent eruptive activity f r o m its origin c. 1.5 million years ago (Bell et al.. 1972) until p e r h a p s as recently as a few h u n d r e d years ago ( A t k i n s et al., 1964). Ten o f the a p p r o x i m a t e l y 25 i n d i g e n o u s vascular p l a n t species are endemic to the island. O f the endemics Sporobolus durus Brongn., Hedyotis adscensionis DC.,']" and Do'opteris ascensionis ( H o o k . ) O. K u n t z e have not been seen since 1889 a n d "1"Linnaeus first described this species as SherardiaJ~'utieosa L. and later de Candolle redescribed it as Hedyotis adscensionis DC. However, although distinctive, it comes closest to the genus Oldenlandia L. emend. Bremekamp, to which it can be transferred. Therefore the name Oldenlandia adscensionis(DC.) Cronk comb. nor'. (basionym: Hedyotis adscensionis DC., Prodr. IV. p.419 (1830): syn. Sherardia ]ruti('osa L. Sp. PI. 1,103 (1753)) must be used (the combination OldenlandiaJruticosa is preoccupied by a synonym of HedyotisJruticosa L.). 207 Biol. Conserv. 0006-3207/80/0017-0207/$02"25 ,~' Applied Science Publishers Ltd, England, 1980 Printed in Great Britain

208

Q.c.a.

CRONK

Anogramma ascension& (Hook.) Diels was last seen in 1958. Sporobolus cae,witosus Kunth, Euphorbia origanoides L., A,wleniurn ascensionis S. Wats., PoO'podium ascensionense Hieron.:~ Pteris adscensionis (Forst.) Sw. and Marattia purpurascens De Vriese were all observed by the author during a stay of three months on the island from 19 September, 1976. A further two species described as endemic by Watson (1891) are synonymous. ~Nephrodium viscidum" was found to be an immature plant of ttypolepis rugosula (Lab.) J. Sm. by Christensen, on examining the type (K) in 1930. it was probably introduced from St Helena where it is native. The other, "Rubus nanus', is probably the asiatic species Rubus rosifolius J. Sm., introduced before 1851 from St Helena (Duffey, 1964), where it is an alien. 1 have not, however, examined the type. Cyperus appendiculatus Brongn. was described from Ascension (Brongniart, 1829) but was later found on Tristan da Cunha and Fernando Noronha. Gordon collected specimens with contracted umbels from dry watercourses between 60 m and 235 m, and plants with laxer umbels on Green Mountain between 610 m and 700 m. Kiikenthal (1936) later distinguished between the plants from Tristan da Cunha, Fernando Noronha and Ascension Island at a varietal level, the plants from Green Mountain becoming var. appendiculatus and the plants from lower altitudes becoming var. /~ gordoni. The extent to which the variation within this group is genetically determined and their relationship with mainland species are much in need of careful investigation. They are thus omitted from the following account. Both varieties are still present on Ascension, having been collected by Duffey (1964) and by myself in 1976. Although the island is situated in the tropics its climate is modified by the southeast trade winds. In the coastal regions maritime desert prevails, while the top of Green Mountain (860 m) is cool and often cloud-capped. From the first permanent settlement of the island in 1815 up to the present day approximately 300 species of vascular plants have been introduced deliberately or by accident. These have had a considerable effect on the vegetation of the island. Goats were introduced by mariners as a food source and they survived on the scanty indigenous vegetation. As early as 1701 Dampier (1709) records them as very common but they became extinct in about 1939 through hunting. Rabbits and donkeys were later introductions which are still present on the island. Over 1000 sheep have had free range on the island for about 100 years and been culled regularly for their meat. The dairy herd which was confined to the top of Green Mountain was transferred to St Helena in 1977. For an account of the ecology and biological history see Duffey (1964). In 1976 almost every 1 km square (UTM grid) of the island was visited with the exception of a few barren and inaccessible coastal squares. Collections of bryophytes and fungi were made and it is hoped that accounts of these will appear in :~This taxon was first described as Pol)podium trichomanoides Sw. var/3 jungermannioides Hook. (non P. jungermannioides KI.), and Hieronymus raised it to specific rank as P. ascensionense Hieron. However, to bring the nomenclature in line with modern trends the name Xiphopteris ascensionense (Hieron.) Cronk comb. nov. must be used (basionym: PoO'podium ascensionense Hieronymus, Hedwigia, 4,1, 93 (1905)).

