The Island Dilemma by Jared Diamond

7/28/2019 The Island Dilemma by Jared Diamond

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T H E I S L A N D D I L E M M A : L E S S O N S O F M O D E R NB I O G E O G R A P H I C S T U D I E S F O R T H E D E S I G N O FN A T U R A L R E S E R V E S

JARED M. DIAMONDPhysiology D epartm ent , University o f Cali fornia Med ical Center,Lo s Angeles, California 90024, U SA

A B S T R A C TA s y s t e m o f n a t u r a l r e s er v e s, e a c h s u r r o u n d e d b y a l t e r e d h a b i t a t , r e s e m b l e s as y s t e m o f i sl an d s f r o m t h e p o i n t o f v i e w o f s pe c ie s r e s tr i c te d t o n a t u r a l ha b it a ts .R e c e n t a d v a n c e s i n i s la n d b i o g e o g r a p h y m a y p r o v i d e a d e t a i le d b a si s . fo ru n d e r s t a n d i n g w h a t t o e x p e c t o f s u c h a s y s t e m o f r es e rv e s. T h e m a i n c o n c l u s i o n sa r e a s f o l l o w s :

T h e n u m b e r o f s p e c i e s t h a t a r e se r v e c a n h o l d a t e q u i l i b r i u m is a . f u n c t i o n o f i t sa r e a a n d i ts i s o la t i o n . L a r g e r r e s e r v es , a n d r e s e r v e s l o c a t e d c l o s e t o o t h e r r e s e r v e s ,c a n h o l d m o r e s p e c ie s .

I f m o s t o f t h e a r e a o f a h a b i t a t i s d e s t r o y e d , a n d a r a c t i o n o f t h e a r e a is s a v e d a s ar e s e r v e , t h e r e s e r v e w i l l i n i t ia l l y c o n t a i n m o r e s p e c i e s t h a n i t c a n h o l d a te q u i l i b r iu m . T h e e x c e s s w i l l g r a d u a l l y g o e x t i n c t . T h e s m a l l e r t h e r es e rv e , th eh i g h e r w i l l b e t h e e x t i n c t i o n r a te s . E s t i m a t e s o f t h e s e e x t i n c t i o n r a t e s f o r b i r d a n dm a m m a l s p e c ie s h a v e r e c e n t l y b e c o m e a v a il a b le i n a f e w c as es .D i f f e r e n t s p e c i e s r e q u i r e d i f f e r e n t m i n i m u m a r e a s to h a v e a r e a s o n a b l e c h a n c e o fs u r v i v a l .

S o m e g e o m e t r i c d e s i g n p r i n c i p l e s a r e s u g g e s t e d i n o r d e r t o o p t i m i s e t h e . f u n c t io no f r e s e r v e s i n s a v i n g s p e c i e s .

INTRODUCTIONFor terrestrial and freshwater plant and animal species, oceanic islands representareas where the species can exist, surrounded by an area in which the species cansurvive poorly or not at all and which consequently represents a distributionalbarrier. Many situations that do not actually involve oceanic islands neverthelesspossess the same distributional significance for many species. Thus, for alpinespecies a mounta in top is a distribut ional 'island' surrounded by a 'sea' of lowlands;129Biol. Conserv. (7) (1975)-- Applied Science Publishers Ltd, England, 1975Printed in Great Britain


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130 JARED M. DIAMOND

for an aquatic species a lake or river is a distributional island surrounded by a sea ofland; for a forest species a wooded tract is a distributional island surrounded by asea of non-forest habitat; and for a species of the intertidal or shallow-water zones,these zones represent distributional islands compressed between seas of land and o fdeep water.

O r ig i n a l e q u i l ib r i u m s i tu a t i o n Situat ion at t ime when protec-t ive measures go into force

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Final equ i l ibr ium si tuat ion

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F i g . 1 . I l l u s t r a t i o n o f w h y t h e p r o b l e m s p o s e d b y d e s i $ n i n g a s y s t e m o f n a t u r a l r e se r v e s a r e s i m i l a r t ot h e p r o b l e m s o f i s l a n d b i o g e o g r a p h y . I n t h e s i t u a t i o n b e f o r e t h e o n s e t o f a c c e l e r a ti n g h a b i t a td e s t r u c t io n b y m o d e r n m a n , m a n y n a t u r a l h a b i t a t s w er e p r e se n t a s c o n t in u o u s e x p a n s e s co v e r in g l a r g ea r e a s ( i n d i ca t e d b y s h a d e d a r e a s o f s k e t c h o n l e ft ). S p e c i es c h a r a c t e r i s ti c o f s u c h h a b i t a t s w e r e s i m i l a r l yd i s t r i b u t e d o v e r l a r g e , r e la t i v el y c o n t i n u o u s e x p a n s e s . B y t h e t i m e t h a t e x t e n s i v e h a b i t a t d e s t r u c t i o n h a so c c u r r e d a n d s o m e o f t h e r e m a i n i n g f r a g m e n t s a r e d ec l a r e d n a t u r a l r e s e r v es , t h e t o t a l a r e a o c c u p i ed b yt h e h a b i t a t a n d i t s c h a r a c t e r i s ti c sp e c ie s i s m u c h r e d u c e d ( c e n t r e s k e tc h ) . T h e a r e a i s a l s o f r a g m e n t e di n t o i s o l a t e d p i ec e s . F o r m a n y s p e ci e s, s u c h d i s t r i b u t i o n s a r e u n s t a b l e . A p p l y i n g t h e le s s o n s o f m o d e r ni s l a n d b i o g e o g r a p h y t o t h e s e i s l an d s o f n a t u r a l h a b i t a t s u r r o u n d e d b y a s e a o f d i s t u r b e d h a b i t a t m a yh e l p p r e d i c t t h e i r f u t u r e p r o s p e c t s .Throughout the world today the areas occupied by many natural habitats, andthe distr ibutional areas of many species, are undergoing two types of change (Fig.1). First, the total area occupied by natural habitats and by species adverselyaffected by man is shrinking, at the expense of area occupied by man-made habitatsand by species benefited by man. Second, formerly continuous natural habitats anddistributional ranges of man-intolerant species are being fragmented intodisjunctive pieces. If one applies the island metaphor to natural habitats and toman-intolerable species, island areas are shrinking, and large islands are beingbroken into archipelagos of small islands. These processes have importantpractical consequences for the future of natural habitats and man-intolerantspecies (Preston, 1962; Willis, 1974; Diamond, 1972, 1973; Terborgh, in press, a, b;Wilson & Willis, in press). Ecologists and biogeographers are gaining increasing


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T H E I S L A N D D I L E M M A 131u n d e r s t a n d i n g o f t h e s e p r o c e s s e s a s a r e s u l t o f t h e r e c e n t sc i en t if ic r e v o l u t i o ns t e m m i n g f r o m t h e w o r k o f M a c A r t h u r & W i l s o n (1 9 6 3 , 1 9 6 7) a n d M a c A r t h u r( 1 9 7 2 ) . I n th i s p a p e r I s h a l l e x p l o r e f o u r i m p l i c a t i o n s o f r e c en t b i o g e o g r a p h i c w o r kf o r c o n s e r v a t i o n p o l i ci e s : ( l ) T h e u l t i m a t e numbe r o f s p e c i e s t h a t a n a t u r a l r e s e r v ew i l l s a v e i s l i k e l y t o b e a n i n c r e a s i n g f u n c t i o n o f t h e r e s er v e 's a r e a . ( 2 ) T h e ra te a tw h i c h s p e c i e s g o e x t i n c t i n a r e s e r v e i s l i k e l y t o b e a d e c r e a s i n g f u n c t i o n o f th er e s e rv e ' s a r ea . ( 3 ) T h e r e l a t i o n b e t w e e n r e s e rv e d a r e a a n d p r o b a b i l i t y o f a sp e c i e s's u r v i v a l i s c h a r a c t e r i s t i c a l l y d i f f e r e n t f o r d i ff e r e n t s p e c i es . ( 4 ) E x p l i c i t s u g g e s t i o n sc a n b e m a d e f o r t h e o p t i m a l g e o m e t r i c d e s i g n o f r es e rv e s.

H O W M A N Y S P EC I E S W I L L S U R V I V E ?

L e t u s f ir st e x a m i n e t h e r e l a t io n b e t w e e n r e se r v e a r e a a n d t h e n u m b e r o f s p ec i est h a t t h e r e se r v e c a n h o l d a t e q u i l i b r i u m . A s a p r a c ti c a l i l l u s tr a t i o n o f t h i s p r o b l e m ,c o n s i d e r t h e f a c t t h a t w e s u r e l y c a n n o t s a v e a l l t h e r a in f o r es t o f t h e A m a z o n B a s i n .W h a t f r a c t io n o f A m a z o n i a m u s t b e l e f t a s r a i n f o r e s t t o g u a r a n t e e th e su r v i v a l o fh a l f o f A m a z o n i a ' s p l a n t a n d a n i m a l s p e c ie s , a n d h o w m a n y s p e c i e s w i l l a c t u a l l ys u r v i v e i f o n l y 1% o f A m a z o n i a c a n b e p r es e r v ed a s r a i n f o r e st ? N u m e r o u s m o d e l

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F i g . 2 . E x a m p l e o f t h e r e la t i o n b e t w e e n s p e c i e s n u m b e r a n d i s la n d a r e a i n a n a r c h i p e l a g o . T h eo r d i n a t e i s t h e n u m b e r o f r e s i d e n t , n o n - m a r i n e , l o w l a n d b i r d s p e c i e s ( S ) o n t h e is l a n d s o f V i t ia z a n dD a m p i e r S t r a i t s n e a r N e w G u i n e a i n t h e s o u t h - w e s t P a c if ic O c e a n , p l o t t e d a s a f u n c t i o n o f is l a n d a r ea( A , i n k m 2) o n a d o u b l e l o g a r i t h m i c s c a l e . T h e p o i n t s r e p r e se n t r e l a t i v e l y u n d i s t u r b e d i s la n d s . T h es t r a i g h t l i n e S = 18 . 9A 0"15w a s f i tt ed b y l e a s t m e a n s q u a r e s t h r o u g h t h e p o i n t s f o r t h e s e i sl a n d s . N o t et h a t s p e c i e s n u m b e r i n c r e a s e s r e g u l a r l y w i t h i s l a n d a r e a . T h e t w o p o i n t s O r e fe r t o L o n g a n d Ri tt e rI s l a n d s , w h o s e f a u n a s w e r e r e c e n t ly d e s t r o y e d b y v o l c a n i c e x p l o s i o n s a n d w h i c h h a v e n o t y e t r e ga in e dt h e i r eq u i l i b r i u m s p e c i e s n u m b e r .


