R~EARCH I ~Sl1ITUTB. NEW alP NLK-H-3 IARI-lO 5.55.

456 Pages·2012·37.91 MB·English

INDIAN AGRICULTURAl. .
R~EARCH I ~Sl1ITUTB. NEW DELHI
I.A.A 1.6.
alP NLK-H-3 I.A.R.I.-lO 5.55.-1',000
THE
JOURNAL OF ECOLOGY
VOLUME 30
CAMBRIDGE UNIVERSITY PRESS
LONDON, BENTLEY HOUSE
BOMBAY, CALCUTTA, MADRAS: MACMILLAN
All rights rt!se,."",d
THE
JOURNAL OF ECOLOGY
EDITED FOR THE
BRITISH ECOLOGICAL SOCIETY
BY
W. H. PEARSALL
VOLUME 30
1942
WITH TWENTY-SEVEN PLATES, AND NUMEROUS
FIGURES IN THE TEXT
CAMBRIDGE
AT THE UNIVERSITY PRESS
1942
PRINTED IN GREAT BRITAIN
CONTBN'fS
rAG};
ORIGINAL PAPERS
Burtt, B. D. ilw·tt. Memorial Supplonwnt. Sorno East, African Vegetut.ioTl COIll
muuitie8. l~ditod hy C. H. N .•T ltoksOTl. Wit.h It foroword by W. H. PoM.s.
(With Plutfls 2-27, eoutainillg Photographs 1--52) 65
Chu, S. P. 1'h" Intlllf'lIeO of t.he M.illfll'UI Composition of tho Medium OIl tho (ll'owt,h
of Plankt.onie Alga.('. I. Mt't,IHHIH and ('ult,lIf(l Mndia. (With eight. Figuros ill til(>
TI1xt.) 2H4-
Harley, J. L. 1111(1 Yemm, E. W. l';('olngieal AS[ltlet.s of Pilat. AecllnmlMion.
T. Thornt.oll Min'. YOl'k,dlir(·,. (Wit.h ('iVhtllOIl Figtll'(ls ill t.h!) Text.) . 17
Hepburn, Ian. Tho Vegej"ltion of t.)l(' Hal'lln(~k Htmltl QUUl'l'i(I;;. A Htwly of 1110
Vllgot.ut.ioll uf t,ho Nort,jntlllptonshil'(' .Tumssie LiImlstollo. (With Plat,,, I and
t.wo Figures in the 'j'tlxt) . 57
Laurie, E. M. O. The Disi·mlved Oxygen of an Upland Pond and its In!lowing
Htr·(,am. at. Y~tllrnt'lH'n, North CUJ'digaw;hil'tl, ·Walos. (\viUI Ilight. Figlll'tls Il1
the T('xl.) . 357
Lund, .J. W. G. Tho Ml11'ginal Algae of Certain PoneL'!, with Hpooial j{tlfor(llle(l t.o t.11('.
Boj.t.orn UnjJositt-l. (With UlI'!ltl Figlll'llt-l ill j,JJe Tuxt) 245
Matins Smith, A. TIlt' Algal' of Milo~ H.ollgh Bog, Bradford. (Wit,h 0110 Figure in
the 'i'oxj) . 341
Mortimer, Clifford H. Tlie Exehungo of l>iHHoivwi Hlli>;;tmwfls bdwt'-oll Mud I1ntI
VI/utp!, in LakpH. (With twollt.y-fiw Figul'flK in UwTuxt) 147
Pennington, Winifred. Exp0l'inwnt.:; on f,h" llf ili~a.t.ioll of Nit I'Oglll1 in Frosh Wu.kI'.
