How are diversity and productivity related, ML Rosenzweig, Z Abramsky

Tags: diversity, Patrick 1963, Michael L. Rosenzweigand Zvika Abramsky, Yount 1955, experiments, coefficient of variation, hypothesis, enrichment experiments
Content: HowAreDiversityandProductivitRy elated?
Michael L. Rosenzweigand Zvika Abramsky
THE ParrsnN
Two groups str"rdiedit, and each discerned a piece of the
The relationshipof primary productivityand speciesdi- w h c r l e r e l a t i o n s h i p ( f i g . - 5 . 6 ) ,B r o w n ( 1 9 7 5 ) d i s c o v e r e d
v e r s i t yo n a r e g i o n a sl c a l e( 1 0 " k m r ) i s n o t s i m p l e .B u t within sr-rcrhegions,and perhapsevenlarger ones,a par-
that diversity grows with productivitl'-ar leasr in parrs o f t h e S o u t h w c s t .A n d O w e n ( l 9 8 l l ) d em o n s t r a r e dt h a ri r
t e r n j s e m e r g i n ga: s p r o d u c t i v i t yr i s e s ,6 r s t d i v er s i t y i n - declines-at least in Texas. fJrown's data for the Great
creasest,hen it dcclines.Someecologistsalreadyaccept Basin actually show two dara points in decline, but with
t h i s p a t t c r na s a n e m p i r i c a lp h e n o m e n o nA. t l e a s ro n c only those tu'o points, he conservativelv interpreted the
standardecr>logytextbook reports it in detail (Begon, declineas an island effect. Now that we know that rodent
H a r p e r ,a n dT i r w n s e n d1 9 9 0 ) .
diversity declines as productivity increases all across
Evidenccfor the hump-shirperdegionapl atrernis accu- Texas, we need no longer resort to thc island eff'ectto ex-
mulating(seealso\il/right,Clurrie,and Maurer,chap.6). plain the deviation from thc overall pattern, becauseno
It was first proposcdfor planrs(Vhittaker and Niering d c v i r i t i o r rc x i s t s .
1 9 7 5 ) .T i l m a n( 1 9 8 2 )d c v e l o p ead r h e o r yt h a tp r c d i c t st h e
l)escrt rodent speciesdiversity increascswith prccipita-
pattern,and he suggesretdhat variousdata fronr plant tion (the surrogirte variable for prodrrctivity in deserts:
c o m m u n i t i e sf i t i t . ( S e eT i l m a n a n d P a c a l ac, h a p .2 , f o r R o s e n z ' " v e i g1 9 6 8 ) u n t i l r a i n f a l l r e a c h e sa b o u t 3 - 5 0m m
someof theseexamples. Br-rnt otethat theyreston contro- per year (which happens in southeastcrn Arizorra). Then
v e r s i i rsl u r r o g a t ev a r i a b l eos f u n k n o w na b i l i t yt o s t a n di n rodent diversity begins its long dccline ircrossthe rvhole
f o r p r o d u c t i v i t y .P) e r h a ptsh c l c a s tc o n r r o v c r s i apll a n tc x - statc of Texas. Rodcnt spccics diversity drarnple comes from Mediterranean vascular plants thirds as ;rroductivitiesrise to the levelsfound rn eastern
( S h m i c l aE, v e n a r i a, n d N o y - M e i r 1 9 8 6 ;S h m i d a u, n p u b - T e x a sa l o n g t h e L o u i s i a n ab o r d e r .
l i s h e dd a t a ;A r o n s o na n dS h m i d a1 9 9 0 )( f i g . . 5 . 1 ) .
The pattern irr the lJnited Sfirfesresemblesthe one in
The humped-shapcdptrfcrn exisrsin Middle Eastcrn the Negcv desert (Abrarnsky, Brand, and Roscnzwerg
desertrodents(fig.5.2),both in rhc psammophilicassern- 1 9 8 . 5 )A. l s o , t h e i n c r e a s ep h a s er n a t c h e st h e ( ) n et h a t M e s -
b l a g ea n d i n t h e l i t h o p h i l i ca s s e m b l a g(eA b r a m s k ya n d e r v c a n d G l a n z ( 1 9 7 8 )f o u n d a l o n g a 1 0 0 0 - k m l a t i r u d i n a l
RosenzweigI 984).Owen (19tt8)docur.nenteitdfor Tcxas t r a n s e c ti n a r i d b i o m e so f C h i l c ( f i g L r r e- 5 . 6 A ) .
c a r n i v o r e s( f i g .- 5 . 3 ) .