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132 JARED M. DIAMONDs y s t e m s t o s u g g e s t a n s w e r s t o t h e s e q u e s t i o n s a r e p r o v i d e d b y d i s t ri b u t i o n a l s t u d i eso f v a r i o u s p l a n t o r a n i m a l g r o u p s o n v a r i o u s a r c h i pe l a g o s t h r o u g h o u t t h e w o r l d . I fo n e c o m p a r e s i s l a n d s o f d i ff e r e n t s i ze b u t w i t h s i m i l a r h a b i t a t a n d i n t h e s a m ea r c h i p e l a g o , t h e n u m b e r o f s p e ci e s S o n a n i s la n d i s u s u a l ly f o u n d t o i n c re a s e w i thi s l a n d a r e a A i n a d o u b l e l o g a r i t h m i c r e l a t i o n :

S = SoA z (1 )w h e r e S o is a c o n s t a n t f o r a g i v e n s p e c i e s g r o u p i n a g i v e n a r c h i p e l a g o , a n d z u s u a l l ya s s u m e s a v a l u e i n t h e ra n g e 0 . 1 8 - 0 . 3 5 ( P r e s t o n , 1 96 2; M a c A r t h u r & W i ls o n , 1 96 3 ,1 96 7; M a y , i n p re s s ). A r o u g h r u l e o f t h u m b , c o r r e s p o n d i n g t o a z v a l u e o f 0 . 30 , ist h a t a t e n f o l d i n c r e a s e i n i s l a n d a r e a m e a n s a t w o f o l d i n c r e a s e i n t h e n u m b e r o fs p e c ie s . F i g u r e 2 i l lu s t r a te s t h e s p e c i e s / a r e a r e l a t i o n f o r t h e b r e e d i n g l a n d a n df r e s h w a t e r b i r d s p e c i e s o n th e i s la n d s o f t h e B i s m a r c k A r c h i p e l a g o n e a r N e wG u i n e a a n d s h o w s t h a t t h e n u m b e r o f b i r d s p e c ie s i n c re a s e s r e g u l a r l y w i t h i s la n da r e a . I f o n e c o m p a r e s i s l a n d s o f s i m i l a r a r e a b u t a t d i f fe r e n t d i s t a n c e s f r o m t h ec o n t i n e n t o r l a r g e is l a nd t h a t s e r v es a s th e m a i n s o u r c e o f c o l o n i s a t i o n , t h e n o n ef in d s t h a t t h e n u m b e r o f s p e c ie s o n a n i s la n d d e c r e a s e s w i t h i n c r e a s i n g d is t a n c e .T h i s f e a t u r e is il l u s tr a t e d b y F i g . 3, w h i c h s h o w s t h a t t h e n u m b e r o f b ir d s p e c i es o n

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o.zs- ~ ~ a c t u a l S _ % o near-island S 0 0

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n I I I t I ~ I u I F0 2,000 4,000 6,000 8,000 10,000D I S T A N C E ( k m )

Fig. 3. Example of the relation between species number and island distance from the colonisationsource in an island archipelago. The ordinate is the nu mber of resident, non-marine, lowland birdspecies S on tropical south-west Pacific islands more than 500 km from New Guinea, divided by thenumber of species expected on an island of equivalent area less than 500 km from New Guinea. Theexpected near-is land S was read o ff the species/area relation for such islands (Fig. 5). The abscissa is theisland distance from New Guinea. Note that S decreases by a factor of 2 per 2600 km distance from NewGuinea. (After Diamond, 1972.)


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THE ISLAND DILEMMA 133

i s l a n d s o f t h e s o u t h - w e s t P a c i f i c d e c r e a s e s b y a f a c t o r o f 2 f o r e a c h 2 6 00 k m o fd i s t a n c e f r o m N e w G u i n e a . F o r p l a n t s o r a n i m a l s w i t h w e a k e r p o w e r s o f d i s p e r s a lt h a n b i r d s , th e f a l l - o f f i n s p e c ie s n u m b e r w i t h d i s t a n c e i s ev e n m o r e r a p i d .

S i m i l a r f in d i n g s a r e o b t a i n e d i f , i n s t e a d o f o c e a n i c i s la n d s , o n e c o m p a r e s h a b i t a t' i s l an d s ' w i th i n a c o n t i n e n t o r l a r g e i s l a n d. F o r e x a m p l e , i s o l a t e d a s e n c l a v es w i t h i nt h e r a i n f o r es t t h a t c o v e r s m o s t o f N e w G u i n e a a r e t w o s e p a r a t e a r e as o f s a v a n n a ,w h i c h r e c e iv e d m o s t o f th e i r p l a n t a n d a n i m a l s p e c i e s f r o m A u s t r a l i a ( S c h o d d e &C a l a b y , 1 97 2; S c h o d d e & H i t c h c o c k , 1 97 2) . T h e s a v a n n a w h i c h is l a r g er a n d a l soc l o s e r t o A u s t r a l i a s u p p o r t s t w i c e a s m a n y s a v a n n a b i r d s p e c i es a s t h e s m a l l e r a n dm o r e r e m o t e s a v a n n a ( F i g . 4 ) . O t h e r e x a m p l e s a r e p r o v i d e d b y m o u n t a i n s r i s in go u t o f t h e ' s e a ' o f l o w l a n d s , s u c h a s t h e i s o l a t e d m o u n t a i n r a n g e s o f A f r i c a , S o u t hA m e r i c a , N e w G u i n e a a n d C a l i f o r n i a . T h u s , t h e n u m b e r o f b i r d s p ec i e s o n e a c h' i s l an d ' o f a l p i n e v e g e t a t i o n a t h i g h e l e v a t i o n s i n t h e n o r t h e r n A n d e s i n c r e as e s w i tha r e a o f a l p i n e h a b i t a t a n d d e c r e a s e s w i th d i s t a n c e f r o m t h e l a rg e a l p i n e so u r c e a r e ai n t h e A n d e s o f E c u a d o r ( V u i l l e u m i e r , 1 97 0) .

,~o 14'o E ~oKt~ 0 , , 2o0. . . ~ M Io , , 2o0

- o : R ~ ,,. , . , ~ o -

Fig. 4. Exam ple of the re la t ion be tween a rea o f 'habi ta t i s lands ' and the numb er of charac te r is t icspecies they supp ort . M ost of New G uinea is covered b y rain forest , but tw o sepa rate areas on the southcoast (shaded in the f igure) sup por t savann a wo odland . The cha racter ist ic bird species of these savann asare m ost ly der ived f rom Au st ra l ia (the nor thern t ips of Aus t ra l ia a re jus t v isib le a t the lower border ofthe f igure) . The so-called T rans -Fly savanna ( lef t) not on ly has a larger area than the so-ca lled PortM oresby savann a ( right) , but i s a lso c lose r to the colonisa t ion source of Aust ra l ia . As a resul t, the / ra ns -F ly savann a su ppor ts twice as many bi rd spec ies charac te r is t ic of savanna w oodland (c. 30 comp aredwith 15 species) as does the Port Moresby savanna.


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Why is it tha t species number increases with increasing area of habitat butdecreases with increasing isolation? In explanation of these findings, Preston(1962) and MacArthur & Wilson (1963, 1967) suggested that species number S onan island is set by (or approaches) an equilibrium between immigration rates andextinction rates. Species immigrate into an island as a result of dispersal of colonistsfrom continents or other islands; the more remote the island, the lower is theimmigration rate. Species established on an island run the risk of extinction due tofluctuation in population numbers; the smaller the island, the smaller is thepopulation and the higher the extinction rate. Area also affects immigration andextinction rates in several other ways: through its relation to the regional magnitudeof spatial and temporal variation in resources; by being correlated with the varietyof available habitats as stressed by Lack (1973); and by being correlated with thenumber of 'hot spots', or sites of locally high utilisable resource production for aparticular species (Diamond, in press). On a given island, extinction rates increase,and immigrat ion rates decrease, with increasing S. The S value on an island in thesteady state is the number at which immigration and extinction rates become equal.The larger and less isolated the island, the higher is the species number at which itshould equilibrate.