(Wit,11 one Figlll'l~ ill till' Toxf.) . 321)
Polunin, Nicholas. U. K Nidlols-An Apprtleiat.ioll 202
Vesey~Fitz~erald, D. Furt.hol' Ht.mlios 011 t,ll(' \,pgnt.n.t.ion on J~dI1THIK ill t.ho IlHlial1
Oool1,n. (Wit.h UIJl'. Map ill tho Tt,xt) I
REVTEWR
Journnt of A nirnal ]!Je%!!!" \'01. 10, 1':0. 2 206
An Introduction to the Stu.d.'lld Al(/n(J 207
Journal oj Animal ]!Jn%(lY, Vol. ll, No.1
Ecological Orol1 Geography
PAPERH OF J~COLOGll'AL INTEREST RECEIVED 208
Content8
VI
PAGE
BIOLOGICAL FLORA 0«' THE BRITISH ISLES
Accounts Published or in Proparation 209
Cladium Mar/scus (L.) R.Br. by V. M. Conway 211
Zostera L. by T. G. Tut,in 217
AStilT Tripolium (1..) by A. R. Clapham, W. H. Poarsall and P. W. Richar<L-l 3S5
HRITJSH 1<1COLO(JTCAL SOCIETY
Annual Ml't·t.ing ill Mil' UniVfll'Hity Department, of Bot,any. OxfoJ'(l 227
List of Mombers 2:H
Accounts 243
VOLUME 30, No.1 'FEBRUARY 1942
-------.---.. -------"'--------'_:;~,'~_+l'~~~.,j." .:.,, .,., "
1- _______" __
FURTHER STUDIES OF THE VEGETATION ON
ISLANDS IN THE INDIAN OCEAN
By D. VESEY-FITZGERALD
(With one Map in the Text)
INTRODUCTION
THl!J floras of the elevated granitic islands comprising the Seychelles have been
described in a previous paper. The present paper deals with a further series of
islands which come under the same administration. These islands form col
lectively those known as the Amicantes and the Aldabra Islands. The former
are sand cays at present worked as coconut estates, and so their primary
vegetation has been destroyed. The latter are limestone islands whiCh still
largely retain their former vegetation, and it is these which will form the subject
of the present paper. In order to make the study complete, however, in
addition to the Aldabra Islands proper, namely, Aldabra, Assumption, Astove
and Cosmoledo, it is necessary to include the more distant islands of St Pierre
and Farquhar which also belong to the limestone series.
Fryer (1910) has given a description of the geological structure of these
islands, so in the present study it is only necessary to note that they are islands
of coral origin which at some time in the past have become raised in relation to
the surfaee of the sea and are at the present time in the process of being worn
down. Before recording the vegetation, however, the forces acting upon these
islands must be considered.
There is a very marked wet and dry season, the latter extending from March
to November. The islands are not big enough to have rivers, so there is no
washing of the land surface into stream channels. The islands have a very small
rainfall, e.g. on Aldabra about 38 em. (15 in.) per annum, whieh rapidly perco
lates into the ground. The precipitation has the effect, however, of etching the
metamorphic limestone into a fretted surface which gives it a very rugged
appearance known locally as 'champignon'. In the main the surface of the
islands is level and at an elevation of about 5 m. above sea-level. Around the
seaward coast the pounding of the waves eats into the land rim, breaking it
down or forming overhanging cliffs. In the centre of most of the islands there
is a lagoon. The shores of these lagoons are being continually dissolved away so
that the lagoon surfaces are increasing and the land surfaces decreasing. Around
the open coasts of the islands fragments of coral in various degrees of disintegra
tion are deposited as shingle and sand. In places where the waves have broken
down the coastal cliffs the wind is able to blow the finer coral sand up on to
the island. The prevailing winds of the latitudes in which the islands are
J. Eool. 30 1
2 Studies of the vegetation on Islands in the Indian Ocean
situated are from the south-east, and so sand dunes are built up along the
exposed coasts which have an easterly or southerly aspect. Hence on most of
the islands there is a peripheral coastal fringe of sea-washed detritus which may
even be thrown up on to the low cliffs especially around the mouths of blow
holes, and on the windward coasts dunes are often found. The best developed
dune, reaching a height of 30 m., is on Assumption Island. These dunes are
characterized by a. gently sloping seaward face and a steeply sloping landward
face under the lee of which is a sheltered zone. Deep wind-scoured gullies
divide the dunes into numerous minor ridges and valleys. In places where the
cliffs are 8till intact and too high for sand to be thrown up in any great quantity
the 8hore is bathed in an almost continual shower of spray.
