ATOLL RESEARCH BULLETIN NO. 231
THE TERRESTRIAL VEGETATION OF AN INDIAN OCEAN CORAL ISLAND: WILINGILI, ADDU ATOLL, MALDIVE ISLANDS
I. TRANSECT ANALYSIS OF THE VEGETATION by R. A. Spicer and D. McC. Newberry 11. A LIMITED QUANTITATIVE ANALYSIS
OF THE VEGETATION DISTRIBUTION by D. McC. Newberry and R. A. Spicer
Issued by
THE
SMITHSONIAN INSTITUTION Washington, D. C., U.S.A.September
1979THE TERRESTRIAL VEGETATION OF AN INDIAN OCEAN CORAL ISLAND: WILINGILI, ADDU ATOLL, RIALDIVE ISLANDS
I. TRANSECT ANALYSIS OF THE VEGETATION
by R.A. s p i c e r l and D.McC. ~ e w b e r ~ ~
ABSTRACT
The vegetation of a l i t t l e - d i s t u r b e d Indian Ocean c o r a l i s l a n d , W i l i n g i l i , Addu A t o l l , i s described both s u b j e c t i v e l y and i n terms of t h r e e sea t o lagoon t r a n s e c t s . Exposure and edaphic f a c t o r s were measured and t h e p o s s i b l e e f f e c t s of t h e s e on v e g e t a t i o n a l p a t t e r n discussed. I t i s concluded t h a t because s o i l m a t u r i t y and exposure a r e both dependent on proximity t o t h e s t r a n d s , i n v e s t i g a t i o n of t h e r e l a t i v e importance of t h e i n f l u e n c e of t h e s e v a r i a b l e s on
v e g e t a t i o n a l p a t t e r n i s impossible from t r a n s e c t a n a l y s i s i n t h i s c a s e . However, t h e t h r e e t r a n s e c t s spanning d i f f e r e n t widths o f t h e i s l a n d probably r e v e a l v e g e t a t i o n a l d i f f e r e n c e s determined by t h e s a l i n i t y o f t h e ground waters.
A l i s t o f p l a n t s c o l l e c t e d i n t h e course of t h e work i s presented which r e p r e s e n t s p r a c t i c a l l y t h e complete f l o r a o f W i l i n g i l i and
i n c l u d e s e i g h t confirmed new r e c o r d s f o r t h e Maldive I s l a n d s .
INTRODUCTION
The problems concerning t h e low f l o r a l d i v e r s i t y o f oceanic i s l a n d s and t h e f a c t o r s i n f l u e n c i n g t h e d i s t r i b u t i o n of v e g e t a t i o n
Formerly Department of Botany, Imperial College, London; p r e s e n t address, Department of Geology, Imperial College, London.
Formerly Department of Botaqy, Imperial College, London; p r e s e n t address, Department of Botany, The University, B r i s t o l .
(Manuscrip$ received November 1975
--
Eds.)have, t o d a t e , received l i t t l e a t t e n t i o n beyond a s u b j e c t i v e a p p r a i s a l . I n t h e work r e p o r t e d h e r e one i s l a n d , W i l i n g i l i , was s e l e c t e d f o r d e t a i l e d study i n a more q u a n t i t a t i v e manner. W i l i n g i l i i s a c o n s t i t u e n t i s l a n d of Addu A t o l l ( 0 ~ 3 8 ' ~ .
,
73O10'~.) which l i e s a t t h e southern extremity of t h e Maldive I s l a n d chain.The J. Stanley Gardiner expedition of 1899-1900 was t h e f i r s t t o make e x t e n s i v e p l a n t c o l l e c t i o n s i n t h e Maldive group. Soon
a f t e r w a r d s Agassiz (1903) remarked "The vegetation....
.
of W i l i n g i l i....
[is] perhaps, a s l u x u r i a n t a s t h a t of any i s l a n d i n t h e Maldives."U n t i l 1972 t h e most r e c e n t work i n t h i s a r e a was c a r r i e d o u t by an expedition l e d by D.R. S t o d d a r t i n 1964. For p o l i t i c a l reasons work was confined t o t h e h e a v i l y populated i s l a n d s of Gan and Hittadu and it was suggested t h a t t h e v e g e t a t i o n of a l e s s d i s t u r b e d i s l a n d should be s t u d i e d f o r comparison (Sigee 1966).
W i l i n g i l i with i t s l a c k of permanent human population provided a s u i t a b l e s i t e f o r such work which was undertaken by t h e a u t h o r s during t h e i r s i x week (16 August t o 28 September 1972) expeditionary v i s i t t o t h e A t o l l .
GEOMORPHOLOGY
The t o t a l land a r e a of Addu A t o l l (Fig. 1) measures some
11.19 sq. km, with W i l i n g i l i , t h e s i x t h l a r g e s t i s l a n d i n the group, occupying 0.79 sq. km. I n 1836 W i l i n g i l i w a s c h a r t e d a s two s e p a r a t e i s l a n d s b u t by 1900 (Gardiner 1903) it had become one and remains s o t o this day. S t o d d a r t (1966) c l a s s i f i e s t h e i s l a n d a s a seaward edge type s i n c e t h e seaward beach, l i e s only 90 m from t h e r e e f edge. This beach
4 s
mainly composed of rounded c o r a l fragments 20 cm. o r more i n diameter.
The margins of c o r a l i s l a n d s u s u a l l y show t h e h i g h e s t r e l i e f due t o t h e formulation of unconsolidated r i d g e s of c o r a l d e b r i s along seaward and, o c c a s i o n a l l y , lagoon, shores. The m a t e r i a l comprising t h e marginal r i d g e s v a r i e s from f i n e sand through t o cobbles and boulders (Fosberg & C a r r o l l 1965). The W i l i n g i l i seaward r i d g e was noted by Agassiz (1903) a s being "perhaps t h e h i g h e s t we have seen i n t h e Maldives1'. The v e g e t a t i o n , o f t e n c o n s i s t i n g e n t i r e l y of Pemphis a c i d u l a (Table l ) , a c t s a s a wind break with t h e r e s u l t t h a t a d e p o s i t of wind blown sand forms on t h e landward s i d e of t h e seaward r i d g e . Unlike many P a c i f i c i s l a n d s t h e marginal r i d g e of W i l i n g i l i contained no l a r g e boulders, presumably because t h e high energy a g e n t s of
t r a n s p o r t and d e p o s i t i o n , e.g. typhoons, a r e never experienced on Addu A t o l l . No lagoon r i d g e was observed on W i l i n g i l i b u t t h e r e was some evidence of e r o s i o n along t h e southern lagoon shore which, l i k e most o f t h i s beach i s composed of f i n e c o r a l sand.
Although drainage through unconsolidated c o r a l d e b r i s i s p r a c t i c a l l y p e r f e c t , t h e porous n a t u r e of t h i s m a t e r i a l allows
r e t e n t i o n of o f t e n considerable amounts of r a i n water. The presence of a well (Fig. 2) i n d i c a t e s t h a t a non-saline ground water l e n s i s p r e s e n t , a s on many o t h e r c o r a l i s l a n d s , and approaches t h e s u r f a c e i n numerous shallow hollows within t h e Pisonia f o r e s t (Fig. 2 ) .
S u b s t a n t i a l amounts of f r e s h water a r e a b s e n t however i n t h e narrower p a r t s of t h e i s l a n d and some hollows a r e a t l e a s t p e r i o d i c a l l y f i l l e d with b r a c k i s h water.
CLIMATE
I t was n o t p o s s i b l e t o make extensive meteorological o b s e r v a t i o n s during t h e p e r i o d of our s t a y on W i l i n g i l i , b u t t h e expedition o f 1964 obtained d a t a from t h e R.A.F. Staging P o s t on Gan and t h e following i s summarized from S t o d d a r t (1966)
.