The correctness of this interpretation has been established by several types ofstudy. One has involved observing the increase in species number on an islandwhose fauna and /o r flora have been destroyed. The most famous such study wasprovided by a 'natural experiment', the colonisation by birds of the vocanic islandof Krakatoa after its fauna had been destroyed by an eruption in 1883(Dammerman, 1948; see MacArthur & Wilson, 1967, pp. 43-51). Similar 'naturalexperiments' are provided by the birds of Long Island near New Guinea, whosefauna was destroyed by a volcanic eruption two centuries ago (see Fig. 2), and bythe birds of seven coral islets in the Vitiaz-Dampier group near New Guinea, whena tidal wave destroyed the fauna in 1888 (Diamond, 1974). Simberloff & Wilson(1969) created an analogous 'artificial experiment' by fumigating several mangrovetrees standing in the ocean offthe coast of Florida and observing the recolonisationof these trees by arthropods . In all these studies, the number of species on the islandreturned within a relatively short time to the value appropriate to the island's areaand isolation, confirming that this value really was an equilibrium value. Naturally,the rate of approach to equilibrium depends on the plant or animal group studiedand the island's location: for example, successive surveys have shown the numberof plant species on Krakatoa still to be rising and not yet to have reachedequilibrium (Docters van Leeuwen, 1936; MacArthur & Wilson, 1967, p. 49).

Another type of test of th[ MacArthur-Wilson interpretation is provided byturnover studies at equilibrium. According to the MacArthur-Wilsoninterpretation, although the n u m b e r of species on an island may remain near anequilibrium value, the iden t i t i e s of the species need not remain constant, because


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T H E I S L A N D D I L E M M A 135

n e w s p e c ie s a r e c o n t i n u a l l y i m m i g r a t i n g a n d o t h e r s p e c i e s a r e g o i n g e x ti n c t .E s t i m a t e s o f i m m i g r a t i o n a n d e x t i n c t i o n r a t e s a t e q u i l ib r i u m h a v e b e e n o b t a in e db y c o m p a r i n g s u r v e y s o f a n i s l a n d i n s e p a r a t e y e a r s . S u c h s t u d i e s h a v e b e e n c a r r i e do u t f o r t h e b i r d s o f t h e C h a n n e l I s l a n d s o f f C a l i f o r n i a ( D i a m o n d , 1 96 9; H u n t &H u n t , 1 97 4; J o n e s & D i a m o n d , i n p r e s s) , K a r k a r I s la n d o f f N e w G u i n e a( D i a m o n d , 1 97 1), V u a t o m I s la n d o f f N e w B r i ta i n ( D i a m o n d , in p r e ss ) , a n d M o n aI s l a n d o f f P u e r t o R i c o ( T e r b o r g h & F a a b o r g , 1 97 3). A l l t h e s e s t u d i e s f o u n d t h a t ac e r t a i n n u m b e r o f s p ec i e s p r e s e n t in t he e a r l ie r s u r v e y h a d d i s a p p e a r e d b y t h e t im eo f t h e l a t e r s u r v e y , b u t t h a t a s i m i l a r n u m b e r o f o t h e r s p e c i e s i m m i g r a t e d i n t h ei n t e r v e n in g y e a rs , so t h a t t h e t o t a l n u m b e r o f s p e ci e s r e m a i n e d a p p r o x i m a t e l yc o n s t a n t u n l es s t h e re w a s a m a j o r h a b i t a t d i s t u r b a n c e . A s e x p e c t e d f r o mc o n s i d e r i n g t h e r is k o f e x t i n c t i o n i n r e l a t i o n t o p o p u l a t i o n s iz e , m o s t o f t h ep o p u l a t i o n s t h a t d i s a p p e a r e d h a d i n i t i a ll y c o n s i s t e d o f f e w i n d i v i d u a ls . T h et u r n o v e r r a t e s p e r y e a r ( i m m i g r a t i o n o r e x t i n c t io n r a te s ) o b s e r v e d i n t h e se s tu d i esh a v e b e e n i n t h e o r d e r o f 0 . 2 - 6 % o f th e i s l a n d ' s b i rd s p e c i e s f o r i s l a n d s o f3 0 0 - 4 0 0 k m 2 a r e a .

T h u s , t h e n u m b e r o f s p e c i e s t h a t a r e s e r v e c a n ' h o l d ' a t e q u i l i b r i u m is l i k e ly to b es e t b y a b a l a n c e b e t w e e n i m m i g r a t i o n r a t e s a n d e x t i n c t i o n r a t e s. T h e s e t - p o i n t w il lb e a t a l a r g e r n u m b e r o f s p e c i e s, t h e l a r g e r th e r e s e r v e o r t h e c l o s e r it is t o a s o u r c e o fc o l o n i s t s :

! . I f 9 0 % o f th e a r e a o c c u p i e d b y a h a b i t a t i s c o n v e r t e d b y m a n i n t o a n o t h e rh a b i t a t a n d t h e r e m a i n i n g 1 0 % is s a v e d a s a n u n d i v i d e d r e se r v e, o n e m i g h t e x p e c tt o s a v e r o u g h l y a b o u t h a l f o f th e s p e c i e s r e s t r ic t e d t o t h e p r e s e r v e d h a b i t a t ty p e ,w h i l e t h e p o p u l a t i o n s o f t h e r e m a i n i n g h a l f o f th e s p e c ie s w ill e v e n t u a ll y d i s a p p e a rf r o m t h e r e s e r v e . I t s h o u l d b e s t r e ss e d e x p l i c i tl y t h a t i n c r e a s e d h a b i t a t d i v e r s i t y isp a r t o f t h e r e a s o n , b u t n o t t h e o n l y o n e (c f p . 1 3 4 f o r o t h e r s ) , w h y l a r g e r a r e a s h o l dm o r e s p e c i es . T h u s , e v e n i f a r e s e r v e d o e s i n c l u d e s o m e o f t h e t y p e o f h a b i t a tp r e f e r r e d b y a t h r e a t e n e d s p e c ie s , t h e s p e c i es m a y s ti ll d i s a p p e a r b e c a u s e o fp o p u l a t i o n f l u c tu a t io n s , s p a t i a l o r t e m p o r a l v a r i a t i o n i n r e s o u rc e s , a n d t o o f e w o rt o o s m a l l ' h o t s p o t s ' .

2 . I f o n e s a v e s t w o r e s e r v e s , t h e s m a l l e r r e s e r v e w i ll r e t a i n f e w e r sp e c i e s if i t isr e m o t e f r o m t h e l a r g e r r e s e r v e t h a n i t w o u l d i f i t w e r e n e a r t h e l a r g e r r e s e r v e .

3 . A s t h e c o n t r a s t i n c r e a s e s b e t w e e n th e p r e s e r v e d h a b i t a t t y p e s a n d t h es u r r o u n d i n g h a b i t a t t y p e s , o r b e t w e e n t h e e c o lo g i c a l r e q u i r e m e n t s o f a th r e a t e n e ds p e c i e s a n d t h e r e s o u r c e s a c t u a l l y a v a i l a b l e i n a r e a s l y i n g b e t w e e n r e s e r v e s , t h er e s u lt s o f i s l an d b i o g e o g r a p h i c s t ud i e s b e c o m e i n c re a s i n g ly r e l e v a n t . T h e g r e a t e rt h i s c o n t r a s t , t h e l o w e r w i ll b e t h e p o p u l a t i o n d e n s i t y o f th e t h r e a t e n e d s p e c i e s i nt h e a r e a b e t w e e n r e s e r v e s , a n d t h e l o w e r w i ll b e t h e s p e c i e s' d i s p e r s a l r a t e b e t w e e nt h e r e s e rv e s . T o s o m e s p e c ie s t h e i n t e r v e n i n g a r e a m a y b e n o b a r r i e r a t a ll , w h i l e t oo t h e r s p e c ie s it m a y b e a s m u c h o f a b a r r i e r a s th e o c e a n i s t o a f li g h tl e ss m a m m a l .


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136 J A R E D M . D I A M O N D

H O W R A P I D L Y W I L L S PE C IE S G O E X T I N C T .9

S u p p o s e t h a t 9 0 % o f a h a b i t a t i s d e s t r o y e d a n d t h e r e m a i n i n g 1 0 % is s a v e d a s af a u n a l r e s e r v e . T h e r e s e r v e w il l i n i t ia l l y s u p p o r t m o s t , t h o u g h n o t a ll , s p e c ie sr e s t r ic t e d t o t h e o r i g i n a l e x p a n s e o f h a b i t a L ( T h e a c t u a l p r o p o r t i o n o f t h e s p ec i e sp r e s e n t i n s u c h a p o r t i o n o f a l a rg e r h a b i t a t i s d i s c u ss e d o n p p . 9 - l 0 a n d 1 6 o fM a c A r t h u r & W i l s o n ( 19 6 7 ). ) H o w e v e r , w e h a v e j u s t s e e n t h a t a t e q u i l ib r i u m t h er e s e r v e w il l s u p p o r t o n l y a b o u t h a l f t h e sp e c ie s o f t h e o r i g in a l e x p a n s e o f h a b i t a t.T h u s , a t t h e t i m e t h a t t h e r e s e r v e i s s e t a s i d e , i t w i l l c o n t a i n m o r e s p e c i e s t h a n i t sa r e a c a n s u p p o r t a t e q u i l i b r iu m a s a n i sl a n d . S p e c i e s w i ll g o e x t i n c t u n t i l th e n e we q u i l i b r i u m n u m b e r is r e a c h e d . S u c h a r e s e rv e w il l c o n s t i t u t e t h e e x a c t c o n v e r s e o fa n i s la n d w h i c h h a s h a d i ts f a u n a d e s t r o y e d : e q u i l ib r i u m o f s p e c ie s n u m b e r w i ll b ea p p r o a c h e d f r o m a b o v e , b y a n e x c e ss o f e x t i n c t io n o v e r i m m i g r a ti o n , r a t h e r th a nf r o m b e l o w , b y a n e x c e s s o f i m m i g r a t io n o v e r e x t in c t io n . T h e i m p o r t a n t p r a c t ic a lq u e s t i o n t h u s a r i se s : h o w r a p i d l y w i ll s p ec i es n u m b e r ' r e la x ' t o t h e n e w e q u i l i b r i u mv a l u e ? I f e q u i l i b r i u m t i m e s w e r e o f t h e o r d e r o f m i ll io n s o f y e a r s, t h e se e x t i n c t i o n sw o u l d n o t b e a m a t t e r o f p r a c t i c a l c o n c e r n , w h e r e a s a r e s e r v e t h a t l o s t h a l f o f itss p e c ie s in a d e c a d e w o u l d b e u n a c c e p t a b l e .