Tropical Vertebrates
Productivityis the ratear which energyflowsthrough an ec()system(e.g.,kj/nrzlyr).But in all the caseswe cire, ecologistsusedan indexof productivityratherthan meas u r i n gi t d i r c c t l yI.n a r i da n ds e m i a r i dl a n d s c i r p epsr,e c i p i ^
\il/ho would haveguessedthirt higher productivitieds epresstr()picalbird arrdtropical mammal cliversitiesY?er we know rwo studiesthat suggestthey do. One (69.,5.7)
tation supplies thc indcx. Actual evapotranspinrion is straightforwardA: r-rstraliarnropicalmammirldiversity
works for terrestriasl ystemswith a wide varietyrisesto a peakfrom the dry-wettropicsto thewet tropical highlancisI.t then declinessubstantiallyin the wet lowlands(Rosenz"veiagnd Braithwaite,unpublishcddara). The secondstudyrequiresa bir of explanationE. very
On the ocean'sfloor,deprhitself(whichdeterrninetshe availabilityof light for photosynthesisg)overnsproduc-
birdwatcherc, veryfan of public televisiona, nd certainly everyecologistknows that bird speciesfairly oozefrom
t i v i t y ( e . g . ,S m i t h 1 9 7 8 ) .R e x ( 1 9 8 1 )s h o w e dt h a t r n a n y the lowlandtropics.Nobody haseverfound anotherplace
bottorn-dwellingmarinerara showthe htrmp-shapepdat- tern: Gastropoda,PolychaetaP, rotobranchaC, umacea, invertebratemegafaunaa, nd fishesF. igure5.4 showsthe cumaceanp:rttern.In figure5.5,we seea groupnot mentioned in Rex'sreview,brachiopodsin Antarcticwaters (thispatterncomesfrom data in Foster1974).
rhat comesclose.Censuseos f birds in a lorvlandfloodplain forestin the Amazonbasinof Perureveal319speciesin a 97-hastudysite,and point diversitiesofover150 species(Terborghet al. 1990).Suchpoint diversitieesxceedthe highestof thoseof any urriformhabitatin Norrh Americaby at least300% (Terborghet al. 1990).Yetwe
are about to claim that in one sensec. omnaredwith the
Rodents of the Southwestern United States
lessproductivehigherelevationst,he lowlandsaredepauperate!
Among the most curiouscasesof the hump-shapedpat-
Remen.rbepr,roductivity is not the only influenceondi-
tern is that of rodentsof the soutl-rwesteUrnnitedStates. versityP. erhapsthe besrestablisheidnfluenceof allisarea,
'l H O \ ( / A R E D I V E R S I T Y A N D P R O D U C T I V I T Y R E L A T E D ? (1]
G ,1+
a llJ nn l t Oi llJ A40' .i
a uJ
LIJ o-
Rain (cm)
F i g u r c5 .I l ) l a t r ts p c c i c sc l i v e r s i t yi n 0 . | - h a P l 1 ' ! :P : " k : i r t r c g i o t r s w i l h i , r t c r , l c d i a t "t , i i n f " l l i n I s r a c l , f u r k c y , r t n c lS p a i n ' ( l ) a t a f r o n r S h m i d a1 9 1 . l . 5S;h n r r t l a ,u n p u b l i s h c t l c l ; r t a ;A r o n s o n r r n c l S h n r i d a 19 9 0).