The annual r a i n f a l l f l u c t u a t e s between 300 cm and 188 cm with a mean of 229 cm and e x h i b i t s no c l e a r seasonal p a t t e r n . The
temperature, l i k e t h e high humidity, i s c o n t r o l l e d by t h e surrounding ocean and has a mean of 2 8 ' ~ throughout t h e year. The d i u r n a l
temperature v a r i a t i o n of 5 . 5 ' ~ f a r exceeds t h e annual range of 1 . 6 6 ' ~ . Surface winds show t h e following p a t t e r n :
January February March A p r i l May June July August September October November December
WNW
-
NNEN
-
NEW W W
SW
-
SE S-
SE S-
SE SWSW
-
WW
WSW
-
NNEGales and s t r o n g winds a r e r a r e b u t winds up t o 36 m/sec have been recorded during s q u a l l s .
TRANSECT METHODS
Three t r a n s e c t l i n e s were s e t up i n t h e p o s i t i o n s shown i n Fig. 2 . D e t a i l e d measurement of microclimate was n o t p o s s i b l e , b u t an e f f e c t i v e e s t i m a t e o f t h e r e l a t i v e exposure i n d i f f e r e n t p a r t s of t h e i s l a n d was found from t h e l o s s of m a t e r i a l t a t t e r e d from 25 c m b y 4 0 cm c l o t h f l a g s (method of Lines & Howell 1963). Two f l a g s were s e t up a t the mid-point and e x t r e m i t i e s of the t h r e e t r a n s e c t s and t a t t e r i n g
proceeded over 35 days (21 August 1972 t o 25 September 19721, a f t e r which t h e a r e a l o s t was measured f o r each f l a g . The mean values f o r each s i t e a r e shown i n Fig. 3.
I n t h e following s t u d i e s of t h e v e g e t a t i o n r e p r e s e n t a t i v e
specimens of each s p e c i e s encountered were c o l l e c t e d and r e t u r n e d t o t h e B r i t i s h Museum (Natural History) where they a r e now lodged, and were determined by M r . E.W. Groves u n l e s s otherwise s t a t e d . I t i s considered t h a t t h e s p e c i e s l i s t e d h e r e (Appendix 1) r e p r e s e n t t h e m a j o r i t y of those s p e c i e s t o be found on W i l i n g i l i a t the p r e s e n t time.
Records of t h e v e g e t a t i o n along each t r a n s e c t were made on a simple presence and absence b a s i s f o r each metre i n t e r v a l o f t h e t r a n s e c t l e n g t h .
S o i l p i t s , 30
-
40 cm deep, were dug every 20 m along t h e middle and southern t r a n s e c t s and every 40 metres along t h e n o r t h e r n t r a n s e c t . pH measurements and s o i l samples (approx 150 g n e t weight) were taken a t 5, 10 and 20 cm depths supplemented by observations on t h eappearance of t h e s o i l s . I t was n o t p o s s i b l e t o d r y t h e s o i l s
completely i n t h e f i e l d , and t h e r e f o r e they were s t o r e d wet and d r i e d on r e t u r n t o England. Analysis of t h e s o i l s was then c a r r i e d o u t a s described i n Newbery and S p i c e r ( t h i s i s s u e ) .
SUBJECTIVE DESCRIPTION OF THE VEGETATION
The ' s o i l ' of t h e southern t i p of W i l i n g i l i was composed of c o r a l blocks 10
-
20 cm i n diameter i n which S c a e v o l a t a c c a d a , T o u r n e f o r t i a a r g e n t e a , G u e t t a r d a s p e c i o s a , and Pemphis a c i d u l a survived.To t h e north t h e v e g e t a t i o n i n c r e a s e d i n h e i g h t t o about two metres with S. t a c c a d a and G . s p e c i o s a predominating. Approximately 200 m from t h e southern t i p Pemphis a c i d u l a was abundant under a canopy of coconut palms which continued northwards b u t with a n i n c r e a s i n g l y dense understorey of S . t a c c a d a . Unlike t h e r e s t of t h e i s l a n d t h e v e g e t a t i o n here was f r e e of P a s s i f l o r a s u b e r o s a . The seaward r i d g e supported Pemphis a c i d u l a which commonly grew down to, and o f t e n below, high water l e v e l .
Approximately 600 m from t h e southern t i p of t h e i s l a n d i s o l a t e d i n d i v i d u a l s of mature Pandanus sp. were encountered along t h e lagoon shore though nowhere on W i l i n g i l i d i d this p l a n t grow i n abundance.
Some 400 m south from t h e southern t r a n s e c t i n t h e c e n t r e of t h e i s l a n d a depression i n t h e c o r a l was found, covered with a pure s t a n d of Pemphis a c i d u l a . I t may be t h a t t h i s depression i s p e r i o d i c a l l y f i l l e d with brackish water.
Further north coconut palms formed dense v e g e t a t i o n , r i c h i n such s p e c i e s a s ~ a l a n c h . 3 e p i n n a t a and Euphorbia sp.
The a r e a o f t h e southern t r a n s e c t was r i c h i n S c a e v o l a t a c c a d a , many of t h e p l a n t s being covered i n the p a r a s i t i c l e a f l e s s C a s s y t h a f i l i f o r m i s . Many of t h e densely covered S . t a c c a d a had t h e appearance of being e t i o l a t e d with long v e r t i c a l stems crowned with a r o s e t t e of
small c h l o r o t i c leaves. Thuarea i n v o l u t a and L e p t u r u s r e p e n s formed t h e ground cover i n t h e c l e a r i n g s .
A number of i n d i v i d u a l s of C o r d i a s u b c o r d a t a were seen growing above cemented beach rock of a small bay on t h e Lagoon shore north of t h e southern t r a n s e c t . Northwards t h e coconut palms became s p a r s e and s t u n t e d and t h e v e g e t a t i o n was composed mainly of S . t a c c a d a , P . a c i d u l a and G . s p e c i o s a with i s o l a t e d p l a n t s of Pandanus sp.,
Morinda c i t r i f o l i a and H i b i s c u s t i l i a c e u s , many o f which were covered i n P a s s i f l o r a s u b e r o s a . D i g i t a r i a h o r i z o n t a l i s and B o e r h a v i a d i f f u s a occurred along t h e edges of trampled ground.
The open scrub v e g e t a t i o n reached a s f a r n o r t h a s t h e b r a c k i s h water l a k e i n t h e c e n t r e o f t h e i s l a n d (Fig. 2 ) . Here t h e mangroves R h i z o p h o r a mucronata and B r u g u i e r a c y l i n d r i c a w e r e found i n abundance spreading north behind t h e seaward r i d g e .
The canopy of t h e northern p a r t of t h e i s l a n d was formed by P i s o n i a g r a n d i s and i n t h e extremely high humidity of t h e f o r e s t
community C a r i c a papaya
,
Morinda ci t r i f o l i a,
Neisosperma o p p o s i t i f o l i a,
A s p l e n i u m n i d u s and Tacca l e o n t o p e t a l o i d e s were t o be found. Almost t h e e n t i r e understorey of t h e f o r e s t a r e a was covered with t h e c r e e p e r P a s s i f l o r a s u b e r o s a
.
The most e x t e n s i v e coconut groves occurred on t h e lagoon s i d e of t h e P i s o n i a f o r e s t . These a r e a s a r e n o t p l a n t e d a s such b u t t h e growth of any germinating coconut appears t o be encouraged by c l e a r i n g t h e undergrowth around it.
An a r e a of t h e f o r e s t e d region has been c l e a r e d and supports crops of Zea mays and c a s t o r o i l R i c i n u s communis. Wherever t h e ground h a s been d i s t u r b e d t h e weed W e d e l i a b i f l o r a was found i n abundance.
The north-western t i p of t h e i s l a n d appeared t o be s i m i l a r t o t h e southern t i p i n t h a t t h e c o r a l blocks comprising t h e extremely juvenile s o i l a r e s u b j e c t t o p e r i o d i c marine disturbance and consequently only P . a c i d u l a , S . t a c c a d a and G . s p e c i o s a were found. I n p l a c e s however C a s s y t h a f i l i f o r m i s was seen t o be growing over b a r e c o r a l d e b r i s and S c a e v o l a t a c c a d a .
RESULTS
The occurrences of t h e various s p e c i e s encountered along t h e t r a n s e c t s a r e given i n Fig. 4.