EXTINCTION AND SURVIVAL OF ENDEMIC VASCULAR FLORA

209

due course. Studies of the alien plants indicated that accidental and deliberate introduction by man is continuing. However, special attention was paid to the endemics and the available information on them is summarised in the following account under four headings: (a) (b) (c) (d)

the taxonomic status of the endemic; major historical records, including those giving habitat and distribution data, but making no attempt to be complete; the status of the species in 1976, preceded by the IUCN terminology after Lucas & Synge (1977), based on the author's observations; suggested causes of extinction or reasons for survival with ecological notes pertaining to the present status of the plant.

In category (b) where no source has been given, the records have been extracted from herbarium sheets at the Royal Botanic Gardens, Kew (K), where the greater part of the material collected from the island is situated.

Sporobolus durus BRONGNIART (GRAMINEAE) (a) This is a distinct species and its affinities are not clear. (b) It was first found by D'Urville (see Brongniart, 1829). In August 1886 H. J. Gordon, a naval surgeon stationed on the island, sent material of this species to Kew. It is from the lee side of Weather Post Hill, at 460 m and consists of plants about 30 cm high. Since that time no specimens have been found. The grass referred to by Duffey (1964) as S. durus is S. caespitosus. (c) Endangered, possibly Extinct. This species cannot yet be definitely considered extinct. In the past it may have been overlooked because of its vegetative similarity to the introduced S. aji'icanus (Poir.) Robyns and Tournay, an aggressive species now dominant on the northern and western sides of Green Mountain between 610 m and 765 m. However, if populations of S. durus still exist, they must be extremely small and are probably decreasing through competition. (d) S. durus has probably suffered primarily from competition with the African grass Melinis minutiflora P. Beauv., now dominant in the locality recorded by Gordon and over large areas between 305 m and 610 m. This species often forms dense pure stands, in places over one metre high. Higher up the mountain competition with S. aJi'icanus may have been equally significant. The introduction of herbivores, notably goats and sheep, may also have contributed to the decline.

Sporobolus caespitosus KUNTH (GRAMINEAE) (a) The spikelets of this species are very similar to those ofS. durus although it is very different in habit. As no intermediates have been collected it is unlikely that the plants are depauperate phenotypes of S. durus. (b) It was first collected by D'U rville (Kunth, 1829). In August 1889, H. J. Gordon

210

Q . c . B . CRONK

sent specimens to Kew from the weather side of Green Mountain between 610 m and 825 m. In 1958 Duffey (1964) collected it growing in 'short thick tufts on bare rock slopes; N side of Elliott's Pass, Green Mt" at 730 m, and also "a few scattered tufts of this grass in bare red ash; SE spur of Green Mt" at 610m. (c) Endangered. In 1976 scattered tufts were found on the northern and eastern sides of Green Mountain peak at around 730m. No plants were found at lower altitudes and an estimate of approximately 70 tufts for the world population of this species would be reasonably accurate. (d) This tiny perennial species, always less than 10cm tall, only grows on exposed cinder banks subject to cold, misty and very strong winds as well as low rainfall. The inflorescence is always hidden in the protective tuft of leaves. Dead leaves are brittle and do not decompose quickly; often the only living material in a tuft consists of a few leaves and inflorescences in the centre protected from the wind by dead growth. In 1976 all the plants were found growing on slopes of between 45 degrees and vertical where competition is minimal. The cover on these slopes is about 50 ~oand is composed of grasses, hepatics with Xiphopteris ascensionense amongst them, lichens and the introduced Centella asiatica (L) Urban. On shallower slopes in the same area denser communities of introduced plants are formed by Sporobolus aji'icanus, Axonopos compressus Beauv., Pa~palum conjugatum Bergius, stunted Juniperus bermudiana L., Gnaphalium purpureum L. and Centella asiatica; the decrease of S. caespitosus is attributable to the introduction of these latter plants. The cutting of Elliott's Pass around the Mountain in 1839 (formally opened by Admiral Elliott in 1840) must have destroyed some S. caeapitosus plants but it created many nearly vertical slopes free from competition which have subsequently been colonised by S. cae~pitosus in places.