A n a t u r a l e x p e r i m e n t t h a t p e r m i t s o n e t o a s s es s ' r e l a x a t i o n r a t e s ' a s a f u n c t i o n o ft h e r e s e r v e ' s a r e a is p r o v i d e d b y s o -c a l le d l a n d - b r i d g e i s la n d s ( D i a m o n d , 1 97 2,1 9 73 ). D u r i n g t h e l a te P l e i s t o c e n e , w h e n m u c h s e a - w a t e r w a s l o c k e d u p i n g l ac i er s ,t h e o c e a n l e ve l w a s a b o u t 1 0 0 - 2 0 0 m l o w e r t h a n a t p r e s e n t. C o n s e q u e n t l y , i s la n d ss e p a r a t e d f r o m c o n t i n e n t s o r f r o m l a r g e r i sl a n ds b y w a t e r le s s t h a n 1 00 m d e e pf o r m e d p a r t o f th e c o n t i n e n t s o r l a r g e r is l an d s , a n d s h a r e d t h e c o n t i n e n t a l f a u n a sa n d f lo r a s. E x a m p l e s o f s u c h ' l a n d - b r i d g e i s l a n d s' a r e B r i ta i n o f f E u r o p e , A r u a n do t h e r i s la n ds o f f N e w G u i n e a, T a s m a n i a o f f A u s t r a l ia , T r in i d a d o f f S o u t h A m e r i ca ,B o r n e o a n d J a v a o f f s o u t h - e a s t A s i a, a n d F e r n a n d o P o o f f A f r ic a . W h e n r is in g se al e v e ls s e v e r e d t h e l a n d - b r i d g e s a b o u t 1 0 ,0 0 0 y e a r s a g o , t h e s e l a n d - b r i d g e i s l a n d sm u s t h a v e f o u n d t h e m s e l v e s s u p e r s a t u r a t e d ; t h e y i n it ia l ly s u p p o r t e d a s p e c ie s - ri c hc o n t i n e n t a l f a u n a r a t h e r t h a n t h e s m a l l e r n u m b e r o f s p e ci e s a p p r o p r i a t e t o t h e i ra r e a a t e q u i l i b r iu m . G r a d u a l l y , s p e ci es m u s t h a v e b e e n l o s t b y a n e x c e s s o fe x t i n c t i o n s o v e r im m i g r a t i o n s . F i g u r e 5 i ll u s tr a te s h o w f a r t h e a v i f a u n a s o f t h es a t e l l i t e l a n d - b r i d g e i s l a n d s o f N e w G u i n e a h a v e r e t u r n e d t o w a r d s e q u i l i b r i u m i n1 0 ,0 0 0 y e a r s . T h e l a r g e r la n d - b r i d g e i s l a n d s , w i t h a r e a s o f s e v e r a l h u n d r e d t os e v e r a l t h o u s a n d k m % s ti ll h a v e m o r e b i r d s p e c ie s t h a n p r e d i c t e d f o r t h e i r a r e af r o m t h e s p e c i e s / a r e a r e l a t i o n b a s e d o n i s l a n d s a t e q u i l i b ri u m , t h o u g h t h e y d o h a v ec o n s i d e r a b l y f e w e r b i r d s p e c ie s t h a n N e w G u i n e a i t se lf . T h a t i s, t h e l a r g e r la n d -b r i d g e i s l a n d s h a v e l o s t m a n y b u t n o t a ll o f t h e i r e x c e s s s p e c i e s i n 1 0 , 00 0 y e a r s .H o w e v e r , l a n d - b r i d g e i s la n d s s m a l l e r t h a n a b o u t 2 5 0 k m 2 a t p r e s e n t h a v e t h e s a m en u m b e r o f b i r d s p e c i e s a s s im i l a r- s iz e d o c e a n i c i sl a n d s t h a t n e v e r h a d a la n d -b r id g e . T h u s , t h e s m a l l e r l a n d - b r i d g e i s la n d s h a v e l o s t t h e i r e n t i r e e x c e ss o f b ir ds p e c i e s i n 1 0 , 0 0 0 y e a r s .


9/18

T H E I S L A N D D I L E M M A 1 3 7

0 11601 78+5 +4

L 9+ +6 l@ _ e 3 ~ e 40 4 0 J

11 + ~ ' . J 6 @ A 1o / g l ' o ~ l o ~ / / ' e - i 1 o" I I u i I10', ~ 10 1 0 0 1 , 0 0 0 1 0 , 0 0 0 1 0 0 , 0 0 0 t ,0 0 0 , 0 0 0A R E A ( k i n z )

F i g . 5 . E x a m p l e o f h o w o n e c a n u s e l a n d - b r i d g e is l a n d s t o e s t i m a t e e x t i n c t i o n ra t e s in t h e f a u n a s o fn a t u r a l r e s e rv e s . T h e o r d i n a t e i s t h e n u m b e r o f r es i d e n t , n o n - m a r i n e , l o w l a n d b i r d s p e c i es o n N e wG u i n e a s a t e ll i t e is l a n d s , p l o t t e d a s a f u n c t i o n o f i s l a n d a r e a o n a d o u b l e l o g a r i t h m i c s ca l e . T h e p o i n t s a r e i s l a n d s w h i c h h a v e n o t h a d a r e c e n t l a n d - c o n n e c t i o n t o N e w G u i n e a a n d w h o s e a v i f a u n a s a rep r e s u m e d t o b e a t e q u i l i b r iu m . T h e n u m b e r e d p o i n t zx ( 1) r e fe r s t o a r e c e n t ly e x p l o d e d v o l c a n o w h o s ea v i f a u n a h a s n o t y e t r e t u r n e d t o e q u i l i b ri u m ; p o i n t s + ( 4 - 1 2) , t o i s l a n d s c o n n e c t e d t o N e w G u i n e a b yl a n d - b r i d g e s a t t i m e s o f l o w e r s e a - l e v e l 10 ,0 00 y e a r s a g o ; p o i n t s ( 1 3 - 1 6 ) , t o i s l a n d s f o r m e r l yc o n n e c t e d b y l a n d - b r i d g e s t o s o m e o t h e r l a r g e i s l a n d b u t n o t t o N e w G u i n e a i t s e l f; a n d p o i n t s [] ( 2 - 3 ) , t oi s l a n d s t h a t l i e o n a s h a l l o w s h e l f a n d h a d a m u c h l a r g e r a r e a a t t i m e s o f l o w e r s e a -l e v e l. U p t o t h e t i m et h a t t h e l a n d - b r i d g e s w e r e s e v e r e d b y r i s i n g s e a -l e v e l , t h e N e w G u i n e a l a n d - b r i d g e i s l a n d s ( + , 4 - 1 2 )m u s t h a v e s u p p o r t e d n e a r ly t h e f u ll N e w G u i n e a q u o t a o f 3 25 l o w l a n d s p e ci e s ( p o i n t i n t h e u p p e r r i g h t -h a n d c o r n e r ) . A t p r e s e n t n o n e o f t h e s e l a n d - b ri d g e i s l a n d s s u p p o r t s a n y t h i n g c l o s e to 3 2 5 s p e c ie s ; t h el a q g e r o n e s ( + , 4 - 9 ) d o , h o w e v e r , s t il l h a v e m o r e s a e c i e s t h a n e x p e c t e d a t e q u i l i b r i u m ( a s g i v e n b yp o i n t s a n d t h e s t r a i g h t l in e )- a n d t h e s m a U e r o n e s ( + 1 0 - 1 2 ) a l r e a d y h a v e a b o u t t h e n u m b e r o f s p e c i e se x p e c t e d a t e q u i li b r i u m . T h e ' c o n c l u s i o n i s t h a t n o l a n d - b r i d g e is l a n d h a s b e e n a b l e t o h o l d m o r e t h a nh a l f i t s i n i ti a l n u m b e r o f s p e c i e s , b u t t h a t t h e l a r g e r i s l a n d s h a v e b e e n a b l e t o h o l d a n e x c e s s o f s p e c i e s f o rl o n g e r . T h e s a m e c o n c l u s i o n f o l l o w s f r o m p o i n t s a n d [ ] . ( F r o m D i a m o n d , 1 97 2.)