S i m i l a r l y t h e r e s u l t s of t h e s o i l a n a l y s i s a r e presented i n Figs. 5, 6 and 7 , w i t h pH measurements i n Table 1.
Table 1. pH measurements of the s o i l s sampled along t h e t h r e e t r a n s e c t s Northern Transect
D 5cm
e
p lOcm t
h 20cm
Middle Transect
D 5cm
e
p lOcm t
h 20cm
Southern Transect
D 5cm
e
p lOcm t
h 20cm
Sample S i t e s
C D E
7.9 8.2 8.2
7.8 8.5 8.0
7.9 8.4 8.5
Sample S i t e s
C D E
7.8 7 . 7 7.6
8.3 7.8 8.2
8.3 8.5 8.3
Sample S i t e s
C D E
8.1 8.5 8.0
8.3 8.5 8.0
8.2 8.7 8.4
DISCUSSION
The f l o r a of W i l i n g i l i , a s with many other oceanic i s l a n d s , i s extremely r e s t r i c t e d , being almost t o t a l l y represented by t h e 72 species presented here. Consequently t h e expression, by the
vegetation of environmental v a r i a b l e s i s o f t e n concealed (Stone 1953).
The primary sources of land material a r e r e e f organisms (except f o r pumice which i s common on a number of a t o l l s (Sachet 1955), and which i s l o c a l l y abundant on W i l i n g i l i )
,
and it i s only with time t h a t t h e c o r a l d e b r i s becomes a l t e r e d t o provide a v a r i e t y of h a b i t a t s t h a t may be r e f l e c t e d by vegetational p a t t e r n s . This d i f f e r e n t i a t i o n i n t o communities might be expected t o r e f l e c t proximity t o t h e sea and maturity of s o i l . Naturally these two f a c t o r s a r e strongly linked.A s can be seen from the t a t t e r f l a g data (Fig. 3) proximity t o t h e sea i s a l s o associated with exposure, thus it may be d i f f i c u l t t o
i s o l a t e the e f f e c t s of s i n g l e environmental f a c t o r s on the vegetation,
from simple t r a n s e c t a n a l y s i s .
I f percentage organic m a t t e r i s an i n d i c a t o r of maturity i n a t o l l s o i l s then the s o i l s of t h e northern t r a n s e c t a r e t h e most developed
(Fig. 6 ) . Their most s t r i k i n g f e a t u r e i s t h e high phosphate l e v e l s a s s o c i a t e d w i t h t h e more mature P i s o n i a g r a n d i s f o r e s t (Figs. 4 and 6).
Such a n a s s o c i a t i o n i s commonly found i n p a c i f i c a t o l l s (Fosberg 1953) and i s probably t h e r e s u l t of guano from r o o s t i n g s e a b i r d s . I t i s not l i k e l y t h a t P . g r a n d i s i s dependent on t h e phosphate f o r i t s growth a s has been suggested by Shaw (1952) s i n c e it i s a l s o found on
r e l a t i v e l y low phosphate s o i l s e.g. sample s i t e F. The a s s o c i a t i o n of P . g r a n d i s , phosphorus, and a l a y e r of r e l a t i v e l y a c i d raw humus has been discussed by Fosberg (1954, 1957a). He has pointed o u t t h a t t h e a c i d i t y of raw guano (pH 6.0) i s n o t s u f f i c i e n t t o take t h e calcium phosphate p r e s e n t i n guano i n t o s o l u t i o n and it i s only when guano f a l l s onto t h e t h i n , a c i d , raw humus l a y e r , c h a r a c t e r i s t i c of mature P. g r a n d i s f o r e s t s , t h a t t h e phosphate passes i n t o s o l u t i o n l a t e r t o be p r e c i p i t a t e d a s a phosphate hardpan when t h e underlying c o r a l d e b r i s
i s reached. Wiens (1962) concludes t h a t while P . g r a n d i s does not n e c e s s a r i l y r e q u i r e phosphate and a c i d s o i l s f o r germination i t
undoubtedly t h r i v e s i n such an environment s i n c e i t s r e l a t i v e l y shallow r o o t system i s o f t e n d i r e c t l y exposed t o such conditions. However the development of a raw humus l a y e r and a c i d conditions c h a r a c t e r i s t i c of Jemo s o i l s , a s described by Fosberg (19541,are not f u l l y developed on W i l i n g i l i (Table 1).
By comparison t h e middle and southern t r a n s e c t s r e p r e s e n t more open scrub vegetation. Here the i s l a n d is only a t h i r d of t h e width of t h e northern t r a n s e c t , thus a s u b s t a n t i a l f r e s h water l e n s cannot be supported (Stone 1953), w i t h obvious consequences i n times of r e l a t i v e drought.
The absence of a non-saline groundwater l e n s is i n d i c a t e d i n t h e southern t r a n s e c t by t h e widespread d i s t r i b u t i o n of t h e s a l t t o l e r a n t bushes S c a e v o l a t a c c a d a and G u e t t a r d a s p e c i o s a . These p l a n t s do however provide cover and l o c a l l y humid c o n d i t i o n s t o allow P a s s i f l o r a s u b e r o s a t o survive.
The coconut palms on t h e southern h a l f of t h e i s l a n d appear t o be l e s s vigorous and s e t f u r t h e r a p a r t than on t h e northern lagoon shore and it has been noted by Wiens (1962) t h a t t h e d e n s i t y of coconut palms tends t o be l e s s on d r i e r a t o l l s . However Hatheway (1953) has s t a t e d t h a t t h e coconut t h r i v e s over an extremely wide range of environmental c o n d i t i o n s , consequently i t i s " u t t e r l y worthless a s an i n d i c a t o r of environmental d i f f e r e n c e s " . Although t h i s i s t r u e , p a r t i c u l a r l y w i t h r e f e r e n c e t o ground water s a l i n i t y , t h e fronds appear t o s u f f e r from s e a spray damage. I t i s thus a common p r a c t i c e t o p r o t e c t coconut groves from d i r e c t spray by leaving a band of e s s e n t i a l l y n a t u r a l v e g e t a t i o n on t h e seaward s i d e of an i s l a n d , t h u s t h e g r e a t e s t d e n s i t y of coconut palms i s found along t h e lagoon shore of W i l i n g i l i .
I t may be r e a d i l y seen from F i g . 5 t h a t t h e c o n c e n t r a t i o n of magnesium i s c o n s i s t e n t l y higher on both lagoon and s e a s t r a n d s . Stone (1953) has noted t h a t some r e e f organisms, e.g. Lithothamnion, may c o n t a i n up t o 25% magnesium carbonate. However t h i s i s r e l a t i v e l y
s o l u b l e and becomes leached from t h e o l d e r s o i l s , hence i t s lower concentration i n t h e middle of t h e i s l a n d . This hypothesis i s a l s o supported by a g e n e r a l i n c r e a s e i n magnesium c o n c e n t r a t i o n w i t h depth.
Sodium, however, shows a uniform d i s t r i b u t i o n a c r o s s t h e I s l a n d a p a r t from t h e mangrove p e a t . Undoubtedly t h e c o n t r i b u t i o n of sodium i n spray must be considerably higher towards the s t r a n d s b u t t h i s i s n o t r e f l e c t e d i n t h e a n a l y s e s . One i s l e d t o conclude, t h e r e f o r e , t h a t any c o n t r i b u t i o n by spray i s r a p i d l y leached from t h e s u r f a c e s o i l s .