Oldenlandia adscensionis (DC.) CRONK (RUBIACEAE) (a) This shrub is the most distinct of all the Ascension Island endemics, showing little resemblance to any other species of its mainly herbaceous genus. If this is the result of a long period of isolation O. adscensionis must have survived many periods of volcanic upheaval. The flowers are strongly dimorphic but nothing is known about its breeding biology. (b) Dampier (1709) when ship-wrecked in the island noted some'shrubby trees' on the southeast slope of Green Mountain. This is probably the first notice of O ldenlandia adscensionis. Forster (1789), visiting the island with Captain Cook, wrote 'Scherardia (sic) /ruticosa... Stirpem unicam mense maio 1775, emortuam huius speciei legimus'. However, Forster only went to the base of Green Mountain. Higher up the mountain other, more flourishing plants may have been present. In 1843 J. D. Hooker, returning with H M S Erebus & Terror after their Antarctic voyage, collected some compact plants of this species with very short internodes; he

EXTINCTION AND SURVIVAL OF ENDEMIC VASCULAR FLORA

211

describes it as 'here and there' on Green Mountain (Hooker, 1867). In April 1851 Seemann collected both this compact type and shoots of a much larger plant with long internodes. Presumably this latter form came from a moister and less goatgrazed place higher up Green Mountain (Seemann climbed to the top). The variability of this species is also apparent from the discrepancies between the descriptions of Linnaeus (1753) and de Candolle (1830). In August 1874, StiJder (1889) found some plants in a dry watercourse south of Sisters' Peak between 185 m and 215 m. In 1876 Mosely of the Challenger expedition collected the compact form (Hemsley, 1885). G o r d o n collected this species in 1889 but it has not been seen since. (c) Extinct. This plant is almost certainly extinct although a few plants may survive on the cliffs west of Green Mountain at about 610 m. It was probably never very common but existed as scattered shrubs between 356 m and 680 m on the northern and western slopes of Green Mountain. Very occasional plants in the coastal lava plains may have arisen from seeds washed down by the rains (vide Duffey, 1964, p. 231). (d) Before 1815 the main forage of the goats would have been Oldenlandia adscensionis and Portulaca oleracea L. on the sides of Green Mountain. This grazing probably started the decline but the introduction of plant competitors would have hastened the extinction. For instance, a dense growth of the African grass Melinis minutiflora, over I m deep in places, now occurs on much of the northern and western mountain slopes between 305 m and 550 m. Dense thickets of the shrub Leucaena leucocephala (Lam.) De Wit occur on the northwestern slope of the mountain between 335m and 700m. L. leucocephala is now the dominant vegetation in the dry gulley where Strider recorded the OIdenlandia. The bushes are regularly spaced about 1 m apart and appear to utilise all the available soil water; this water stress would probably preclude the existence of less vigorous shrubs. The L. leucocephala here is all severely grazed by sheep which may also have contributed to the extinction of O. adscensionis.

