AN EVALUATION OF CORAL REEF SURVEY METHODS BY COMPUTER SIMULATION
R. A. K i n z i e I11
INTRODUCTION
A computer program package h a s been developed t o g e n e r a t e s i m u l a t e d c o r a l d i s t r i b u t i o n s on a " r e e f . " These " r e e f s " can b e d e s i g n e d t o i n - c o r p o r a t e v a r i a t i o n s i n s p e c i e s number, s i z e of c o r a l h e a d s of each
s p e c i e s , and t h e p o s i t i o n of e a c h s p e c i e s i n a n " h i e r a r c h y of a g g r e s s i o n "
(Lang, 1 9 7 3 ) . The o u t p u t from t h e r e e f - g e n e r a t i n g program g i v e s t h e l o c a t i o n and s i z e of e a c h head a s w e l l a s i n f o r m a t i o n f o r each s p e c i e s . The s p e c i e s i n f o r m a t i o n i n c l u d e s c o v e r , a v e r a g e head s i z e , and v a r i a b i - l i t y of head s i z e . These " r e e f s " w i t h known p a r a m e t e r s a r e t h e n
"sampled" u s i n g s i m u l a t i o n s of a number of t e c h n i q u e s p r e s e n t l y used by r e e f workers. The methods s i m u l a t e d a r e t h e c h a i n l i n k method ( P o r t e r , 1972, 1974; Loya, 1 9 7 2 ) ; a l i n e - p o i n t method w i t h two sampling i n t e n s i - t i e s : 40 p o i n t s p e r 10 m and 20 p o i n t s p e r 10 m (Roy and Smith 1971);
a p o i n t - q u a d r a t method u s i n g 25 u n i f o r m l y p l a c e d p o i n t s i n a l m q u a d r a t
2
t h a t i s p l a c e d a t t h r e e p o s i t i o n s a l o n g t h e 1 0 m t r a n s e c t l i n e ; and a count method t h a t simply c o u n t s t h e heads of e a c h s p e c i e s t h a t f a l l w i t h i n t h e t h r e e q u a d r a t s .
A l l of t h e s e methods e x c e p t t h e quadrat-count t e c h n i q u e g i v e cover e s t i m a t e s f o r each s p e c i e s . From f i e l d s t u d i e s , w e ranked t h e c o v e r e s t i m a t i n g methods a c c o r d i n g t o e f f o r t . The c o n c e p t o f e f f o r t i n c l u d e s t h e time s p e n t under w a t e r c o u n t i n g and t h e work i n v o l v e d i n p l a c i n g t h e g e a r on t h e r e e f . From most d i f f i c u l t t o e a s i e s t t h e y a r e : c h a i n l i n k , p o i n t - q u a d r a t , 40 p o i n t s , and 20 p o i n t s . The f i r s t q u e s t i o n we asked was whether t h e r e was enough improvement i n a c c u r a c y i n t h e u s e of t h e more d i f f i c u l t methods t o j u s t i f y t h e added e f f o r t (and hence reduced number of t r a n s e c t s t h a t c o u l d b e r u n i n a g i v e n amount of t i m e ) .
The second q u e s t i o n we asked was whether t h e r e was a s i g n i f i c a n t q u a d r a t - s i z e e f f e c t and, i f s o , whether d i f f e r e n c e s i n t h e d e n s i t i e s of c o r a l s i n q u a d r a t s of d i f f e r i n g s i z e s can b e used i n d e t e r m i n i n g t h e
d i s p e r s i o n p a t t e r n s of t h e c o r a l s ( M o r i s i t a , 1959; Grieg-Smith, 1 9 6 4 ) . Another measure of d i s p e r s i o n p a t t e r n s was s i m u l a t e d u s i n g a number of n e a r e s t - n e i g h b o r t e c h n i q u e s ( C l a r k and Evans, 1954; P i e l o u , 1 9 6 0 ) .
Department of Zoology, U n i v e r s i t y o f H a w a i i , Honolulu, Hawaii, USA
METHODS
We g e n e r a t e d t h r e e s e r i e s of " r e e f s " i n o r d e r t o s t u d y t h e e s t i m a t e s of c o r a l cover and c o r a l abundance. The r e e f p a r a m e t e r s we v a r i e d were c o v e r , head s i z e , and abundance. Keeping any one o f t h e s e c o n s t a n t and l e t t i n g t h e o t h e r two v a r y , we developed t h r e e s e r i e s : (A) cover of e a c h s p e c i e s c o n s t a n t ; (B) head s i z e o f e a c h s p e c i e s c o n s t a n t (each of t h e f o u r s p e c i e s had a mean r a d i u s two t i m e s t h a t of t h e n e x t s m a l l e r s p e c i e s ) ; and ( C ) r e l a t i v e head s i z e and abundance were c o n s t a n t w h i l e t h e cover was reduced by a f a c t o r of two i n each of t h e " r e e f s " i n t h e s e r i e s . The r e a s o n f o r t h e s e s e r i e s was t o s e e i f t h e methods were d i f f e r e n t i a l l y s e n s i t i v e t o s p e c i e s which might have e q u a l cover b u t which d i f f e r e d i n head s i z e and abundance.
I n o r d e r t o e v a l u a t e t h e d i s p e r s i o n e s t i m a t e s , we g e n e r a t e d a n o t h e r s e r i e s of " r e e f s " t h a t ranged from uniform through random t o clumped.