CONCLUSIONS
Coral i s l a n d s a r e g e n e r a l l y considered t o e x h i b i t a s t r o n g
environmental g r a d i e n t from windward t o leeward and from s e a shore t o lagoon shore. On W i l i n g i l i t h e former g r a d i e n t i s n o t e v i d e n t due t o t h e v a r i a b l e n a t u r e of t h e p r e v a i l i n g wind d i r e c t i o n . Exposure
however i s considerably l e s s i n t h e c e n t r e of t h e i s l a n d than along t h e s t r a n d s and because of t h e proximity of t h e seaward r i d g e t o t h e r e e f edge, the e f f e c t of spray i s more marked on t h e seaward s i d e . The maturity of s o i l s a s i n d i c a t e d by percentage o r g a n i c matter and
a s s o c i a t e d i o n i c a v a i l a b i l i t y , i s mainly c o r r e l a t e d with d i s t a n c e from t h e s t r a n d s , hence it i s d i f f i c u l t to i s o l a t e exposure e f f e c t s from edaphic f a c t o r s . This argument a l s o a p p l i e s t o t e x t u r a l d i f f e r e n c e s i n the s o i l s . However t h e r e a r e marked v e g e t a t i o n a l d i f f e r e n c e s between t h e northern t r a n s e c t and t h e two more s o u t h e r l y ones which a r e probably determined by t h e s a l i n i t y of ground waters. Where t h e i s l a n d i s wide enough t o support a f r e s h ground-water l e n s the
development o f a P i s o n i a f o r e s t has i n t u r n g r e a t l y modified t h e s o i l . Superimposed upon t h i s n a t u r a l v e g e t a t i o n p a t t e r n a r e t h e
a c t i v i t i e s of man e s p e c i a l l y with r e g a r d t o t h e managing o f coconut groves and ground disturbance. Where t h i s i s most marked weeds such a s Wedelia b i f l o r a a r e a b l e to grow i n abundance.
ACKNOWLEDGEMENTS
A s members o f t h e 1972 Imperial College Maldive I s l a n d s Expedition we should l i k e t o r e c o r d our thanks t o t h e many who supported and financed us, e s p e c i a l l y t h e Imperial College
Exploration Board, The Royal Geographical S o c i e t y , The Royal A i r Force, t h e Maldivian Government and t h e B r i t i s h Museum (Natural H i s t o r y ) . We would a l s o l i k e t o t a k e t h i s opportunity of expressing our g r a t i t u d e t o M r . E.W. Groves f o r determining p l a n t specimens and preparing t h e s p e c i e s l i s t . S i n c e r e thanks go t o D r . A.J. Morton o f t h i s department
f o r h e l p f u l d i s c u s s i o n and P r o f e s s o r A.J. R u t t e r f o r u s e o f f a c i l i t i e s f o r s o i l a n a l y s i s .
REFERENCES
A g a s s i z , A. 1903. C o r a l R e e f s o f t h e Maldives. Memoirs o f the Museum o f C o m p a r a t i v e Z o o l o g y a t Harvard C o l l e g e , 29 : 1-168,
79 p l a t e s .
Fosberg, E.G. 1953. V e g e t a t i o n o f C e n t r a l P a c i f i c A t o l l s , a b r i e f summary. A t o l l R e s e a r c h B u l l . 23 : 1-26.
Fosberg, F.R. 1954. S o i l s o f t h e N o r t h e r n M a r s h a l l A t o l l s w i t h s p e c i a l r e f e r e n c e t o t h e Jemo s e r i e s . S o i l S c i . 78 : 99-107.
Fosberg, F.R. 1957. D e s c r i p t i o n and o c c u r r e n c e o f a t o l l phosphate r o c k . Amer. J. S c i . 255 : 584-92.
Fosberg, F.R. and C a r r o l l , D. 1965. T e r r e s t r i a l s e d i m e n t s and s o i l s o f the Northern M a r s h a l l I s l a n d s . A t o l l R e s e a r c h B u l l . 113 : 1-156.
G a r d i n e r , J.S. 1903. T h e f a u n a and g e o g r a p h y o f the M a l d i v e and L a c c a d i v e A r c h i p e l a g o e s , b e i n g the a c c o u n t o f the work c a r r i e d o u t and o f collections made b y an e x p e d i t i o n d u r i n g the y e a r s 1 8 9 9 and 1 9 0 0 . Cambridge: U n i v e r s i t y P r e s s . Volume 1.
i - i x , 1-471.
Hatheway, W.H. 1953. The Land v e g e t a t i o n o f Arno A t o l l , M a r s h a l l I s l a n d s . A t o l l R e s e a r c h B u l l . 1 6 , 1-68.
L i n e s , R. and Howell, R.S. 1963. The use o f f l a g s t o e s t i m a t e the r e l a t i v e exposure of t r i a l p l a n t a t i o n s . Forest R e c o r d s 5 1 :
1-31.
S a c h e t , M.-H. 1955. Pumice and o t h e r e x t r a n e o u s v o l c a n i c m a t e r i a l s o n c o r a l a t o l l s . A t o l l R e s e a r c h B u l l . 37, 1-27.
Shaw, M.K.A. 1952. On t h e d i s t r i b u t i o n o f P i s o n i a g r a n d i s R. B r . (Nyctaginaceae) w i t h s p e c i a l r e f e r e n c e t o Malaysia. Kew B u l l . : 87-97.
S i g e e , D.C. 1966. P r e l i m i n a r y a c c o u n t o f t h e l a n d and marine v e g e t a t i o n o f Addu A t o l l . A t o l l R e s e a r c h B u l l . 116 : 61-75.
S t o d d a r t , D.R., Ed. 1966. Reef s t u d i e s a t Addu A t o l l , Maldive I s l a n d s . P r e l i m i n a r y r e s u l t s o f a n e x p e d i t i o n t o Addu A t o l l i n 1964.
A t o l l R e s e a r c h B u l l . 116 : 1-121.
S t o n e , E.L., Jr. 1953. Summary of i n f o r m a t i o n o n a t o l l s o i l s . A t o l l R e s e a r c h B u l l . 22 : 1-6.
Wiens, M.J. 1962. Atoll environment and ecology. N e w Haven, Conn. and London: Yale U n i v e r s i t y P r e s s . 532 pp.
Appendix 1. Botanical Specimens collected on the Imperial College of Science & Technology (London) Expedition t o the Maldive Islands, Indian Ocean, 1972.
Material determined by M r . E.W. Groves, Dept. of Botany, B r i t i s h Museum (Natural History) London unless otherwise s t a t e d . The c o l l e c t i o n has been deposited i n t h e Herbarium of t h a t Museum.
*
i n d i c a t e s new species recorded t o the Maldives.ANGIOSPERMS DICOTYLEDONS Portulacaceae
P o r t u l a c a t u b e r o s a (Roxb
.
) T r i n . GuttiferaeC a l o p h y l l u m i n o p h y l l u m L.
Malvaceae
H i b i s c u s t i l i a c e u s L.
S i d a h u m i l i s Willd.
Tiliaceae
* ~ r i u m f e t t a procumbens Forst. f
.
C o r c h o r u s a e s t u a n s L.
Surianaceae
S u r i a n a m a r i t i m a L.
Oleaceae
*Ximenid americana L. var. americana
Rhamnaceae
C o l u b r i n a a s i a t i c a (L
.
Brongn.Leguminosae
(det. F.R. Fosberg July 1973)
~ a e s a l p i n i a bonduc (L
.
Roxb.
C a n a v a l i a c a t h a r t i c a Thou.
C a s s i a o c c i d e n t a l i s L.
V i g n a m a r i n a (Burm.) M e r r i l l
Crassulaceae
Kalanchoe pinnata (Lam. 1 Pers.
Rhizophoraceae
Bruguiera c y l i n d r i c a [L. ) B I
.
Rhizophora mucronata Lam.
*Ceriops t a g a l (Perr
.
) C .B. Robinson (det. F .R. Fosberg July 1973) LythraceaePemphis a c i d u l a Forst.
Turneraceae
Turnera u l m i f o l i a L.
Passif loraceae
P a s s i f l o r a sube2osa L.
Caricaceae
C a r i c a papaya L .
Cucurbi taceae
Cucumis melo L .
Aralisceae
P o l y s c i a s g u i l f o y l e i (Bull.) Bailey ( d e t . F.R. Fosberg July 1973) Rubiaceae
Morinda c i t r i f o l i a var b r a c t e a t a Hook. f . Guettarda s p e c i o s a L.
Compositae
Launaea p i n n a t i f i d a Cass
.
Wedelia b i f l o r a (L.) DC.
T r i d a x procumbens L
.
Conyza b o n a r i e n s i s (L. Crong
.
Vernonia c i n e r e a var
.
parvi f l o r a (B1.1 DC.