Euphorbia origanoides

L. (EUPHORBIACEAE)

(a) This is a distinct species although similar to E. glaucophylla Poir. of the Guinea Coast. (b) Osbeck collected it on his voyage to China (Osbeck, 1771), probably from near Cross Hill which he climbed. It was seen by most of the early travellers (e.g. Cunninghame, 1699; Forster, 1789), possibly indicating a greater abundance than is the case today. Rudmose Brown (1906) was the first person to record localities and Duffey (1964) made a map of its distribution in 1958. (c) Rare. In 1976 its distribution was found to follow fairly closely that recorded by Duffey, with a few notable exceptions (Fig. 1). For instance, Duffey records a small population on the lava plains near Wideawake Fairs; this has now disappeared but a large colony is now present nearby on the sides of Cotar Hill and Gannet Hill.

Fig. 1.

\

/

,~ ~

~

'\

'

•

,,

~

-

'

'"

HILL

t,

7'

" L L

'

.

~

.

\_,~

....

•

f

3O5rn

..- --

1

"

KM,

"

150m

CRICKET VALLEY ....

N E BAY

G~RE~E~["

~

'

( _ i ~ , - % ~ . _

\ ~-

"

......

~

~ 150

2",

05

/ PEAKX~

_--

"--,- - . s 7 . _ . ,

t

~

/

~

f

I

/

~

t cD

~

HEAD

i t

Map showing the distribution of Euphorbia origanoides in 1958 (horizontal hatching; after Duffey, 1964) and 1976 (vertical hatching). The approximate localities of Rudmose Brown are also shown (crosses) along with some places mentioned in the text.

. y "

l

l U ~ ' - ;i5O

CROSS

•

C Z

@

b,9 Ix9

EXTINCTION AND SURVIVAL OF ENDEMIC VASCULAR FLORA

213

This instability might indicate that E. origanoides has a very narrow ecological tolerance with slight changes in the habitat causing shifts in distribution. For this reason, although the population of E. origunoides is fairly stable at present, it may have a precarious future. (d) Its survival is probably due to the fact that it inhabits those areas of Ascension Island which have been least affected by the alteration of the habitats over the last 150 years. It occurs in the deserts of pyroclastic deposits in the north and west of the island, where the only plants in the vicinity are species such as Waltheria indica L., Enneapogon cenchroides (Roem. et Schult.) C. E. Hubb. and Aristida congesta Roem. at Schult. (Fig. 2). The rainfall is so low (132 ram/annum (Duffey, 1964)), that these plants occur very sparsely and the community is open. It is possible though, that competition for soil water has caused its extinction in some sites. Euphorbia origanoides is, however, very well-adapted to this habitat in that it holds its leaves at an oblique angle to the incident sunlight. All the plants (even ones only 10cm high) are covered with flowers and capsules until death and the amount of seed shed is very large. Considerable variation exists within the species with many of the plants, particularly from the eastern populations, having red stems and red or purple leaves. The cottony-cushion scale lce~3'a purchasi Mask., a serious pest of the introduced

Fig. 2. A small plant of Euphorbiaoriganoidesfrom the lava desert betweenCross Hill aand English Bay with four dead plants of Artstida congesta.