10/18


T h e r e - e q u i l i b r a t i o n o f l a n d - b r i d g e i s la n d s i s th e r e s u l t a n t o f t h e e x t i n c t i o n r a teE ( i n s p e c i e s / y e a r ) e x c e e d i n g t h e i m m i g r a t i o n r a t e I ( in s p e c i e s / y e a r ) u n t i l a ne q u i l i b r i u m s p e c ie s n u m b e r S eq i s a t t a i n e d . B o t h I a n d E d e p e n d o n th ei n s t a n t a n e o u s s p e c i e s n u m b e r S ( t ) , w h e r e t r e p r e s e n t s t i m e ( in y e a r s ) . A s a h i g h l ys i m p l if ie d m o d e l , l e t u s a s s u m e c o n s t a n t c o e f f ic i e n ts K i a n d K e ( in y e a r - 0 o fi m m i g r a t i o n a n d e x t i n c t i o n , r e s p e c t i v e l y :

E = K e S ( t ) (2 )I = K i [S* - - S ( t ) ] ( 3 )

w h e r e S * is t h e m a i n l a n d s p e c ie s p o o l , a n d [ S * - - S ( t ) ] is t h e n u m b e r o f s p e c i e s i nt h e p o o l n o t p r e s e n t o n t h e i s l a n d a t t i m e t , h e n c e a v a i l a b le a s p o t e n t i a l i m m i g r a n ts .A t e q u i l i b ri u m , w h e n d S / d t = 1 - - E = 0 , S e q is g i v e n b y

S e q = K i S * / ( K i + K e ) (4 )I f a l a n d - b r i d g e i s l a n d i n i t i a l ly (a t t = 0 ) s u p p o r t s a s p e c i e s n u m b e r S ( 0 ) t h a te x c e e d s Se q , t h e r a t e a t w h i c h S ( t ) d e c l i n e s f r o m S ( 0 ) t o w a r d s S eq i s o b t a i n e d b yi n t e g r a t i n g t h e d i f f e re n t i a l e q u a t i o n

d S / d t = I - - E = ( K i + g e ) [ K i S * / ( K i + K e ) - - S( t )]

w i th t h e b o u n d a r y c o n d i t io n S( t) -~- S(O) a t t = 0 , t o o b t a i n :IS ( t ) - Seq I / IS (0 ) - - Seq] = exp ( - - t / t r ) (5)T h e r e l a x a t i o n t i m e t r is th e l e n g t h o f t i m e r e q u i r e d f o r t h e s p e ci es e x c e s s[ S ( t ) - - Se q ] t o r e l a x t o 1 e o r 3 6 . 8 % o f t h e i n i ti a l e x c e s s [ S ( 0 ) - - S e q ] , w h e r e e i s t h eb a s e o f n a t u r a l l o g a r i t h m s . R e l a x a t i o n is 9 0 % c o m p l e t e a f t e r 2. 30 3 r e l a x a t i o nt i m e s .A s a n e x a m p l e o f t h e u s e o f t h is f o r m u l a , c o n s i d e r t h e l a n d - b r id g e i s l a n d o fM i s o l n e a r N e w G u i n e a . A t t h e t i m e 1 0 ,0 0 0 y e a r s a g o w h e n i t f o r m e d p a r t o f N e wG u i n e a , M i s o l m u s t h a v e s u p p o r t e d n e a r l y th e f u ll N e w G u i n e a lo w l a n d s f a u n a o f3 2 5 b i r d s p e c i es . W i t h a n a r e a o f 2 0 4 0 k m 2, M i s o l s h o u l d s u p p o r t o n l y 6 5 sp e c ie s a te q u i l i b r iu m , b y c o m p a r i s o n w i t h t h e s p e c i e s / a r e a r e l a t i o n f o r i s la n d s th a t l a c k e dl a n d - b r i d g e s a n d a r e a t e q u i l i b ri u m . T h e p r e s e n t s p e c ie s n u m b e r o n M i s o l is 1 35 ,m u c h l e ss t h a n t h e i n i ti a l v a l u e o f 32 5 b u t s ti ll in e x c e s s o f th e f in a l e q u i l i b r i u mv a l u e . S u b s t i t u t i n g S ( 0 ) = 3 2 5 , S ( t ) = 135 , Seq = 65 , t = 10 ,000 ye ar s i n t o eq n .( 5) y i e ld s a r e l a x a t i o n t i m e o f 7 6 00 y e a r s f o r t h e a v i f a u n a o f M i s o l.

S i m i l a r c a l c u l a t i o n s h a v e b e e n c a r r i e d o u t f o r o t h e r l a n d - b r i d g e i s la n d s f o r m e r l yc o n n e c t e d t o N e w G u i n e a , f o r i s la n d s f o r m e r l y c o n n e c t e d t o s o m e o t h e r la rg es a t e l l i te i s l a n d b u t n o t t o N e w G u i n e a it s e lf , a n d f o r i s la n d s t h a t l i e o n a s h a l l o w -w a t e r s h e l f a n d t h a t f o r m e r l y m u s t h a v e b e e n m u c h l a rg e r in a r e a a l th o u g h w i t h o u t


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c o n n e c t i o n t o a l a r g e r is l a nd . A s i m i l a r a n a l y s i s i n a c o n t i n e n t a l s i t u a t i o n w a s m a d eb y B r o w n ( 1 9 71 ), w h o s t u d i e d d i s t r i b u t i o n s o f sm a l l n o n - v o l a n t m a m m a l s inf o r e s ts w h i c h a r e n o w i s o la t e d o n t h e t o p s o f m o u n t a i n s r is in g o u t o f w e s t e r n N o r t hA m e r i c a n d e s e r t b a s i n s b u t w h i c h w e r e fo r m e r l y c o n n e c t e d b y a c o n t i n u o u s f o r e s tb e l t d u r i n g t i m e s o f c o o l e r P l e i s t o c e n e c l i m a t e s . T e r b o r g h ( in p r e s s , a , b ) h a s m a d ea s i m i l a r a n a l y s i s o f t h e a v i f a u n a s o f C a r i b b e a n i s la n d s a n d h a s d r a m a t i c a l l yc o n f i r m e d t h e a c c u r a c y o f h is c a l c u l a t io n s b y s h o w i n g t h a t t h e y c o r r e c tl y p r e d i c tt h e e x t in c t i o n r a t e s o b s e r v e d w i t h i n t h e p r e s e n t c e n t u r y o n B a r r o C o l o r a d o I s la n d( W i l li s, 1 9 74 ). B o t h T e r b o r g h ' s a n a l y s e s o f C a r i b b e a n b i r d s a n d m i n e o f N e wG u i n e a b i r d s s h o w t h a t r e l a x a t i o n t i m e s i n c r e a s e w i t h i n c r e a s in g i sl a n d a r e a . B o t ha n a l y s e s a l s o s h o w t h a t e q n s . ( 2) a n d ( 3) a r e o v e r s i m p l i f ie d : K i a c t u a l l y i n c r e a s e sw i t h S(t), a n d K i d e c r e a s e s w i t h S(t).

T h u s , t h e g r a d u a l d e c l i n e o f s p e c ie s n u m b e r f r o m a h i g h i n i t ia l v a l u e t o a l o w e re q u i l i b ri u m v a l u e o n l a n d - b r i d g e i s la n d s m a y f u rn i s h a m o d e l f o r w h a t c o u l dh a p p e n w h e n a f r a c t i o n o f a n e x p a n s e o f h a b i t a t is se t a s id e a s a r es e r v e a n d t h er e m a i n i n g h a b i t a t i s d e s t r o y e d . A s m a l l r e se r v e n o t o n l y w i ll e v e n t u a l l y c o n t a i n f e ws p e c i e s b u t w i l l a l s o i n i t i a l l y l o s e s p e c i e s a t a h i g h r a t e . F o r r e s e r v e s o f a f e w k m 2,e x t i n c t i o n r a t e s o f s e d e n t a r y b i rd a n d m a m m a l s p e c ie s u n a b l e t o c o l o n i se f r o m o n er e s e r v e to a n o t h e r a r e s o h i g h a s t o b e e a s i ly m e a s u r a b l e i n a f e w d e c a d e s . W i t h i n af e w t h o u s a n d y e a r s e v e n a r e s e r v e o f 1 00 0 k m 2 w i ll h a v e l o s t m o s t s u c h s p e c i e sc o n f i n e d t o t h e r e s e r v e h a b i t a t . T h e s e e s t i m a t e s a s s u m e t h a t m a n ' s la n d - u s ep r a c t i c e s d o n o t g r o s s l y a l t e r t h e p r e s e rv e d h a b i t a t . M o r e r a p i d c h a n g e s i n s p e ci e sc o m p o s i t i o n a r e l ik e l y t o o c c u r i f s y l v ic u l t u re o r o t h e r h u m a n u s e c h a n g e s t h eh a b i t a t s t r u c t u r e .

WHAT SPECIES WILL SURVIVE?

I n t h e p r e c e d i n g p a g e s w e h a v e c o n s i d e r e d t h e p r o b l e m o f su r v i v a l f r o m a s ta t is t ic a lp o i n t o f v i e w : w h a t f r a c t i o n o f i ts in i ti a l f a u n a w i l l a r e s e r v e e v e n t u a l l y s a v e , a n dh o w r a p i d l y w i l l t h e r e m a i n d e r g o e x t in c t ? W e h a v e n o t y e t c o n s i d e r e d t h e s u r v i v a lp r o b a b i l i t i e s o f i n d i v i d u a l s p e ci e s. I f e a c h s p e c i es h a d e q u a l p r o b a b i l i t i e s o fs u r v i v a l , t h e n i t w o u l d b e a v i a b l e c o n s e r v a t i o n s t r a t e g y t o b e s a ti s fi e d w i t h l a r g en u m b e r s o f s m a l l r e s e rv e s . E a c h s u c h v e s t - p o c k e t r e s er v e w o u l d l o s e m o s t o f it ss p ec i es b e f o r e r e a c h i n g e q u i l i b r i u m , b u t w i t h e n o u g h r e se r v e s a n y g i v e n sp e c ie sw o u l d b e l i k el y to b e a m o n g t h e s u r v i v o r s i n a t l e a s t o n e r e s e r v e . I n t h i s s e c t i o n w es h a l l e x a m i n e t h e f la w i n t h i s s t r a t e g y : d i f f e r e n t s p e c ie s h a v e v e r y d i f f e r e n t a r e ar e q u i r e m e n t s f o r s u rv i v a l .