Goodeniaceae
Scaevola taccada (Gaertn
.
) Roxb.
Apocynaceae
N e i s o s p e m m o p p o s i t i f o l i a (Lam.) Fosberg & Sachet.
( o c h r o s i a o p p o s i t i f o l i a (Lam. IS. Schum)
.
Boraginaceae
T o u r n e f o r t i a a r g e n t e a L
.
f.
C o r d i a s u b c o r d a t a Lam.
Convolvulaceae
Ipomoea p e s - c a p r a e L.
Ipomoea n n c r a n t h a R. & S.
~ a c q u e m o n t i a p a n i c u l a t a (Burm. f
.
) Hallier f.
Scrophulariaceae
Bacopa m o n n i e r i (L.1 Wettst.
Nyctaginaceae
P i s o n i a g r a n d i s R. Br.
B o e r h a v i a d i f f u s a L.
Amaranthaceae
~l t e r n a n t h e r a sessilis (L
.
) R.
Br.
A c h y r a n t h e s a s p e r a L.
Lauraceae
C a s s y t h a f i l i f o r m i s L.
Hernandiaceae
Hernandia p e l t a t a Meissn.
Euphorbiaceae
R i c i n u s communis L.
Euphorbia c ya t h o p h o r a Murray
*Euphorbia i n d i c a Lam.
* L a p o r t e a interrupts (L.) Chew (det. F.R. Fosberg, July 1973) A g y n e i a b a c c i f o r m i s (L. ) Juss
.
P h y l l a n t h u s u r i n a r i a L.
Moraceae
F i c u s b e n g h a l e n s i s L.
MONOCOTYLEDONS A m a r y l l i d a c e a e
Crinum a s i a t i c u m L . T a c c a c e a e
Tacca 1 e o n t o p e t a l o i d e s (L. 1 0 . K t z e
.
P a n d a n a c e a e
Pandanus sp. ( j u v e n i l e m a t e r i a l ) A r o i d e a e
C o l o c a s i a e s c u l e n t a L.
C y p e r a c e a e
Cladium jamaicense C r a n t z . var c h i n e n s e ( N e e s ) Koyama Cyperus l i g u l a r i s L.
Cyperus conglomeratus R o t t b . f o r m a pachyrrhi z u s ( N e e s ) K u k e n t h . Cyperus d u b i u s R o t t b .
*~imbrist y l i s cymosa ssp. s p a t h a c e a ( R o t h ) Koyama F i m b r i s t y l i s f e r r u g i n e a (L. ) V a h l
G r a m i n e a e
E r a g r o s t i s t e n e l l a (L. ) B e a u v . Apluda m u t i c a L .
*Digi t a r i a h o r i z o n t a l i s Willd.
D a c t y l o c t e n i u m a e g y p t i u m (L. 1 Willd.
Thuarea i n v o l u t a ( F o r s t ) R . B r a L e p t u r u s r e p e n s ( F o r s t . f . 1 R.Br.
*Oplismenus sp. ( p o s s i b l y 0 . c o m p o s i t u s )
PTERIDOPHYTES P s i l o t a c e a e
A s p l e n i a c e a e
Asp1 e n i um n i d u s L
.
N e p h r o l e p i s h i r s u t u l a ( F 0 r s t . f . ) P r e s l
F i g . 1. Addu A t o l l , Maldive I s l a n d s .
Lagoon
NORTHERN TRANSECT
MIDDLE TRANSECT
SOUTHERN TRANS
Sea
Fig. 2. W i l i n g i l i , Addu A t o l l , showing t h e p o s i t i o n s o f t h e t r a n s e c t s . T a t t e r f l a g s were p o s i t i o n e d i n t h e c e n t r e and a t t h e ends of t h e t h r e e t r a n s e c t s .
160
-
140
-
N
E
0 120
-
-
0
LU
100- I- I-
a
8 0 - C3
a
1 LL 6 0 -
LL
0
a
4 0 -[I
a
20
-
0
-
Fig. 3.
DISTANCE ALONG TRANSECT
Three dimensional representation of relative exposure at nine points on the %and as measured by tatter flags.
L
z
a
L
$ a
I I
j ERAGROSTIS TENELLA-
BOERHAVIA DIFFUSA
-
S r A F V n l A T A r C A n A C
CORDIA SUBCORDATA BRUGUIERA CYLlNDRlCA PEMPHIS ACIDULA HERNANDIA PELTATA CARICA PAPAYA ASPLENIUM N l D U S NEISOSPERMA OPPOSlTlFOLlA PlSONlA GRANDIS
MORINDA ClTRlFOLlA 'SCAEVOLA TACCADA
APLUDA M U T I C A GUETTARDA SPECIOSA BOERHAVIA DIFFUSA PASSIFLORA TUBEROSA CYPERUS LlGULARlS THUAREA INVOLUTA WEDELIA BIFLORA COCOS NUCIFERA HIBISCUS TlLlACEUS
M E T R E S LAGOON
PEMPHIS A f l n l l l A
w
&
p
- - -
- - -
- - -
- - -
- -- -
-- ---- --- -- -
- - -
m e - --- - - - -- - - -.- - .-- - - - -
- - - -- --
- -
- - - - - -- - - - - -- - -
- -- - - - - ---
- -- -- ---- - - -
- - - -
- -- -- -
- -- -- - - -
A 10 2 0 3 0 B 5 0 6 0 7 0 C 9 0 100 110 D 130 140 150 E 170 180 1 9 0 F 210 2 2 0 2 3 0 G 2 5 0 2 6 0 2 7 0 2 8 0 2 9 0 3 0 0 310 3 2 3 H
-
. - . . . . . . . - . .
-
. - - - . .Fig. 4. The d i s t r i b u t i o n of s p e c i e s along t h e t r a n s e c t s . The
PORTULACA TUBEROSA IPOMOEA MACRANTHA GUETTARDA SPECIOSA APLUDA MUTICA PANDANUS sp CASSYTHA FlLlFORMlS PASSIFLORA TUBEROSA - COCOS NUCIFERA HIBISCUS TlLlACEUS THUAREA INVOLUTA
I-
3
V)
5
aI- I I-
presence of a s p e c i e s i n every 1 metre s e c t i o n of t h e
- - - - -
- -- - - -
---- - - - - -
- - -
- - - - -
- - - -
- - - - ---
t r a n s e c t i s marked by a h o r i z o n t a l bar.
--..----.. -
M E T R E S LAGOON
TOURNEFORTIA ARGENTEA CORDIA SUBCORDATA IPOMOEA MACRANTHA COCOS NUCIFERA HIBISCUS T l L l A C E U S SCAEVOLA TACCADA GUETTARDA SPECIOSA MORINDA ClTRlFOLlA PASSIFLORA SUBEROSA BOERHAVIA DIFFUSA SASSYTHA FlLlFORMlS
SlDA H U M l L l S
M E T R E S LAGOON
A 10 B 3 0 C 5 0 D 7 0 E 9 0 100 F SEA J
- - - - -
- - - -
- - - -- - --- -
- - -- - - - - - - -- -
--- - - - - -
- -
A 10 B 3 0 C 5 0 D 7 0 E 9 0 1 0 0 F SEA
N a+
M$'
-
c
a+-
Fig.
NORTH M IDDLE SOUTH
CALE
TRANSECT TRANSECT TRANSECT
5 . The distribution o f the elements Sodium, Magnesium and Calcium i n the s o i l samples from three transects spanning
the width of W i l i n g i l i . The values represent the t o t a l amounts of the elements present.
IEPTH SCALE NORTH M l DDLE SOUTH TRANSECT TRANSECT TRANSECT
c m M%O,.
SOIL
A I B I C I D I E I F I G I H
A ~ B ~ C [ D ~ E ~ FA ~ B ~ C I D I E J F
Fig. 6. Total Phosphate and Nitrogen l e v e l s , together with
percentage organic matter i n the s o i l s from t h r e e t r a n s e c t s .