214

Q . c . B . CRONK

Casuarina equisetiJblia L. trees, has spread to many other plants on the island, including the E. origanoides populations on Cross Hill and Sisters Peak. It has caused considerable mortality in these populations, and if it spreads could cause extensive reductions of the E. origanoides populations. However, in December 1976 a predator (Rodolia cardinalis (Mulsant)) was introduced and this may control the I. purchasi. If the introduction of plants to the mountain has increased the rainfall on the lower mountain slopes (as the steady increase of Opuntia and other shrubs in the zone perhaps indicates) there is a possibility of more vegetation spreading into the desert areas and posing a threat to E. origanoides. Dryopteris ascensionis (w. J. HOOKER)O. KUNTZE (ASPLENIACEAE) (a) This is a distinct plant most closely resembling the St Helenian species D. cognata (Pr.) O. Kuntze and D. napoleonis (Bory) O. Kuntze. (b) Hooker found D. ascensionis in 1843, at the top of Green Mountain. Lyall also found it, as did Seemann in 1851. The last person to collect it was H. J. Gordon in August 1889, who records it between 765 m and 825 m on the weather side of Green Mountain. (c) Endangered. No trace of this species was found in 1958 (Duffey, 1964), but in 1976 Mr Eric Timm told me of a fern, clearly this species, that he had seen the previous year. The densely paleaceous rhizome and stipe distinguish this fern from any other on Ascension Island. It was growing on a steep, bryophyte-covered slope of compacted cinders beside Rupert's Path near the farm, at 700 m on the lee side of Green Mountain. The fact that it was not present at that site in 1976 is perhaps attributable to the drought that had been affecting the Mountain Farm during much of 1976. Either this species is now extinct or very nearly so; it may survive in some of the moist ravines on the weather side of Green Mountain. (d) When Hooker found this species in 1843 it was probably part of the 'carpet of ferns" that he reported as covering the summit of Green Mountain. Since that time numerous trees, bushes and grasses have been introduced. The peak is now covered with a dense thicket of bamboo, while much of the mountain top is now Paspalum/Eleusine pasture for cattle, and many of the ravines on the weather side of Green Mountain now support thick scrub of Buddleia madagascariensis Lain. and Vitex triJblia L. This destruction of large areas of the indigenous plant communities has probably been the major factor in the decrease of D. ascensionis.

Asplenium ascensionis s. WATSON (ASPLENIACEAE) (a) This species is a member of the variable African A. lunulatum group, and is probably best regarded as a variety of A. erectum Bory ex Willd. (b) in 1843 Hooker found it forming part of the 'carpet of ferns" (Hooker, 1867) on the peak and weather side of Green Mountain and also in rock crevices. The plants

EXTINCTION AND SURVIVAL OF ENDEMIC VASCULAR FLORA

215

he collected were large, and proliferating at the tips of the fronds; he identified them as A. erectum vat. proliJerum. Loomis of the US Eclipse Expedition collected it in 1889, and Watson (1891) described it as a new species. Rudmose Brown of the Scottish National Antarctic Expedition collected in 1904 a plant identified as A. lunulatum Sw. which is almost certainly referable to A. ascensionis. Duffey collected it from a wet ash slope on Elliott's Pass and from a rock crevice on Middleton Ridge of Green Mountain. (c) Rare. Although local on the mountain, it was quite common in 1976 on walls and ash slopes especially along Elliott's Pass and Bell's Path. It was also found growing in a crevice of trachyte near the summit of White Horse Hill. (d) No plants collected since Hooker's visit match the luxuriance of his specimens. The massive introductions of the latter half of the 19th century seem to have destroyed it as a member of the ground vegetation of the summit of Green Mountain. It has only survived because of its capacity to colonise steep slopes where competition is minimal. On White Horse Hill it was found growing through a foliose hepatic with no vascular plants in the vicinity. On Green Mountain it grows on the nearly vertical cinder slopes, such as those created by the cutting of Elliott's Pass, with mosses, sporeling ferns, stunted Christella dentata (Forssk.) Holttum and grasses.

Xiphopteris ascensionense

(HIERONYMUS) CRONK (GRAMMITIDACEAE)

(a) This is a distinctive fern although similar to X. trichomanoides (Sw.) Copel from which it differs, amongst other characters, by the lack of a rufous indumentum (Hieronymus, 1905). (b) It was first found by Hooker in 1843 amongst moss on banks at the summit of Green Mountain (855 m). Fleet Surgeon C. G. Matthew collected it from a tree on Green Mountain in 1895 and it was also collected on Elliott's Pass in 1904 (Rudmose Brown, 1906). In 1958, Duffey (1964) recorded it as common on the peak in rock crevices and on trees. (c) Rare. In 1976 it was only found above 730m, mainly on trees but also in rock crevices. It was always found growing amongst moss, often the endemic species Campy~opus smaragdinus (Brid.) Jaeg. and Calymperes ascensionis C. Muell., even on the moss-covered nodes of the bamboos at the peak. (d) This is one of the few indigenous species to have benefited from the mass introduction of plants in the 19th century. It was able to colonise the habitats created by the bryophyte colonisation of the introduced shrubs and trees, it is, however, extremely Iocalised on the mountain, and seems to require the constant mists of the very peak for its growth and survival.