10 0 0
F i g u r c . 5 . 2 I ) i v c r s r t y a n c l p r o d u c t i v i t y i n M i c l d l c F ) i l s t c r nr o L i c r l t , , s s c n r l t l a g c rI.l a i n f a l l i s , r g i , o d i n c l c x o f p n r c l u c t i v r t y . i ns r r c ha r i c l a n r l s c n r r a r i thl a b i t a t s .( A f t c r A b r a l n s k v a n d R o s c n z w ei g l 9 t l ' l ' )
|0'20 uJ
ut 10
(L a
'.,1 "
". .../
<" t
"----- \
+ts 0'
DePth(thousandsof m)
F i g u r c . 5 . 4 A t l a n t i c c t l l l t r l c c r ' l lrl sc a c hp c a k d i v c ' r s i t i east i l l t c r t r l t c l t -
a t c . l c p t h s .l ) r o c l u c t r v i t yc o r r e l r r t e si n v c r s e l yw i t h d c p t h T h e s c a n i n , a l s l i u c ( ) n t h c b ( ) t t o n l r r n d w c r c s a n r p l c dw i t h a n c p i b c n t h i c s l c d
in :r trirnscctfron.r(iiry Hcad to Be-rnrudaO' nly sar.r.rplewsitlr at lcast
o IJJ 0- a
1 0 0 i n c l i v i c l u a lasp p c r t ri n t h c g r a p h ' T h i s i s o n c o f r t n u t r b c r o f c i t s c s c i t c d b y I { e x ( 1 9 ' 8 l ) s h o w i n g b t ' n t h i c 'm a r i n e i n v , c r t c b r r r tc l i v c r s i t y p " " f . i r t g o v e r i n t c r m c d i r r t ei e p t h t . ( D a t a f r o r r . r. [ o n e sa n c l S a n d e r s t972..\
16 0 0
Fi hi
g n
ure ped
5.3 sha
Diversity ;rnd p ei s e v i d e n t ,b u
productiv t thc peak
ity .in Texas i s d i s p l a c t dt
c o
ar a
nivorct' Thc productivity
o f a b o u t 1 4 0 0 g / m t / y r - a n o r d e r o f r . n a g n i t u d eh i g h e r t h a n t h a t f o r
t h e r o d e n t s ' p c a k s i n f i g u r e s- 5 . 2a n d - 5 . 6 .P r o d u c t i v i t y i s e s t i m a t e d
b v a f u n c t i o n o f a c t u a l e v a p o t r a n s p i r a t i o n(.A f t e r O w c n 1 9 8 8 ' )
In any biogeographical province' lilrger areas harbor
-o.. ,p".i.i. tWe ecologists have been proving area's im-
ou.tun.. for decades (reviews include Connor and Mc-
C , o y\ 9 7 9 ; M c G u i n n e s s1 9 8 4 a ) .S o w e m u s t n o t r g n o r e
area when tropicalup
it la
comestime to ndsand lowland
sc .o' Wmepmaur estthf aecdt oi vreorusti
tie its
so f ef-
f..i b.furi we decidewhat productivityis doing' Rahbek
(C. Rahbek,personalcommunication)has done this for
neotropicalbirds (fig. 5.8). (As you read the following
summary of his findings,keep in mind that elevation
I n
o ul o TU (L a t+
F ' i g u r e- 5 . . 5 A n t a r c t i c b r a c h i o p o d ss h ; r r c t h c c l i v c r s i t yp a t t e r n o f m a n v o t h c r n r a r i n ei n v c r t eb r a t et a x a l i k e C u m a c e : r( { i g .. 5 . a )P. r o d u c t i v i t y c o r r c l : r t e si n v c r s e l y w i t h d e p t h . D i v c r s i t y i s t h e n r r x i r n u n r n u n r b c ro f s p t ' c i c sk n o w n a t a g i v e nd c p t h i n A n t a r c t i cw a t c r s ,r a t h e r than the nunrbcr in a collection.l)epths we report in the figurc arc c a c l r t h c u p p c n r t ( ) s to r l o w c r n r o s tc l c p t hk n o r v n f o r ; r t l e : r s to n c r > f t h e s p c c i c s .( D e t r rf r o m F o s t c r 1 9 7 4 . )
probably is a good inverseindex of productivity in the
wet troplcs.)
Rahbekfoundthat, in theNeotropicsl,owlandtropical
area far exceedsthat of any other elevation.But, over
similar-sizedareas,the more productivelowlands have
fewerbird speciesthan subtropicael levationsdo. For ex-
ample,a lowland areaof 105km2hasabout 526 species,
but at subtropicalelevations,an area of that size has
8 - 5 5s p e c i e s .
The temperate-elevatiobnird species-arecaurveis like
thelowlands';a 10s-km2regionat temperateelevationhas
451 spe four; its
ciesT. he high-elevat 10'-knr2regionhas o
io nl
ync u1 r. 7v e6si sptehcei el os'wI. ' heus sto.ffr ot hme
high elevationsdown to subtropicalelevationst.he more
productivitSthe more bird diversityB. ut the evengreater
productivityat low elevationsdoesnot add to diversity;
it decreasedsiversity.We seeso many rnorebirds in the
lowlandsmerelybecausethe lowlandsare so extenslve.