NORTHERN TRANSECT
50 5
0
50 10
0
50 20
0
M I D D L E TRANSECT
M E C H A N I C A L FRACTIONS
I S A M P L E SITES
SOUTHERN TRANSECT
S A M P L E S I T E S
Fig. 7. Mechanical f r a c t i o n s of t h e t r a n s e c t s o i l s . The a s t e r i s k s denote those samples composed e n t i r e l y of organic Mangrove p e a t .
THE TERRESTRIAL VEGETATION OF AN INDIAN OCEAN CORAL ISLAND: WILINGILI, ADDU ATOLL, W I V E ISLANDS
11. A LIMITED QUANTITATIVE ANALYSIS OF THE VEGETATION DISTRIBUTION
by D.McC. ~ e w b e r ~ l and R.A. s p i c e r 2
ABSTRACT
The d i s t r i b u t i o n of t h e v e g e t a t i o n of W i l i n g i l i was a s s e s s e d by measurement of 45 s p e c i e s and 51 s o i l v a r i a b l e s a t each o f twenty chosen s i t e s . P r i n c i p a l components a n a l y s i s (PCA) was used t o i d e n t i f y t h e main g r a d i e n t s of t h e v e g e t a t i o n , t h e component s c o r e s being c o r r e l a t e d with t h e f l o r i s t i c and environmental v a r i a b l e s t o d e t e c t a s s o c i a t i o n s between t h e s e v a r i a b l e s .
Two g r a d i e n t s were i d e n t i f i e d . F i r s t l y , a mid-island lagoon t o s e a shore g r a d i e n t of decreasing sandiness and i n c r e a s i n g sodium along which a change from coconut communities, through bush t o s t r a n d
v e g e t a t i o n was seen. The second, a n o r t h t o south g r a d i e n t
r e p r e s e n t e d changes from bush t o f o r e s t accompanied by i n c r e a s i n g i r o n , phosphorus and n i t r o g e n with decreasing sandiness, magnesium and
carbonate.
A simple model r e l a t i n g t h e main t y p e s of v e g e t a t i o n , t h e p o s i t i o n of t h e g r a d i e n t s and an approximate r e a l i s a t i o n t o t h e s u c c e s s i o n a l development of t h e i s l a n d i s discussed.
Formerly Department o f Botany, Imperial College, London; p r e s e n t address, Department o f Botany, The University, B r i s t o l .
Formerly Department of Botany, Imperial College, London ; p r e s e n t address, Department of Geology, Imperial College, London.
(Manuscript received November 1975
--
Eds.)INTRODUCTION
The general f e a t u r e s of t h e v e g e t a t i o n on W i l i n g i l i have been described i n t h e previous paper, i n which an i d e n t i f i a b l e lagoon-to sea-shore g r a d i e n t was s t u d i e d . Here, we adopt an a l t e r n a t i v e approach by sampling t h e v e g e t a t i o n over a l l t h e i s l a n d with a view t o t h e recognition of t h e main f l o r i s t i c and environmental g r a d i e n t s i l l u s t r a t e d by t h e use of o r d i n a t i o n techniques.
The d a t a c o l l e c t e d a r e not extensive and s o t h e a p p l i c a t i o n of numerical m u l t i v a r i a t e techniques and t h e i r i n t e r p r e t a t i o n w i l l be l i m i t e d . This paper, we hope, w i l l s e r v e a s an i n t r o d u c t i o n t o t h e , a s y e t , l i t t l e s t u d i e d t o p i c of t h e ecology of t e r r e s t r i a l c o r a l i s l a n d vegetation.
METHODS
Vegetation and s o i l measurements of stand s i t e s
Twenty s u b j e c t i v e l y chosen s i t e s , h e r e a f t e r r e f e r r e d t o a s stand s i t e s , were s t u d i e d i n d e t a i l with t h e primary aim being t o record t h e major v e g e t a t i o n types p r e s e n t on t h e i s l a n d , and secondly t o
i n v e s t i g a t e t h e p o s s i b l e occurrence of a l o n g i t u d i n a l g r a d i e n t from t h e southern t i p t o t h e northern head of t h e i s l a n d (Fig. 1).
Furthermore f o u r stand s i t e s (numbers 2, 3 , 4 and 5; Fig. 1) were included t o show t h e s t a g e s i n t h e common phenomenon of coconut grove regeneration a f t e r disturbance, which was super-imposed on t h e more n a t u r a l vegetation of t h e i s l a n d . This disturbance was c h i e f l y t h e r e s u l t of bush c l e a r i n g and of trampling. On f i e l d observation a l l stand s i t e s , with t h e p o s s i b l e exception of s i t e 11, could be
considered a s members of a continuum, r a t h e r than d i s t i n c t groups of d i f f e r i n g vegetation.
A t each s i t e a 1 0 m by 10 m a r e a square was pegged o u t with s t r i n g qnd within i t more s t r i n g s l a i d t o subdivide t h e a r e a i n t o 1 0 0 1 m b y l m quadrats. T h i r t y f i v e of t h e s e quadrats were randomly s e l e c t e d , and i n each, t h e presence o r absence of each s p e c i e s was noted, s o t h a t on summation f o r each stand s i t e a l o c a l frequency e s t i m a t e f o r each s p e c i e s was a t t a i n e d , a s a s c o r e o u t of t h i r t y - f i v e . These d a t a , r e f e r r e d t o a s t h e f l o r i s t i c v a r i a b l e s matrix, appear i n Table 1.
Fig. 1 shows t h e r e l a t i v e p o s i t i o n of each s i t e on t h e i s l a n d .
A s o i l p i t was dug i n t h e c e n t r a l a r e a of each stand s i t e , i n a s i m i l a r manner t o t r a n s e c t s o i l p i t s (described i n t h e previous p a p e r ) . S o i l pH was recorded i n situ a t each of t h e t h r e e depths, 5, 10 and 20 cm depth (Table 2, col.1)
.
S o i l samples of approximately 150 grammes wet weight were a l s o taken a t each of these depths i n each s t a n d s i t e .Chemical a n a l y s i s of s o i l s
On r e t u r n t o England the s o i l samples were a i r d r i e d i n t h e
laboratory. I t had been necessary t o s t o r e the s o i l s wet i n t h e field.
Because of the somewhat unusual nature of the ' s o i l ' a mechanical f r a c t i o n s c a l e had t o be s p e c i a l l y devised. The s i x f r a c t i o n s ,
MF/l/X ...MF/ 6/x where x i s the depth c o l l e c t e d , correspond t o t h e following p a r t i c l e s i z e s : 1, > 2 0 mm; 2 , 20-3.35 mm; 3, 3.35-2.0 mm;
4 , 2.0-1.0 mm; 5, 1.0-0.5 mm; 6 , c0.5 mm. Each sample was sieved and t h e mechanical f r a c t i o n s recorded a s percentages of t h e t o t a l a i r dry weight. I t w i l l be noted t h a t f o r s i t e 11, each f r a c t i o n ,
because of the peaty nature, was given an equal value of 16.7%.
After t h e s i x weeks storage of t h e s o i l s , wet, and i n an anaerobic s t a t e , t o t a l elemental a n a l y s i s seemed an appropriate
measure of the n u t r i e n t s t a t u s of t h e s o i l r a t h e r than an a n a l y s i s f o r the exchangeable f r a c t i o n . This former measure of s o i l n u t r i e n t s has been successfully employed i n previous s t u d i e s of vegetation whose s c a l e was comparable t o t h a t of W i l i n g i l i (e.g. Austin, Ashton and Greig-Smith, 1972)
.
The s o i l sample was subdivided and a portion (approx. 20 g.) milled i n a Glen Creston Sample Mixer M i l l (M280) using tungsten carbide b a l l s .
Two d i g e s t s were prepared: ( a ) n i t r i c and p e r c h l o r i c a c i d s ( 4 : l mixture), w i t h two r e p l i c a t e s of each sample, and (b) sulphuric a c i d and hydrogen peroxide. These were made up t o 10 per cent acid s o l u t i o n f o r a n a l y s i s .
.Iron, copper, calcium and magnesium were determined by atomic absorption spectroscopy using d i g e s t s ( a ) . These d i g e s t s were a l s o analysed f o r potassium and sodium by flame emission spectroscopy.