Pteris adscensionis SWARTZ (ADIANTACEAE) (a) This species is very close to Pteris dentata ssp. flahellata (Thunb.) Runemark

216

Q.C.B. CRONK

Christensen regarded it as the variety P. dentata var. ascensionis (Forst.) C.Chr. However, many of the Ascension plants are very distinct on both pinnae characters and spore morphology (see Runemark, 1962) and the plants resembling typical P. dentata ssp.flabe//ata may be derived from comparatively recent colonists. Plants approaching P. adwensionis in appearance have very occasionally been found in St Helena and S Africa (Runemark, 1962). (b) Forster (1789) first found this plant at the base of Green Mountain in 1775. Hooker in 1843 collected a plant similar to P. dentata growing in +large harsh tufts" on the weather top of Green Mountain. He also collected a plant closer to typical P. adscensionis, with +more horizontal pellucid fronds" and broader pinnules, growing between 365 m and 550 m amongst rocks in shaded places on Green Mountain. It was also collected by Seeman in 1852, Mosely in 1876 and Loomis in 1889. Duffey in 1958 collected it at 610 m in rock crevices on the southeast spurs of Green Mountain and at 640 m at the toot of a rock lace in Breakneck Valley on Green Mountain. (c) Endangered. In 1976 it was found amongst herbage and rocks on the southeast spur of Green Mountain at around 610 m, and between 460 m and 490 m in the scree gulley on the north-western side of Cricket Valley. The large tufts that Hooker collected near the peak have disappeared although the specimens collected by Mosely and that of Duffey from Breakneck Valley are similar in form. (d) This species survives with the introduced plants on the drier areas of Green Mountain. Its disappearance from the peak is probably due to the increased luxuriance of the vegetation there. The two populations are very small with less than about 10 plants in each.

Anogramma ascensionis (w. J. HOOKER) DIELS (AD1ANTACEAE) (a) This species is completely distinct from all other members of the genus; Hooker (1854) noted that it approached A. leptophylla (L.) Link in size and texture but .4. chaerophylla (Desv.) Link in ramification. (b) Curror found this fern in 1842+ and Hooker found and described it a year later, growing on wet rocks and banks on the weather side of Green Mountain between 365 m and 550 m. Mosely of the Challenger expedition collected it in 1876 (Hemsley, 1885) and it was also found by the U S Eclipse expedition in 1889 (Watson, 1891). In September 1958, Duffey (1964) found it growing in a rock crevice at 730m, on a north-facing slope. (c) Endangered. Despite a thorough search this species was not found in 1976. Its apparent absence may have been due to the drought on the mountain that year. Annual ferns are usually sensitive to drought but the plant may not be extinct as spores on the cinders would germinate at the onset of heavy rains. It may also be present in some of the moister ravines on the weather side of Green Mountain, but the populations (if any) must be precariously small.

EXTINCTION AND SURVIVAL OF ENDEMIC VASCULAR FLORA

217

(d) it appears to be considerably less common now than during Hooker's visit. This may be due to the increase of introduced plant species since then, which must have reduced the number of suitable habitats by covering the bare cinders and shading the crevices.