T h e s u r v i v a l p r o b l e m n e e d s t o b e c o n s i d e r e d f r o m t w o p o i n t s o f v i ew : t h e c h a n c et h a t a r e s e r v e w h e r e a s p e c ie s h a s g o n e e x t i n c t w i ll b e r e c o l o n i s e d f r o m a n o t h e rr e s e r v e , a n d t h e c h a n c e t h a t a s p e c i e s w i ll g o e x t i n c t in a n i so l a t e d r e s e r v e . C o n s i d e rt h e f o r m e r q u e s t i o n f ir st . S u p p o s e t h a t t h e r e a r e m a n y s m a l l r e se r v e s. S u p p o s e n e x t


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140 J A R E D M . D I A M O N Dt h a t a g i v e n s p e c ie s i s i n c a p a b l e o f d is p e r s in g f r o m o n e r e s e r v e t o a n o t h e r a c r o s s t h ei n t e r v e n i n g s e a o f u n s u i t a b l e h a b i t a t . T h e i s o la t e d p o p u l a t i o n s i n e a c h r e s e rv e ru n af in i te r i s k o f e x t i n c t i o n . I f t h e r e is n o p o s s i b i l i ty o f r e c o l o n i s a t i o n , e a c h e x t i n c t i o n i si r r e v o c a b l e , a n d i t is o n l y a q u e s t i o n o f ti m e b e f o r e t h e l as t p o p u l a t i o n o f t h e s p e ci esd i s a p p e a r s . S u p p o s e o n t h e o t h e r h a n d t h a t d i s p e r s a l f r o m o n e r e s e rv e t o a n o t h e r isp o s s ib l e . T h e n , a l t h o u g h a s p e c ie s t e m p o r a r i l y g o e s e x t i n c t in o n e r e s er v e , t h es p e c ie s m a y h a v e r e c o l o n i s e d t h a t r e s e r v e b y t h e t i m e i t g o e s e x t i n c t i n a n o t h e rr e s e rv e . I f t h e r e a r e e n o u g h r e s e rv e s o r h i g h e n o u g h r e c o l o n i s a t i o n r a t es o r l o we n o u g h e x t i n c t i o n r a t e s , t h e c h a n c e s o f t h e s pe c ie s d i s a p p e a r i n g s i m u l t a n e o u s l yf r o m a l l r e s e r v e s a r e l o w , a n d t h e l o n g - t e r m s u r v i v a l p r o s p e c t s a r e b r i g h t . D i s p e r s a la b i l it y o b v i o u s l y d i f fe r s e n o r m o u s l y a m o n g p l a n t a n d a n i m a l s p e c ie s . F ly i n ga n i m a l s t e n d t o d i s p e r se b e t t e r t h a n n o n - f l y in g o n e s ; p l a n t s w i t h w i n d - b o r n e s e e dst e n d t o d i sp e r s e b e t t e r t h a n p l a n t s w i t h h e a v y n u t s . T h e m o r e s e d e n t a r y t h e s p e ci es ,t h e m o r e i r r e v o c a b l e i s a n y l o c a l e x t i n c t i o n , a n d t h e m o r e d i f f i c u l t w i l l i t b e t od e v is e a s u c c e s s f u l c o n s e r v a t i o n s t r a t e g y . T h u s , c o n s e r v a t i o n p r o b l e m s w i ll b em o s t a c u t e f o r s lo w l y d i sp e r s i n g s p e ci e s i n n o r m a l l y s t ab l e h a b i t a t s , s u c h a st r o p i c a l r a i n f o r es t . E v e n p o w e r o f fl ig h t c a n n o t b e a s s u m e d t o g u a r a n t e e h i g hd i s p e r s a l a b i l i ty . F o r in s t a n c e , 1 34 o f t h e 3 2 5 l o w l a n d b i r d s p e c i e s o f N e w G u i n e aa r e a b s e n t f r o m a ll o c e a n i c i s l a nd s m o r e t h a n a fe w k m f r o m N e w G u i n e a , a n d a r ec o n f i n e d t o N e w G u i n e a p l u s i s la n d s w i t h r e c e n t l a n d - b r i d g e c o n n e c t i o n s t o N e wG u i n e a . S i m i l a r ly , m a n y n e o t r o p i c a l b i r d f a m i l ie s w i t h d o z e n s o f sp e c ie s h a v e n o te v e n a s i n g l e r e p r e s e n t a t i v e o n a s i n g l e N e w W o r l d i s l a n d l a c k i n g a r e c e n t l a n d -b r i d g e t o S o u t h o r C e n t r a l A m e r i c a ; a n d n o t a s in g le m e m b e r o f m a n y l a r g e A s i a nb i r d f a m i l ie s h a s b e e n a b l e t o c ro s s W a l la c e ' s L i n e s e p a r a t i n g t h e S u n d a S h e l f l a n d -b r i d g e i s l an d s f r o m t h e o c e a n i c is l a n ds o f I n d o n e s i a . S u c h b i r d s p e c ie s h a v ei n s u p e r a b l e p s y c h o l o g i c a l b a r r i e r s t o c ro s s i n g w a t e r g a p s , a n d a r e g e n e r a l l yc h a r a c t e r i s t ic o f s ta b l e fo r e s t h a b i ta t s . T h u s , l o w r e c o l o n i s a t i o n r a te s m a y m e a ne i t h e r t h a t a s p e c i e s c a n n o t c r o s s u n s u i t a b l e h a b i t a t s ( a m o u n t a i n f o r e s t r o d e n tf a c e d b y a d e s e r t b a r r i e r ), o r t h a t it w i l l n o t c r o s s u n s u i ta b l e h a b i t a t s ( s o m e t r o p i c a lf o r e s t b ir d s f a c e d b y a w a t e r g a p ) .

H a v i n g s e e n t h a t s p e ci e s v a r y i n t h e i r a b i li t y t o r e c o l o n i s e , l e t u s n o w c o n s i d e rh o w s p e c i e s v a r y i n e x t i n c t i o n r a t e s o f l o c a l p o p u l a t i o n s . T h e N e w G u i n e a l a n d -b r i d g e i s la n d s a g a i n o f f e r a c o n v e n i e n t t e s t s it u a t i o n ( D i a m o n d , 1 97 2, in p re s s ).R e c a l l t h a t t h e s e i s l an d s i n i ti a ll y s u p p o r t e d m o s t o f t h e N e w G u i n e a l o w l a n d sf a u n a , t h a t t h e l a n d - b r i d g e s w e r e s e v e r e d a b o u t 1 0 ,0 0 0 y e a r s a g o , t h a t 1 34 N e wG u i n e a l o w l a n d s b i rd s p e c i es d o n o t c r o s s w a t e r g a p s , a n d t h a t a n y e x t i n c t i o n s o fp o p u l a t i o n s o f th e s e s p e c ie s o n t h e l a n d - b r i d g e i s la n d s c a n n o t t h e r e f o r e h a v e b e e nr e v e r s e d b y r e c o l o n i s a t i o n . V i r t u a l ly a ll t h es e s p e c ie s a r e n o w a b s e n t f r o m a ll l a n d -b r i d g e i s la n d s s m a l l e r t h a n 5 0 k m 2, b e c a u s e e x t i n c t i o n r a t e s o n s m a l l i s la n d s a r e s oh i g h t h a t v i r t u a l l y n o i s o l a te d p o p u l a t i o n s u r v iv e s 1 0 ,0 0 0 y e a r s . H o w e v e r , th e s e1 3 4 s p e c i e s v a r y g r e a t l y t o d a y i n t h e i r d i s t r i b u t i o n o n t h e s e v e n l a r g e r( 4 5 0 - 8 0 0 0 k m 2) l a n d - b r i d g e i s la n d s . A t t h e o n e e x t r e m e , s o m e s p e c i e s , s u c h a s t h e


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f r i l l e d m o n a r c h f l y c a t c h e r (Monarcha telescophthalmus), h a v e s u r v i v e d o n a l ls e v e n i sl a n ds . A t t h e o t h e r e x t r e m e , 3 2 s p ec i es h a v e d i s a p p e a r e d f r o m a l l s e v e ni s la n d s , a n d m u s t b e e s p e c i a ll y p r o n e t o e x t i n c t i o n i n i s o la t e d p o p u l a t i o n s . M o s t o ft h e s e 3 2 s p e c i e s fi t i n t o o n e o r m o r e o f t h r e e c a t e g o r i e s : b i r d s w h o s e i n i t i a lp o p u l a t i o n s m u s t h a v e n u m b e r e d f e w i n d i v id u a l s b e c a u s e o f v e r y l a r g e t e r r i to r yr e q u i r e m e n t s (e.g. t h e N e w G u i n e a h a r p y e a g le (H arpyopsis novaeguineae)); b i r d sw h o s e i n it ia l p o p u l a t i o n s m u s t h a v e n u m b e r e d f e w i n d iv i d u a l s b e c a u s e o fs p e c i al is e d h a b i t a t r e q u i r e m e n t s (e.g. t h e s w a m p r a il (Megacrex inepta)); a n d b i r d sw h i c h a r e d e p e n d e n t o n s e a s o n a l o r p a t c h y f o o d s o u r ce s a n d n o r m a l l y g o t h r o u g hd r a s t i c p o p u l a t i o n f l u c t u a t i o n s (e.g. f r u i t - e a t e r s a n d f l o w e r - f e e d e r s ) .