The organic matter content of t h e s o i l s was determined by l o s s on i g n i t i o n a t 3750C (Ball 1964) with subsequent s u b t r a c t i o n f o r a i r d r i e d s o i l moisture. Samples of pure calcium carbonate heated t o this temperature l o s t no weight, i n d i c a t i n g t h a t organic matter l o s s e s were not confounded by any l o s s e s i n carbonate from t h e samples. This method was preferable t o a wet oxidation procedure, which i n agreement w i t h Orphanos (1973) was fouqd t o be unsuitable f o r such carbonate r i c h s o i l s . Carbonate i t s e l f was assessed gravimetrically by the method of Bauer, Beckett and Bie (1973)
.
Phosphate ( i n t h e n i t r i c : p e r c h l o r i c a c i d d i g e s t s ) was estimated by the molybdenum blue and t o t a l nitrogen ( i n the sulphuric acid:
peroxide d i g e s t s ) , by t h e alkaline-phenol r e a c t i o n on a Technicon Auto Analyzer. The r e s u l t s of t h e analyses are summarised i n Table 2 a s
the means of each v a r i a b l e over t h e t h r e e depths sampled. Variables
f o r each of t h e t h r e e depths c o n s t i t u t e the environmental v a r i a b l e s matrix (51 v a r i a b l e s )
.
NUMERICAL ANALYSIS
The raw d a t a m a t r i c e s t o be considered i n t h e ensuing a n a l y s i s a r e :
(i) Matrix X I t h e f l o r i s t i c v a r i a b l e s matrix, Table 1 (45 spp.)
,
and,(ii) Matrix Y , t h e environmental v a r i a b l e s matrix (51 v a r i a b l e s ) a condensed form of which appears a s Table 2 .
P r i n c i p a l components a n a l y s i s (PCA) was employed i n a numerical a n a l y s i s of t h e d a t a , t o e l u c i d a t e those f l o r i s t i c and environmental v a r i a b l e s most important i n t h e d i s t r i b u t i o n o f , o r r a t h e r changes i n , t h e v e g e t a t i o n of t h e i s l a n d . The method of a n a l y s i s was Q-type and used t h e weighted s i m i l a r i t y c o e f f i c i e n t of O r l o c i (1966).
C l a s s i f i c a t i o n techniques have n o t been employed i n t h e work r e p o r t e d h e r e . Computer programmes were w r i t t e n i n F o r t r a n 1 V by D r . A. J.
Morton and one of us (D .McC
.
N. ).
Stand Ordinations
S t a n d a r d i s a t i o n f o r s t a n d s ( t o zero mean and u n i t variance f o r each s t a n d ) was s u p p l i e d t o matrix X I r e s u l t i n g i n matrix A1. PCA of A1 provided a s t a n d o r d i n a t i o n based on f l o r i s t i c v a r i a b l e s (Fig. 2 ) . Matrix B, t h e r e s u l t of s t a n d a r d i s a t i o n of Matrix Y ( t o zero mean and u n i t v a r i a n c e f o r each environmental v a r i a b l e ) was a l s o s u b j e c t e d t o PCA giving a second s t a n d o r d i n a t i o n , t h i s being based on environmental v a r i a b l e s (Fig. 3)
.
F l o r i s t i c and environmental.variables o r d i n a t i o n s
Matrix X was then s t a n d a r d i s e d f o r s p e c i e s (each t o z e r o mean and u n i t v a r i a n c e ) g i v i n g matrix A2 t h e transpose of which, A ~ was ~ , s u b j e c t e d t o PCA with t h e production of a s p e c i e s o r d i n a t i o n (Fig. 4 ) .
Transposition of matrix B, t o B~ and subsequent PCA of B' produced an o r d i n a t i o n of environmental v a r i a b l e s (Fig. 5 ) .
Table 3 shows t h e percentage variance removed by each of t h e f i r s t t h r e e components i n t h e f o u r o r d i n a t i o n s .
C o r r e l a t i o n a n a l y s i s
To i n v e s t i g a t e which f l o r i s t i c and environmental v a r i a b l e s were a s s o c i a t e d i n t h e d i s t r i b u t i o n of t h e v e g e t a t i o n , a c o r r e l a t i o n procedure was adopted (Flenley 1968, Burden and Randerson 1972).
Considering t h e p r i n c i p a l components removed i n t h e stand o r d i n a t i o n s of matrix A1, each v e c t o r of component s c o r e s (sensu G i t t i n s 1969) :
was c o r r e l a t e d with each vector of f l o r i s t i c v a r i a b l e s ( i . e . s p e c i e s )
and each vector of environmental v a r i a b l e s ,
The following d i f f e r e n t c o r r e l a t i o n s were performed f o r each p a i r of vectors:
C.V. (i) Fy, (ii) Ez, and (iii) log!, ( E d with t h e Pearson product moment c o r r e l a t i o n coej?ficient, and,
C.V. ( i v ) Ez applying t h e Spearman Rank c o r r e l a t i o n c o e f f i c i e n t (Siege1 1961)
Methods (iii) and ( i v ) were included t o cover t h e p o s s i b i l i t y of non-linear r e l a t i o n s h i p s suggested by t h e work of Austin and Noy-Meir
(1971)
.
A s a c r i t e r i o n , with which t o summarise t h e r e s u l t s u s e f u l l y and concisely, those c o r r e l a t i o n c o e f f i c i e n t s with s t a t i s t i c a l s i g n i f i c a n c e of P<0.05 were s e l e c t e d . Table 4 p r e s e n t s such a summary of a
c o r r e l a t i o n a n a l y s i s , based on t h e component s c o r e s of t h e PCA of ' matrix A1.
DISCUSSION Methodology
I n t h e preliminary a n a l y s i s of the d a t a matrices X and Y , a number of p r i n c i p a l component analyses (PCAs) were performed on d i f f e r e n t arrangements of t h e d a t a . Far be it from t h e purpose of t h i s paper t o p r e s e n t a f u l l discussion of t h e comparison of t h e d i f f e r e n t methods.
This has been adequately done by a number of authors, more r e c e n t l y by Whittaker (1973). However, f o r t h i s p a r t i c u l a r and somewhat unusual s e t of d a t a , the reasons f o r t h e f i n a l adoption of s t a n d a r d i s a t i o n s , used i n t h e above s e c t i o n , w i l l be o u t l i n e d b r i e f l y .
The stand o r d i n a t i o n , from a PCA based on unstandardised f l o r i s t i c v a r i a b l e s (noc i l l u s t r a t e d ) , r e s u l t e d i n the formation of a group of s p e c i e s poor s i t e s (stand s i t e s 6-12 i n c l u s i v e , Table 1). This was not advantageous t o i n t e r p r e t a t i o n e s p e c i a l l y a s those s i t e s contained important s p e c i e s which f e a t u r e d i n o t h e r s i t e s . The e f f e c t of
s t a n d a r d i s a t i o n f o r s t a n d s was t o spread o u t t h e s e s p e c i e s poor s i t e s a c r o s s t h e o r d i n a t i o n (Fig. 21, t o g i v e t r u e r r e a l i z a t i o n t o t h e p o s i t i o n of s i t e 11, and t o enable t h e p o s s i b l e d e t e c t i o n of g r a d i e n t along t h e l e n g t h o f t h e i s l a n d .
Treatment of matrix Y , t h e environmental v a r i a b l e s , t o produce a s t a n d o r d i n a t i o n , was achieved most e f f i c i e n t l y by s t a n d a r d i s a t i o n f o r t h e v a r i a b l e s . S t r a i g h t o r d i n a t i o n and an o r d i n a t i o n based on
logarithmic transformation of matrix Y proved u n s a t i s f a c t o r y .
For t h e s p e c i e s o r d i n a t i o n , s t a n d a r d i s a t i o n f o r each s p e c i e s was e s s e n t i a l , a s i n i t s absence, o r with only s t a n d a r d i s a t i o n f o r s t a n d s , a l l r a r e s p e c i e s were clumped a t t h e o r d i n a t i o n s o r i g i n with f a i l u r e t o d i s s e c t o u t t h e main v e g e t a t i o n types. Ordination o f t h e
environmental v a r i a b l e s was again b e s t performed with s t a n d a r d i s a t i o n f o r each s p e c i e s .