Marattia purpurascens DE VRIESE (MARATTIACEAE)

(a) This species is extremely close to the variable M.Ji'axinea Smith, and is possibly best regarded as a subspecies of it. (b) J, D. Hooker found the plant in 1843 and noted that it was 'rather rare on the weather side of Green Mountain near the peak'. It was also collected by Seemann in 1851 and by Mosely in 1876. In 1958 Duffey found it between 730m and 855m, on the steep east slope of the mountain where it was described as 'locally dominant' and qocally abundant'. (c) Rare. In 1976 it was found to occur fairly regularly above 730 m on the weather side of the peak, and it was also present in the bamboo thicket on the summit. It was completely absent both from the lee side of the mountain and at lower altitudes and thus seems to be dependent on the higher precipitation and humidity of the peak. The population seems to have been stable over the last 100 years and must number several hundred plants. (d) M. purpurascens has probably benefited from the increase of humus resulting from the plant introductions that took place in the latter half of the 19th century. Its phenotypic plasticity enables it to grow both in the deep shade of the bamboo thicket and on exposed slopes where it is subject to strong, cold and misty winds. However, the finest plants are those growing amongst Buddleia madagascariensis and Rubus sp. at the edges of Elliott's Pass. This plasticity and the large size of this species (enabling it to compete with the introduced species) have been the major factors in its survival.

DISCUSSION

The main cause of alterations in the numbers of the endemic vascular plants has been the introduction since permanent settlement in 1815, of numerous alien species, The reasons for these introductions were twofold: to provide food for the garrison, through crops and grazing, and to moderate the xeric climate by inducing precipitation and so increasing the amount of fresh water available for drinking, it is not clear from the rainfall data whether this latter aim has been achieved but it seems likely that the condensation on the leaves and stems of the bamboo and other plants of the peak has enabled the mountain to support a more luxuriant flora than would have otherwise been possible. Furthermore the increase of vegetation on the upper

218

Q . C . B . CRONK

laval plains has possibly increased precipitation from the moist winds passing offthe mountain by lessening the effect of rising warm air. This would have had a positive feedback effect (cf. Hare, 1977, p. 87) and would account for the spread of Opuntia in these areas; it may also have contributed to the extinction of OIdenlandia adscensionis. It is possible that this effect will bring more competitors into the proximity of Euphorbia origanoMes populations. Duffey (1964) has suggested that the abandonment of the mountain farm, which keeps some aliens in check, might complete the destruction of the remaining fragments of semi-indigenous plant communities. The farm is now (December, 1977) being abandoned. The re-creation of areas of indigenous community is effectively impossible due to many factors including the isolation of Ascension Island and the difficulty of keeping such an area free of aliens. It is therefore possible that other endemic taxa will become extinct in the wild in the near future, and the only effective conservation measure would be cultivation or gene-banking of the remaining species. Cultivation, however, presents difficulties. The ferns of the peak, notably Xiphopteris ascensionense and Marattia purpurascens, are dependent on cool air with a relative humidity greater than 90 %, which are difficult to reproduce in cultivation. It has been suggested earlier in this paper that Euphorbia origanoides also has extremely narrow limits of ecological tolerance (but in the xeric range) which would also be difficult to reproduce. It is possible that extinction had also taken place before the destruction of the indigenous habitats by man. Baker (1973) suggests that the island has had more volcanic activity in the last 10,000 years than any other in the South Atlantic. Gases, as in Tristan da Cunha (Dickson, 1962), and pyroclastics may have affected the local mountain vegetation. In particular, the formation of the Cricket Valley explosion caldera (c. 0-35 million years ago) and the Green Mountain cindercone may have affected large parts of the island (J. D. Bell, pers. comm.) and thus contributed to the paucity of indigenes and endemics in the vascular flora. ACKNOWLEDGEMENTS

The author wishes to thank the Churchill Fellowships Trust for encouragement and financial support. Thanks are also due to the officers and staff of the British Broadcasting Corporation in London and Ascension for allowing me to visit the island and for hospitality there; to the many people in Ascension Island who gave me their kind and invaluable help during my stay; to the staff of the Royal Botanic Gardens, Kew and the British Museum (Natural History); and to various members of the Botany School, University of Cambridge, for reading and commenting on the manuscript. REFERENCES ATKINS, F. B., BAKER, P. E., BELL, J. D. &SMITH, D. G. W. (1964). Oxford Expedition to Ascension Island. Nature, Lond., 204, 722-4.