The literaturepointsout that ecologistos ftenconfound
areaand productivity(Wright 1983;Turner,Gatelrouse,
and Corey 1987).But somehavetried to separatethem
(Abramskyand Rosenzweig1984; Owen 1988; Turner,
l.ennon,and Lawrenson1988).Rahbek'swnrk teacheus s
how astonishing:rndrewardingit is to disentarrgltehem.
Other studiesalsopoint or:t mid-elevationpeaksin di-
versity along tropical transects.These include ferns
( T r y o n1 9 8 9 )a n d b r y o p h y t e(sG r a d s t e iann d P o c s1 9 8 9 ) ,
a l t h o u g hn o t a n g i o s p c r m(sS t o c k ear n d U n w i n 1 9 8 9 )n o r
a m p h i b i a n sa n d r e p t i l e s( S c o t t1 9 7 6 ) .B u t e a c ho f t h e s e
casesought to be reconsideredin the light of Rahbek's
aut 64 TIJ ftz :+F
d)20 [! o IJJ (L u) 10 lt
' Chile
10 0 0
Figure -5.6 Diversity and productivity in Chilean and southu,estern [ J n i t e dS t a t e sr o d e n t a s s e m b l a g e sL.e f t : D a t a f r o m B r o w n ( 19 7 3 ) a n d M e s e r v ea n d G l a n z ( 1 9 7 8 ) . T h e C h i l e a n d i v e r s i t i e sc o n t a i n a l l s p e cics known for a site; Brcrwn'sdata contain only rhe specieshe actually caught, and thus fall on a lower trend line. Plotting only species actually caught by Meserve and Glanz would bring the two scat-
tergrams in line, but n,ould diminish rhe accuracl' of the Chilean t r e n d . R i g h t : O w e n ' s ( 1 9 8 8 ) g r a p h o f r o d e n t a s s e m b l a g eisn T e x a s . Owen's area of lowest productivity (150 g/mr/yr.) correspondsto a rainfallof 205 mm; thushis data beginon the right end of Brorvn's. Owen's data come from much larger areas,which accountsfor their mueh highernurnbcr:of speeies.
40 I a I 930 t o uJ20I L (L ato L + I 0 0
a a9 I dma @a-P- ^--a-6&f,-*,8,/, &" a 6"k
F i g u r e5 . 7 T r o p i c a l m a m m a l d i v e r s i t yi n a I a r g ev a r i e r yo f w c t a n d w e t - d r l ' r r o p i c a l h a b i t a t s i n A u s t r a l i a . D i v e r s i t y d c c l i n e so v e r t h e h i g h e s tp r o d u c t i v i t i e s .P r o d u c t i v i t yr s e s t i m a t e da s a s o i l f e r t i l i t y i n d e x m u l t i p l i e d b y a c l i m i r t i ci n d e x ; t h e l a t r e r t a k e st e m p e r a t u r ea n d rainfall into irccount.The curve rnerelyapproximaresthe trend. ( D a t af r o m R o s e n z w e i ga n d B r a i t h w a i t e ,u n p u b l i s h c c l . )
@ lrJ
uJ (L
20 [:
^' 4(t s'
-o - L -
- r.;-*;-.-l ,--aia
ol o]
1 I
2>3. 5.5-
k3m. 5eklemv-l
1500 15000
Cumulativeare (thousandsof km2)
F i g u r e5 . 8 F o r a 6 x e d a r e a ,d i v e r s i t yo f n e o t r o p i c a lb i r d s i s h i g h c s t a t s u b t r o p i c a le l e v a t i o n sn, o t i n t r o p i c a l l o w l a n c l s .( D a t a f r o m R a h - bck, unpublished.)