A n o t h e r n a t u r a l e x p e r i m e n t i n d i f fe r e n t ia l e x t i n c t i o n is p r o v i d e d b y N e wH a n o v e r , a n i s la n d o f 1 2 0 0 k m 2 i n t h e B i s m a r c k A r c h i p e l a g o n e a r N e w G u i n e a . I nt h e l a te P l e i s t o c e n e , N e w H a n o v e r w a s c o n n e c t e d b y a l a n d - b r i d g e t o th e l a rg e ri s la n d o f N e w I r e l a n d a n d m u s t t h e n h a v e s h a r e d m o s t o f N e w I r e l a n d 's sp e c ie s .T o d a y N e w H a n o v e r h a s l o s t a b o u t 2 2 % o f N e w I r e la n d ' s s pe c ie s , a f ra c t i o n a l lo s st h a t d o e s n o t s o u n d s e r io u s . H o w e v e r , a m o n g t h e s e l o s t s p e c ie s a r e 19 o f th e 2 6N e w I r e l a n d s p e c i es c o n f in e d t o t h e l a r g e r B i s m a r c k i s la n d s , i n c l u d i n g e v e r ye n d e m i c B i s m a r c k s p e c ie s i n th i s c a t e g o r y . T h a t i s, N e w H a n o v e r d if f e r e n ti a l ly lo s tt h o s e s p e c ie s m o s t i n n e e d o f p r o t e c t i o n . A s a f a u n a l r e s e rv e , N e w H a n o v e r w o u l dr a t e a s a d i s a s t e r . Y e t i t s a r e a o f 1 2 00 k m 2 is n o t s m a l l b y t h e s t a n d a r d s o f m a n y o ft h e t r o p i c a l r a i n f o r e s t p a r k s t h a t o n e c a n r e a l i s t i c a l l y h o p e f o r t o d a y .

A s a f u r t h e r e x a m p l e o f a n a t u r a l e x p e r i m e n t i n d i f fe r e n t ia l e x t i n c t i o n , c o n s i d e rt h e m a m m a l s i so l at ed o n m o u n t a i n t o p s r is in g f r o m N o r t h A m e r i c a n d e s e rt ba s in s ,m e n t i o n e d i n th e p r e v i o u s s e c t i o n . L i k e t h e b i r d s p e ci e s r e s t ri c t e d t o th e N e wG u i n e a l a n d - b r i d g e i sl a nd s , t h e i s o la t e d p o p u l a t i o n s o f t h e s e m a m m a l s p e c ie s h a v eb e e n e x p o s e d t o t h e r i s k o f e x t i n c t i o n f o r t h e p a s t 1 0 ,0 0 0 y e a r s , w i t h o u to p p o r t u n i t y f o r r e c o l o n i s a t io n . T o d a y , s o m e o f th e s e m a m m a l s p e c ie s a r e s ti llp r e s e n t o n m o s t o f th e m o u n t a i n s , w h i le o t h e r s p e c ie s h a v e d i s a p p e a r e d f r o m a l lb u t a f e w m o u n t a i n s . T h e s p e c ie s w i t h th e h i g h e s t e x t i n c t i o n r a te s a r e t h o s e w h o s ei n it ia l p o p u l a t i o n s m u s t h a v e n u m b e r e d f e w i n d i v id u a l s : e i t h e r b e c a u s e t h e s p ec i esi s a c a r n i v o r e r a t h e r t h a n a h e r b i v o r e , o r b e c a u s e i t h a s s p e c i a l i s e d h a b i t a tr e q u i r e m e n t s , o r b e c a u s e i t is a l a r g e a n i m a l ( B r o w n , 1 97 1).

A m e t h o d o f q u a n t i f y i n g t h e s u r v i v a l p r o s p e c t s o f a s p e ci es is to d e t e r m i n e i ts s o -c a l le d i n c i d e n c e f u n c t i o n ( D i a m o n d , i n p r es s ). O n i s la n d s o f t h e N e w G u i n e a r e g i o no n e n o t e s t h a t s o m e b i r d s p e c ie s o c c u r o n l y o n t h e l a rg e s t a n d m o s t s p e c i e s -r i chi s la n d s ; o t h e r s p e ci es a ls o o c c u r o n m e d i u m - s i z e d i sl a n d s; a n d o t h e r s a l s o o c c u r o ns m a l l i s l a n ds . T o d i s p l a y th e s e p a t t e r n s g r a p h i c a l l y , o n e g r o u p s i s la n d s i n t o c l a s se sc o n t a i n i n g s i m i l a r n u m b e r s o f b ir d s p e c ie s (e.g. 1 - 4 , 5 - 9 , 1 0 - 2 0 , 2 1 - 3 5 , 3 6 - 5 0 ,e t c .) ; c a l c u l a t e s t h e incidence J o r f r a c t i o n o f t h e i s la n d s i n a g iv e n c la s s o n w h i c h ap a r t i c u l a r s p e c ie s o c c u rs ; a n d p l o t s i n c i d e n c e a g a i n s t t h e t o t a l s p e c i es n u m b e r S o nt h e i s la n d ( F i g . 6 ). S i n c e S i s c l o s e l y c o r r e l a t e d w i t h a r e a , i n e f fe c t t h e s e g r a p h sr e p r e s e n t t h e p r o b a b i l i t y t h a t a s p e c i e s w i ll o c c u r o n a n i s la n d o f a p a r t i c u l a r s i ze .


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142 J A R E D M . D I A M O N D

F o r m o s t s p e c i e s, J g o e s t o z e r o f o r S v a lu e s b e l o w s o m e v a l u e ch a r a c t er i st ic o f th ep a r t i c u l a r sp e c i e s , m e a n i n g t h a t t h e r e is n o c h a n c e o f su r v i v a l o n i s la n d s b e l o w ac e r t a i n s i ze . T h e s e i n c i d e n c e f u n c t i o n s c a n b e in t e r p r e t e d i n t e r m s o f th e b i o l o g y o ft h e p a r t i c u l a r s p e c i e s ( e . g . i ts p o p u l a t i o n d e n s i t y , r e p r o d u c t i v e s t r a t e g y , a n dd i s p e r sa l a b i li ty ) . F r o m t h e s e in c i d e n c e f u n c t i o n s o n e c a n e s t im a t e w h a t c h a n c e ac e r t a i n s p e c i e s h a s o f s u r v i v i n g o n a r e s e r v e o f a c e r t a i n s i z e .

T h u s , d i f fe r e n t s p e c i e s h a v e d i f f e r e n t p r o b a b i l i t ie s o f p e r s i s t in g o n a r e s e r v e o f ag i v e n s iz e . T h e s e p r o b a b i l it i e s d e p e n d o n t h e a b u n d a n c e o f t h e s p e c i e s a n d t h e

1 .0 - 4 1 1 - - 0

0 . 8 -

O . 6 -

J0 . 4 -

0 . 2 -

o

/ P i t t o e r y t h r o g o s t e r , B- r o m p

- -

P t i l i n o p u s s u p e r b u s ,C- t r o m p

__j,,,,,,,,,~20 4JO0 I Io ~ o S S O , 6o , ~o , 4o

- I . 0

- 0 . 8

- 0 . 6J-O .4

- 0 . 2

F i g . 6 . S o - c a l l ed i n c i d e n c e f u n c t i o n s f o r t w o b i r d s p e c ie s o f t h e B i s m a r c k A r c h i p e l a g o n e a r N e wG u i n e a . T h e i n c i d e n c e ,/ (3 ') i s d e f i n e d a s t h e f r a c t i o n o f t h e i s l a n d s w i t h a g i v e n t o ta l n u m b e r o f b i r ds p e c i e s S t h a t a g i v e n s p e c i e s o c c u r s o n . F o r e x a m p l e , t h e s o - c a ll e d B - t r a m p Pit ta erythrogaster ( e ) i s o na l l i s l a n d s ( i. e . d ~ 1 . 0) w i t h S > 8 0 , o n a b o u t h a l f o f t h e i s l a n d s ( d = = 0 . 5 ) w i t h S a r o u n d 5 5 , a n d o n n oi s l a n d ( J = = 0 ) w i t h S ~-- 4 0 . O t h e r b i r d s p e c i e s o f t h e B i s m a r c k A r c h i p e l a g o h a v e d i f f e r e n t in c i d e n c ef u n c t i o n s : f o r e x a m p l e , t h e s o -c a l l e d C - t r a m p Pt i l i n o p us sup erbus ( ) i s o n a l l i s l a n d s w i t h S > 6 0 a n do n m a n y i s la n d s ( J = = 0 . 3 - 0 . 8 ) w i t h S ~ 3 0 - 5 0 . S i n c e S i s m a i n l y a f u n c t i o n o f is l a n d a r e a , t h e m e s s a g ei s th a t e a c h s p e c i e s r e q u i r e s s o m e c h a r a c t e r i s t i c m i n i m u m a r e a o f i s la n d f o r it t o h a v e a r e a s o n a b l ec h a n c e o f s u r v i v i n g .