C o r r e l a t i o n a n a l y s i s were a p p l i e d t o t h e component s c o r e s of most of t h e a l t e r n a t i v e PCAs mentioned immediately above. Only t h a t
a n a l y s i s , using t h e component s c o r e s from PCA of matrix A1, provided an i n t e l l i g i b l e i n t e r p r e t a t i o n of the d a t a and the r e s u l t s of which were f u r t h e r supported by those environmental v a r i a b l e o r d i n a t i o n s
f i n a l l y adopted. I t was f e l t t h a t t h i s c o r r e l a t i o n procedure was r e l e v a n t only t o t h e component s c o r e s of t h e PCA matrix A1 a s matrix X was based on a complete s e t of f l o r i s t i c d a t a . On t h e o t h e r hand, matrix Y obviously l a c k s many v a r i a b l e s of t h e environment, s i n c e those
a c t u a l l y measured had i n e v i t a b l y been s e l e c t e d by circumstance and i n t u i t i o n based on some p o s t u l a t e of t h e i r importance. A c o r r e l a t i o n a n a l y s i s procedure based on t h e component s c o r e s of t h e PCA of matrix B d i d , i n f a c t , l e a d t o very s p u r i o u s conclusions which n e i t h e r matched t h e o t h e r c o r r e l a t i o n a n a l y s i s i n Table 4, nor complemented t h e
groupings of v a r i a b l e s i n f e r r e d by t h e o r d i n a t i o n s . This l a t t e r - mentioned c o r r e l a t i o n a n a l y s i s procedure i s n o t i l l u s t r a t e d and w i l l n o t be r e f e r r e d t o f u r t h e r
I n t e r r e l a t i o n s of f l o r i s t i c and environmental v a r i a b l e s
Stand o r d i n a t i o n s , from t h e PCA o f matrix A1 (Fig. 21, and from t h e PCA of matrix B (Fig. 3) do n o t suggest any grouping o f s i t e s , though s i t e s which a r e geographically c l o s e (Fig. 1) tend t o l i e near one another on t h e o r d i n a t i o n s and s i t e s 11 and 19 a r e s e p a r a t e d from t h e r e s t , f o r example s i t e s 15, 1 7 and 19 and s i t e s 2, 3 and 4 i n Fig. 2. I n general terms a x i s I (Fig. 3) and a x i s I11 (Fig. 3) tend t o s e p a r a t e o u t t h e s i t e s , w i t h t h e n o r t h e r l y ones a t one end and t h e s o u t h e r l y ones a t t h e o t h e r end. The s t a n d o r d i n a t i o n o f t h e
environmental v a r i a b l e s a l s o i l l u s t r a t e s the d i f f e r e n c e o f s i t e s 11 and 19 from t h e o t h e r s i t e s .
More immediate information i s t o be gained from an examination of t h e v a r i a b l e s o r d i n a t i o n s . From the s c a t t e r of p o i n t s on t h e s p e c i e s o r d i n a t i o n (Fig. 4) it i s e v i d e n t t h a t some s p e c i e s a r e h i g h l y
a s s o c i a t e d a s groups A, B and C . These groups have not been
d e l i m i t e d by a q u a n t i t a t i v e method such a s c l a s s i f i c a t i o n , a s t h i s was considered unnecessary f o r such a small body o f d a t a , b u t seek only t o h e l p v i s u a l i n t e r p r e t a t i o n . Groups A, B and C c o n t a i n t h e following s p e c i e s , c o n s i s t e n t i n t h r e e dimensions:
A: C y p e r u s l i g u l a r i s , P s i l o t u r n nudum, O p l i s m e n u s i m b e c i l i s ,
F i m b r i s t y l i s cymosa
,
C l a d i u m j a m a i c e n s e and Euphorbia i n d i c a.
B: Kalanchoe p i n n a t a , N e p h r o l e p i s h i r s u t u l a , C o l o c a s i a e s c u l e n t a ,
~ e i s o s p e r m a o p p o s i ti f o l i a
,
L a p o r t e a i n t e r r u p t a and F i c u s b e n g h a l e n s i s.
C: P o r t u l a c a t u b e r o s a
,
S i d a h u m i l i s , Launaea p i n n a t i f i d a,
L e p t u r u s r e p e n s and an unknown s p e c i e s (No. 34, Table 1).
Examination o f t h e t h i r d a x i s F i g . 4b, c r e v e a l s f o u r more groups s e p a r a t e d from t h e c e n t r a l mass of p o i n t s i n Fig. 4a. These groups D t o F i n c l u d e :
D: Ipomoea m a c r a n t h a , V e r n o n i a c i n e r e a and E r a g r o s t i s t e n e l l a
E: G u e t t a r d a s p e c i o s a , Pemphis a c i d u l a , R h i z o p h o r a mucronata and T o u r n e f or t i a a r g e n t ea
.
F: C a r i c a papaya
,
P o l y s c i a s g u i l f o y l e i,
A c h y r a n t h e s a s p e r a and unknown s p e c i e s (no. 39, Table 1).
G: P a s s i f l o r a s u b e r o s a , Morinda c i t r i f o l i a , A s p l e n i u m n i d u s and Hernandi a p e l t a t a
.
Group A i n c l u d e s e s s e n t i a l l y t h o s e s p e c i e s which r e c o l o n i s e
d i s t u r b e d coconut groves i n t h e n o r t h of t h e i s l a n d ( s i t e s 2-5) w h i l s t B c o n t a i n s t h o s e s p e c i e s of t h e w e t t e r c e n t r a l f o r e s t c l o s e t o t h e encampment (such a s s i t e 1 8 ) . The two main s p e c i e s o f d i s t u r b e d a r e a s a r e Thuarea i n v o l u t a (no. 3) i n t h e h e a v i l y trampled p a r t s of t h e i s l a n d and W e d e l i a b i f l o r a (no. 1 9 ) , a weed-like s p e c i e s i n h a b i t i n g d i s t u r b e d c l e a r e d ground. (Fig. 5b, c ) . Group C on t h e o t h e r hand i n c l u d e s t h o s e s p e c i e s r e c o g n i s e d from t h e s c r u b s i t e s ( s i t e s 1 3 and 14) t o g e t h e r w i t h C a s s y t h a f i l i f o r m i s (No. 7)
.
The f o r e s t s i t e s a r e t h e f o u r groups D t o F p l u s o t h e r s p e c i e s . The arrangement of t h e s e groups p r e s e n t s a s u c c e s s i o n a l p i c t u r e from t h e shrubs and small t r e e s i n D , ( r e l a t e d t o t h e s p e c i e s o f group C) t o t h e mature hardwood f o r e s t s i t e s of group G. Group G s p e c i e s a r e found i n such s i t e s a s 15, 1 7 and 19 amongst t h e main s p e c i e s ,
P i s o n i a g r a n d i s and P a s s i f l o r a s u b e r o s a . Group E , a g a i n , a r e
intermediary f o r e s t s i t e s b u t d r i e r , t y p i f i e d by s i t e 15 and a s s o c i a t e d with t h e pawpaw, C a r i c a papaya. L a s t l y , group F , may b e i d e n t i f i e d by mid-island s c r u b , i n c l u d i n g t h e most common bush on t h e i s l a n d ,
G u e t t a r d a s p e c i o s a , and t h e mangrove, R h i z o p h o r a mucrona'ta.
The remaining s p e c i e s , s o f a r n o t d i s c u s s e d , a r e probably, i n terms o f biomass t h e most important, having a s s o c i a t i o n s w i t h two o r more s p e c i e s groups between which they l i e . Cocos n u c i f e r a (no. 5)
,
t h e coconut, f o r i n s t a n c e , i n F i g . 5b, l i e s between groups A , B, C , D, and E a s i t s h a r e s a d i s t r i b u t i o n among a l l t h o s e sites whose s p e c i e s
![Table 1. (continued]](https://thumb-ap.123doks.com/thumbv2/123dok/11201710.0/35.1187.21.1125.107.696/table-1-continued.webp)