Z i e g l e r( 1 9 6 5 )n o t e d t h e g o o d c o r r e l a t i < ;onf s e d i n r e n t
rypeand oceandepth:the finer the sedinrentt,he deeper thewaterin which it was depositedH. e and otl.repr aleo-
biologistsbeganto usethat correlationto describeancient e n v i r o n m e n tasn d t h e i r c o m m u n i t i e sA. l t h o u g hp a l e o b i ologistsagreethat sedimentype doesnot parallelocean
10 a ul
d e p t hp r e c i s e l y( S h a b i c a n d B o u c o t1 9 7 6 ;J o h n s o na n d PotterlL)76;Hurst 1976;\Watkins1979\,theyalsoagree
that the correlationis good cnoughto rank the depthof (L
differentdeposits(e.g.,Mikulic and \ii/atkins198l). So, a 5
sedimenttype becon.reasn indexof relativeproductivity. 1*
We haveesrimatesof ir.rvertebradteiversityovcr deptl.r
gradientsfor a number of periodsin the Ordovicianand
the Silurian.AII show the hump-shapedpattern. Even
Llandoverytime of the Lower Silurian,for which we
couldfind no sunrmarvdata on diversitys, howsthe pat-
Salop Homeo lsorth Dicoel Visby
t e r n .Z i e g l e r ,C o c k s ,a n d B a m b a c h( 1 9 5 8 )n o t e t h a t i t s Lingulacommunityhad both the leastdiversityanclthe s h a l l o w e setn v i r o n m e n tT.h c " m o s t d i v er s e "c o n r m u n i t y ,
Shallow +
the Clorinda, was in deeperwater. Lirter,Cocks and
Rickards(1969) identifieda "Marginal Olw
munity,seawardfewereven than the Lingula. Seawardof that wcre the ir>podsD. iversitiesirreirvcraged()ver irn unspecifiednttmberr>ffossil
gruptoliticmudstoneswith, essentiallyn,o shellyspecies
c o l l e c t i o n se, a c ho i w h i c h h a d 1 0 0 t o 2 0 0 s p c c i r n e n sC. l o m n r u n t t t e s : Salopina; Homoeospira/Sphaerirhyncbia;Ixtrthis; Dicoelosia: Vis-
at all.
l t y e l l a .( A f r c r H a n c o c k , H u r s t , a n d F - u r s i c h1 9 7 4 . )
In Figures-5.95, .10,and -5.11 we plot threefossiltran-
sectsW. ereany of theseto standalone,it might not con-
vinceyou. But the pattern occursrepeatedlyT. he combined Silurian and Ordovician investigationsrepresent
the Pattern?
some75 million yearsof fossilhistory.And eachgraph Ecologists have a new generalization to contenrplate.
incorporatesa very largeamount of data. For example, Within regions about the size of small to n.rediunr-sized
each point of the Ludlow series(\Tatkins 19791of six nations, speciesdiversity is often-perhaps usually?-a
communities(fig.5.10) is the meandiversityper 50 indi- unimodal function of productivity (or some we'lWl-aeccepted vidualsper sample.Thereare 200 samplesa, nd no Lud- index of it like rainfall or nutrient supply). do not
low point comprisesfewerthan 11.TheOrdoviciangraph wish to seehow many other casesof this pattern we can
(fig. 5.11) synthesizes"about 2000 samplescollected collect. We want to set the stagefor the explanation of the
throughabout 5 km of stratarepresentinagbout200,000 pattern and its mechanism(s).
i n d i v i d u a li d e n t i f i c a t i o n s "( L o c k l e v1 9 8 3 ) .
Theoreticians and empiricists have long thought that
(1 1 )
15 (35)
Q8) 10 @4) @ tu
PI upsw lpSw Ml Trans Go
ut 10 6 UJ o- U'5 lt 1
+Q 8 o
Shallow ----+ Deep
( .+-
Shallow ---_-> Deep
+ Caradoc
o pre-Caradoc
D ,)
Maybe we should suspectsuch a universally accepted line and support rriches. Moderately productlve areas
explanatidecreaspehasepresentsthe real puzzle: Why, past a cer- compassesa wealth of different sorts of exploitable niche
tainpoint, doesenhancedproductivity tend to reduce the opportunities. Very productive areas irlso have excellent
numberof species?In the next section,we discusshypoth- meirn hrrbitats. But tlreir variance rarely presentssignifi-
esesto accountfor the decreaseohase.
cantly different challengesto life. A productive p:rtch thrlt
falls to half its mern productivity is sti[[ very productive.
Sirnilarly,a patch 0..5km away with half the productivity is still very productive.
The literaturepresentsninc' different hypothesesto ex-
Tilman (1982), using micronutricnt combinariortsas
plainthe decreasephase.For convenience,we hirve orga- the nreasureof habitat spccialization,argues persuasivelv
nizedthern into three arbitrarl' grolrps. The first group f o r t h i s h y p o t h e s i s .M o r e r e c e n t l y ,( T i l m a n 1 9 8 7 1T i l m a n
includeshypotheseswith irn explicit dynarnical model re- a n d P a c a l a ,c h a p . 2 ) h e h a s b e e n e x p l o r i n g a s i r n i l a r
lating productivity to diversity. The second group's rrrhypotheselsack such a model at prescnt. Each of the third light becornesmore and more of a problem for competing
group'shypothesesmaintains that diversity nrerely corre- plants. This setsup a gradient along rvhich Tilman imirg-
lateswirh productivity; it identifiesthc truc crlr.rsaes some ineseach plant speciesto have a specialt,vR. egions of poor
othervariablt-,i.e., time, space, disturbrlnce rilte, or co- p r o d L r c t i v i t yw i l l i n c l u c l el i t t l e o f t h e w h o l e g r a d i e n t a n d ,
varianccof different spccies'population clcnsitics.
therefore, fe',,"o,f the plant speciesthat specializealt'rngit.