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THE ISLAND DILEMMA 14 3

magnitude of i ts populat ion fluctuations, and also on i ts abil i ty to recolonise areserve on which it has once g one extinct. Even o n reserves as large as 10,000 km 2,some species have negligible prosp ects of lon g-te rm survival. Such species woul d bedo ome d by a system of ma ny small reserves, even if the aggregate area of the systemwere large.

WHAT DESIGN PRINCIPLES WILL MINIMISE EXTINCTION RATES IN NATURALRESERVES?In the preceding sect ions we have examined how the eventual num ber of speciesthat a reserve can hold is related to area, how exti ncti on rates are related to area,and how area- depen dent survival prospects vary a mon g species. Given thisbac kgr oun d inf orma tion , let us f inal ly consider what the designer of natur al

b e t t e r w o r s e

0 B O 0O 00 0O 0 CO 0 0 0

0O 0 D 0 0 0F 0 0 0F (

Fig. 7. Suggested geometric principles, derived from island biogeo.graphic studies, for the design ofnatural reserves. In each of the six cases labelled A to F, species extraction rates wil l be lower for thereserve design on the left than for the reserve design on the right. See text for discussion.


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r e s e r v e s c a n d o t o m i n i m i s e e x t i n c t i o n r a t e s ( D i a m o n d , 1 97 2, 1 9 73 ; T e r b o r g h , i np r e s s, a , b , W i l s o n & W i l li s , i n p re s s ). F i g u r e 7 ( m o d i f i e d f r o m W i l s o n & W i ll is , i np r e s s) s u m m a r i s e s a s e r i e s o f d e s i g n p r i n c i p le s , i d e n t i f ie d as A , B , C , D , E a n d F .A l a r g e r e s e r v e i s b e t t e r t h a n a s m a l l re s e r v e ( p ri n c i p le A ) , f o r t w o r e a s o n s : t h el a r g e r e s e r v e c a n h o l d m o r e s p e c ie s a t e q u i l i b r i u m , a n d i t w i ll h a v e l o w e r e x t i n c t i o nr a t e s .I n p r a c t i c e , th e a r e a a v a i l a b l e f o r r e s e rv e s m u s t r e p r e s e n t a c o m p r o m i s e b e t w e e nc o m p e t i n g s o c ia l a n d p o l i t i c a l i n t e re s t s . G i v e n a c e r t a i n t o t a l a r e a a v a i l a b l e f o rr e s e r v e s in a h o m o g e n e o u s h a b i t a t , t h e r e s e r v e s h o u l d g e n e r a l l y b e d i v i d e d i n t o a sf e w d i s j u n c t i v e p i e c e s a s p o s s i b l e ( p r i n c ip l e B ), f o r e s s e n t ia l l y t h e r e a s o n su n d e r l y i n g p r i n c i p le A . M a n y s p ec i es t h a t w o u l d h a v e a g o o d c h a n c e o f s u r v iv i n gi n a s i n g l e l a r g e r e s e r v e w o u l d h a v e t h e i r s u r v i v a l c h a n c e s r e d u c e d i f t h e s a m e a r e aw e r e a p p o r t i o n e d a m o n g s e v e r a l s m a l l e r re s e rv e s . M a n y s p e ci es , e s p e c i a ll y t h o s e o ft r o p i c a l f o r e s t s , a r e s t o p p e d b y n a r r o w d i s p e r s a l b a r r i e r s . F o r s u c h s p e c ie s e v e n ah i g h w a y s w a t h t h r o u g h a r e s er v e c o u l d h a v e t h e e f fe c t o f c o n v e r ti n g o n e l a r g ei s l a n d i n t o t w o h a l f - s i z e d i sl a n d s . P r i n c i p l e B n e e d s to b e q u a l i f i e d b y t h e s t a t e m e n tt h a t s e p a r a t e r e se r v es i n a n i n h o m o g e n e o u s r e g i o n m a y e a c h f a v o u r t h e s u rv i v al o fa d i f f e r e n t g r o u p o f s p ec ie s ; a n d t h a t e v e n i n a h o m o g e n e o u s r e g i o n , s e p a r a t er e s e rv e s m a y s a v e m o r e s p e c i e s o f a s e t o f v i c a r i a n t s i m i l a r s p e ci e s, o n e o f w h i c hw o u l d u l t i m a t e l y e x c l u d e t h e o t h e r s f r o m a s i n g le r e se rv e .

I f t h e a v a i l a b l e a r e a m u s t b e b r o k e n i n t o s e v e r a l d i s ju n c t i v e r e s e rv e s , t h e n t h e s er e s e rv e s s h o u l d b e a s c l o s e t o e a c h o t h e r a s p o s s ib l e , i f t h e h a b i t a t i s h o m o g e n e o u s( p r i n c i p l e C ) . P r o x i m i t y w i l l i n c r e a s e i m m i g r a t i o n r a t e s b e t w e e n r e s e r v e s , h e n c et h e p r o b a b i l i t y t h a t c o l o n i s t s f r o m o n e r e s e r v e w i l l r e a c h a n o t h e r r e s e rv e w h e r e t h ep o p u l a t i o n o f t h e c o l o n i s t s p e ci e s h a s g o n e e x t i n c t .I f t h e r e a r e s e v e r a l d i s j u n c t i v e r e se r v e s , th e s e s h o u l d i d e a l l y b e g r o u p e de q u i d i s t a n t f r o m e a c h o t h e r r a t h e r t h a n g r o u p e d l i n e ar l y ( p r in c i pl e D ) . A ne q u i d i s t a n t g r o u p i n g m e a n s t h a t p o p u l a t i o n s f r o m e a c h re s er v e c a n r e a d i lyr e c o l o n i s e , o r b e r e c o l o n i s e d f r o m , a n o t h e r r e s er v e . I n a l i n e a r a r r a n g e m e n t , t h et e r m i n a l r e se r v es a r e r el a t iv e l y r e m o t e f r o m e a c h o t h e r , r e d u c i n g e x c h a n g e o fc o l o n i s t s .I f t h e r e a r e s e v e r a l d i s ju n c t i v e r e s e rv e s , c o n n e c t i n g t h e m b y s t r i p s o f t h ep r o t e c t e d h a b i t a t ( P r e s t o n , 1 96 2; W i l li s , 1 97 4) m a y s i g n i fi c a n t l y i m p r o v e t h e i rc o n s e r v a t i o n f u n c t i o n a t l i t tl e f u r t h e r c o s t in l a n d w i t h d r a w n f r o m d e v e l o p m e n t( p r i n c ip l e E ) . T h i s i s b e c a u s e s p e c ie s o f t h e p r o t e c t e d h a b i t a t c a n t h e n d i s p e r s eb e t w e e n r e s e r v e s w i t h o u t h a v i n g t o c r o s s a s e a o f u n s u i t a b l e h a b i t a t . E s p e c i a l l y i nt h e c a s e o f s e d e n t a r y s p e c i e s w i t h r e s t r i c t e d h a b i t a t p r e f e r e n c e s , s u c h a su n d e r s t o r e y r a i n f o r e s t s pe c ie s o r s o m e b i r d s p ec ie s o f C a l i fo r n i a o a k w o o d l a n da n d c h a p a r r a l , c o r r i d o r s b e t w e e n r e se r ve s m a y d r a m a t i c a l l y i nc r e as e d is p e r sa lr a t e s o v e r w h a t w o u l d o t h e r w i s e b e n e g l i g i b l e v a l u e s .A n y g i v e n r e s er v e s h o u l d b e a s n e a r l y c ir c u l a r i n sh a p e a s o t h e r c o n s i d e r a t i o n sp e r m i t , t o m i n i m i s e d i s p e r s a l d i s t a n c e s w i t h i n t h e r e s e rv e ( p r i n c ip l e F ) . I f t h e


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T H E I S L A N D D I L E M M A 1 45

r e s e r v e i s t o o e l o n g a t e o r h as d e a d - e n d p e n i n s u l a s , d i s p e r s a l r a t e s to o u t l y i n g p a r t so f t h e r e s e r v e f r o m m o r e c e n t r a l p a r t s m a y b e s u ff i c i e n tl y l o w to p e r p e t u a t e l o c a le x t i n c t i o n s b y i s l a n d - l i k e e f f e ct s .

A C K N O W L E D G E M E N T SF i e l d w o r k i n t h e s o u t h - w e s t P a c i f i c w a s s u p p o r t e d b y t h e N a t i o n a l G e o g r a p h i cS o c i e t y , E x p l o r e r s C l u b , A m e r i c a n P h i l o s o p h i c a l S o c ie t y , C h a p m a n F u n d a n dS a n f o r d T r u s t o f t h e A m e r i c a n M u s e u m o f N a t u r a l H i s t o r y , a n d A l p h a H e l i x N e wG u i n e a P r o g r a m o f t h e N a t i o n a l S c i e n c e F o u n d a t i o n .

REFERENCESB R O WN , J . H . (1971 ) . Mammals on moun ta in tops : nonequ i l ib r ium in su la r b iogeog raphy . A m . N a t . ,105, 467-78.DAMMERMAN, K. W . (1 948 ) . The faun a o f Kra kata u 1883-1933. Ver h . Kon ink l . N ed . Akad .W e t e n s e h a p . A f d e l . N a t u u r k . , 44(2), 1-594.DIAMOND, J . M. (1969) . Av i f auna l equ i l ib r ia and spec ies tu rnov er r a tes on the C hanne l I s lands o fCal i fo rn ia . Pr oc . na tn . Aca d . Sc i . US A, 64, 57-63.DIAMOND, J . M. (1971 ) . Com par i so n o f fauna l eq u i l ib r ium tu rn over r a tes on a t r op ica l i s land and a

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