As a result of all,these forces therefore the land surface becomes secondarily
raised into dunes orlowered into tide-flooded creeks and the vegetation becomes
modified as a consequenee. But besides these physical agents there is a very
important biological agent influencing both the topography and the flora of
the islands. From the remote past to the present day sea birds have nested
on these islands. Years ago their numbers were very much greater than they
are to-day, and as a consequence vast deposits of guano have accumulated in a
semi-fossil state on parts of some of the islands. The present-da.y nesting birds
influence the vegetation hoth by their deposits of fresh guano and by their
trampling. But the greatest recent influence of all has been the exploitation of
the guano, during which process parts of some of the islands have been cleared
of vegetation.
PI,ANT COMMUNrrms
Mixed scrub
This type of vegetation is best developed on the most elevated parts of
the limestone areas of Aldabra Island, and it is also to be found in a restricted
amount on parts of Cosmoledo and Assumption. It occurs on a rugged pave
lllent of rock, the surface of which is characterized by loose flat slahs and basin
like depressions. Crevices and open pits are frequent. Some of these contain
water at the bottom which is in connexion with the lagoon or the open sea.
I n more restricted areas the surface of the limestone is carved into the most
fantastic shapes, and the numerous bosses and pinnacles resound with a
metallic ring when struck. Much of the rock surface is bare, but a fine soil
collects in t.he depressions.
The community is a mixed, partially deciduous scrub of small trees or
Rhrubs reaching a very uneven height from place to place but averaging about
(--5 m. The density of "the growth varies greatly with the nature of the topo
graphy; in parts it is impenetrable, but numerous open lanes usually allow
passage in all directions. The ~rees have small, simple, elliptical or oval leaves.
About one-third of the species are with spines.
D. VESEy-FITZGERALD 3
Sideroxylon inerme is the most characteristic plant and is the species which
extends farthest into neighbouring communities. There are about fifteen other
species more or less constantly present, and these ineJude Elaeodendron sp.,
Gyrnnosporia senega/rnsis, Apodytes rnauritiana, ET.IIthroxylurn acranthum,
Ochna frye·ri, Tarenna supra-ax'illaris, lllacourtia indica, Polysphaeria 'rnult£
flora, Term.inalia fatraea and Scutia 'inyrtina.
The present community finds its optimum conditions on the more sheltered
parts of the land rim of the islands. Near exposed coasts it it: replaced by a
depauperated, wind-hlown phase in which many of the above species are not
present, but are replaced by a more maritime element. The j_"hderoxylon persists
and with it are associated Scae'llola frutescens and Ouettarda speciosa, all of
which are represented by low wind-moulded shrubs less than 1 m. high and
usually with barren spaces between. Doubtless during the rains there are
many herbs in this area, but these are not in evidence in the dry period except
for clumps of the grass Sclerodactylon rnacrostachyurn. Clumps of a Pandanus
are occasionally a feature of this area. In some places this vegetation has
been almost eliminated through the (;ombined destruction caused by goats and
fuel cutting hy the turtle hunters.
It seems probahle that on some of the smaller islands, as for example
St Pierre, a deciduous woodland of Pison1:a grand1:s and Euphorhia aMotti'i
occupied a similar terrain. On such islands there were deep deposits of guano.
This phase of the scrub comlIlunity has, however, now been completely cut out,
partly during the clearing of the land for guano extraction and partly to
obtain fuel for the purpose of distilling drinking water from sea water. At the
present time only relict trees remain, but early visitors to these islands recall
an impassable thicket of these species.
Where the mixed scrub has been cleared prior to guano extraction a
secondary mat of Plumbago aphylla has covered the land, and the original
scrub formation is only represented by islets of shrubs chiefly around open
pits. On parts where the clearing has been more frequent or where there has
been trampling, the Plnmbago has, in its turn, given place to a growth of mixed
exotic weeds such as Stachytarpheta jarn.aicensil~, Dactyloctenillln pilosurn, and
many others.
l>emphis thicket
This community is located on the rather lower-lying areas of metamorphie
limestone, the surface of which is fretted into jagged edges by weathering
agents. This is the formation locally called' champignoll', and it forms a zone
along both the sea and the lagoon coasts of the islands. The former is only a
narrow strip along the tops of the overhanging cliffs which rise a few feet above
high-water mark. The latter is a much wider zone intersected hy creeks and
tidal fiats, and along the windward lagoon shore isolated blocks form islets.
These islets are of small area and their edges are eaten into by the action of
1-2