T h i s was accomplished by v a r y i n g head s i z e , o r d e r of placement o n t h e r e e f , and t h e d e g r e e of " a g g r e s s i v e n e s s " and hence t h e s p a c i n g of t h e h e a d s .
Three s e t s of t e n t r a n s e c t s u s i n g each o f t h e t r a n s e c t methods were r u n on each o f t h e " r e e f s . " The measure of c o r r e c t n e s s was how many t i m e s o u t of t e n t h e cover e s t i m a t e w a s w i t h i n
f
20% of t h e c o r r e c t c o v e r . A mean and s t a n d a r d d e v i a t i o n of c o r r e c t n e s s were o b t a i n e d . The v a r i a b l e s i z e d q u a d r a t method used t h e e n t i r e " r e e f , " s o i t was a c t u a l l y n o t a f 1 s a m p l i n g " method. The sample s i z e of t h e n e a r e s t - n e i g h b o r t e c h n i q u e had t o be v a r i a b l e because of t h e d i f f e r e n t numbers of c o r a l heads of each s p e c i e s .RESULTS
The a c c u r a c y of a l l t h e methods was q u i t e poor. I n no c a s e was t h e a c c u r a c y of a c o v e r e s t i m a t e g r e a t e r t h a n s e v e n o u t of t e n and u s u a l l y i t was l e s s t h a n f i v e o u t of t e n ; t h a t i s , e s t i m a t e s of cover were w i t h i n
2
20% of t h e t r u e v a l u e l e s s t h a n f i v e o u t of t e n t i m e s . There i s a tendency f o r t h e smaller, more abundant c o r a l s t o have t h e i r cover estima- t e d more c o r r e c t l y t h a n l a r g e r , r a r e r c o r a l s even though b o t h s p e c i e s have e q u a l c o v e r (A " r e e f s " ).
A t moderate l e v e l s of cover a l l t h e methods a r e a b o u t e q u a l l y bad;
hence t h e r e i s l i t t l e t o recommend t h o s e methods consuming more t i m e . I n a d d i t i o n , u t i l i z a t i o n of more r a p i d t e c h n i q u e s w i l l a l l o w more t r a n - s e c t s t o b e r u n . While t h e a c c u r a c y of any one of t h e s e w i l l b e low, t h e f a c t t h a t a l a r g e r a r e a o f t h e r e e f under s t u d y w i l l b e sampled w i l l a t l e a s t g i v e a more i n c l u s i v e p i c t u r e of t h e r e e f . I f c o r a l s a r e
r e l a t i v e l y r a r e (10 t o 15% c o v e r ) , t h e more time consuming methods do show more s e n s i t i v i t y t h a n t h e f a s t e r t e c h n i q u e s . I f cover i s l e s s t h a n 5 % , none of t h e methods a r e a t a l l a c c u r a t e .
The r e s u l t s of t h e s t u d i e s o n d i s p e r s i o n estimates a r e less c l e a r . We had n o a b s o l u t e r a n k i n g o f t h e d i s p e r s i o n p a t t e r n s of t h e c o r a l s ( s e e P i e l o u , 1 9 6 0 ) , s o t h e d i f f e r e n c e s d e t e c t e d by t h e methods w e r e n o t
d i r e c t l y t e s t a b l e . However, b o t h methods s i m u l a t e d d i d r a n k t h e s p e c i e s more o r less i n t h e way t h a t w e p r e d i c t e d , w i t h e f f e c t s of " a g g r e s s i o n "
i n c r e a s i n g u n i f o r m i t y , and s m a l l s i z e combined w i t h l a s t c h o i c e of
p o s i t i o n on t h e r e e f c a u s i n g a g g r e g a t i o n s . I n a d d i t i o n , w e had no f i e l d i n f o r m a t i o n comparable t o o u r s t u d i e s on c o v e r e s t i m a t e s on t h e d i f f i - c u l t i e s of t h e u s e of t h e d i s p e r s i o n e s t i m a t e s under w a t e r . We a r e , however, s a t i s f i e d , t h a t o u r s i m u l a t i o n s a r e r e a s o n a b l e a p p r o a c h e s t o t h e a c t u a l f i e l d t e c h n i q u e s .
CONCLUSIONS
When c o r a l s p e c i e s a r e r e l a t i v e l y abundant o r have h i g h c o v e r t h e r e i s l i t t l e t o recommend t h e more l a b o r i o u s and t i m e consuming sampling methods. I n f a c t , t h e f a s t e r methods a r e t o b e p r e f e r r e d , b e c a u s e more
t r a n s e c t s c a n b e r u n i n t h e same amount of time--covering a w i d e r a r e a of t h e r e e f , t h u s improving r e l i a b i l i t y i f n o t a c c u r a c y . When s p e c i e s a r e m o d e r a t e l y r a r e , t h e more t i m e consuming methods may b e j u s t i f i e d , and when s p e c i e s a r e uncommon o n l y v e r y e x t e n s i v e work would b e e x p e c t e d t o g i v e r e a s o n a b l y a c c u r a t e r e s u l t s anyway. We a r e p r e p a r i n g documenta- t i o n f o r t h i s program package and w i l l make i t a v a i l a b l e t o i n t e r e s t e d p a r t i e s . A more d e t a i l e d d i s c u s s i o n of t h e programs and a n e x t e n d e d a n a l y s i s o f t h e r e s u l t s i s p r e s e n t e d e l s e w h e r e ( K i n z i e and S n i d e r , i n p r e s s ) .
ACKNOWLEDGEMENTS
T h i s r e s e a r c h was s u p p o r t e d by NSF G r a n t No. GA35836.
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