EXTINCTION AND SURVIVAL OF ENDEMIC VASCULAR FLORA

219

BAKER, P. E. (1973). Islands of the South Atlantic. in The ocean basins and margins, !, The South Atlantic, ed. by A. E. M. Nairn and F. G. Stehli, Chapter 13. New York, Plenum Press. BELL, J. D., ATKINS, F. B. & BAKER, P. E. (1972), Notes on the petrology and age of Ascension island, South Atlantic (abstract). Trans. Am. geophys. Un., 53, 168. BRONGNIART, A. T. (1829). Botanique ( Phanerogamie ) du Voyage... sur la Coquille pendant les anng'es 1822, 23, 24, 25. Paris, Arthus Bertrand. CANDOLLE, A. P. DE (1830). Prodromus O'stematis naturalis regni vegetabilis, 4, 419. Paris, lreuttel & W~rtz. CUNNINGHAME, J. (1699). A catalogue of shells, etc., gathered at the island of Ascension by Mr James Cunninghame. Phil. Trans. R. Sot., 21(7), 295-300. DAMPlER, W. (1709). A continuation of a voyage to New Holland, &c. m the year 1699. London, James Knapton. DICKSON, J. H. (1962). The effects of the eruption of 1961 on the vegetation of Tristan da Cunha. Phil. Trans. R. Sot'. (Set. B.), 249, 403-24. DUFFEr, E. (1964). The terrestrial ecology of Ascension Island. J. appl. Ecol., 1,219-51. FORSTER, G, (1789). Plantae Atlanticae. Comm. Sot'. Reg. Sci. Gott., 9, 46-74. HARE, F. K, (1977). Connections between climate and desertification. Environ. Conserv., 4, 81-90. HEMSLEV,W. B. (1885). Report on the botany of the Bermudas and various other islands of the Atlantic and Southern Oceans. Rep. Sci. Res. Voy. "Challenger': Botany, !(3), 1-299. HIERONYMUS, G. (1905). Polypodium species novae et non satis notae. Hedwigia, 44, 78 105. HOOKER, J. D. (1867). On insular floras~(British Association 1866). Gardeners" Chronicle, (3), 50. HOOTER, W. J. (1854). Grammitis Ascensionis, Hook. Hooker's Icon. Pl., 10, pl. 967. K~KENTHAL, G. (1936). Cyperaceae-Scirpoideae-Cyperae. Pflanzenreich, IV, 20(101), 436. KUNTFi, C. S. (1829). R~vision des Gramin~es publides dans les nova genera et specie~ plantarum., 2, 423. Paris, Gide. LINNAEUS, C. (1753). Species Plantarum, ed 1, 103. Stockholm, L. Salvus. Luc'As, G. L. & SVNGE, A. H. M. (1977). The IUCN Threatened Plants Committee and its work throughout the world. Environ. Conserv., 4, 179-87. OSBECK, P. (1771). A voyage to China and the East Indies. London, Benjamin White. RUDMOSEBROWN, R. N. (1906). Contributions towards the botany of Ascension. Trans. Proc. Bot. Sot'. Edinb., 23, 199-204. RUNEMARK, H. (1962). A revision of Pteris dentata and related species. Bot. Not., !15, 177-95. ST(~DER, T. (1889). Die Forsehungsreise S.M.S. Gazelle in den 1874 bis 1876. Berlin, Mittler & Sohn. WATSON, S. (1891). Notes on a collection of plants from the Island of Ascension. Proc. Am. Acad. Arts Sci., 26, 161-3.