We treat each hypothesis under threc suhheadings: R e g i o n so f h i g h p r o d u c t i v i t y w i l l l l s o s u p p o r t f e w o f t h e
Firstwe stateit briefly. Then wc explain why it might be specics becausc thcy too include little of the giradient;
true.Then we evaluatc it. Pleaseunderstand that this for- n r o s to f t h c i r s i t e sw i l l c r r u s ei r r t c n s ec o m p c t i t i o n f o r l i g h t .
mat requiresus to play devil's advocate on belralf of a few
A b r i r m s ( 1 9 8 8 ) d e v e l o p e dt h i s n . r o d t -flo r m a l l y . H e d i s -
hypothesest;hat is, we begin by arguing for tlrem as bcst c o v e r c dt h a t i t c o u l c la l s o l c a d t o a r r l ( ) n o t o n i cr i s c i n d i -
we can, although rve dr> not believe thern. lt sccrnsonly vcrsity irs productivitt' grows. But Tilnran and Prrcrrla
f a i r f o r e a c h t o h a v c i t s d a y i n c o u r t . B u t i n t h e e v a l u a - (chap. 2) doubt thirt thc assumptit)rtsnccessaryfor ntotro-
tions,we stlrteour rcal Although losers do emerlle from thc list of hypotheses,
()ncc the plarrt pattcrn appears,a sirnilar animal pat-
you will s(x)n scc that we ilre not surc whether there arc t e r n w i l l e v o l v c .I n n u m e r a b l es t u d i e sr e c o g n i z et h e i m p o r -
trrnccof plant diversity (in physic i c s )t o t h c m a i n t c n a n c eo f a u i n r a ld i v c r s i t y .
Hypothesewsith MechanismIncluded
E n v i r o n m e n t a lH e t e r o g c n c i t y
E v n r . u R l r o N :N ' [ u c he v i d c n c cf a v o r s t h i s h y p o t h c s i s .F o r cramplc, l-lnited Strrtesrodents hrtvc habitat specializa-
HypotHt-.srs:Under conditior-rsof extreme low proclr.rc- tions that suggest it. As procluctivity rises to the point
tivity,there is not much habitat or resourcehcterogeneity; wlrcre covcr apprriaches 100"1'.the lspccts of habitat het-
the landscrrpcis unifornrly l.rarren.The average spot will erogeneity that support mammalirrtt cliversityin placcs of
not sustainarr1,.speciesA. s productivity rises,thr' rrvcragc i n t e r n r c d i a t ep r o d u c t i v i t v s c e m t o d e c l i n c .S h r u b s d i m i n -
v a r i e t yo i m i c r o n u t r i e n t c o r n b i n i r t i o n si n f e r t i l c s r t c si n - ish in favor of grrrsses.Patches oi open grouncl, which
creaseso, r some fcrtile spots have morc light with sparser scern to be crucial for the existence of bipcdal roclctrts,
n u t r i e n t sw h i l e o t h e r sh a v e l c s sl i g h t w i t h r i c h e r n u r r i c n r s . lrecome srnaller (Rosenzweig 1977b: Lenrcn and Rosenz-
In eithcr case,plant divcrsitv increilsesand plant physiog- w c i g 1 9 7 8 ; B r o w n , R c i c h m a n , : r n d D a v i d s o n 1 9 7 9 ;
diversify, allowing rrnin.raldiversity to incrcasc K o t l c r l 9 t l 5 ) . B o t h l { o s e n z w e i g( 1 9 7 . i ) a n d W h i t f o r d e t
a l . ( 1 9 7 8 ) s h o w c d t l r a t e x p e r i m e n t r r l l ys i m p l i f y i n g t l r e
P l s t a c e r t a i np o i n t , m < l r ep r o d u c t i v i t y h a st h e o p p o s i r c structurc of the plant contniunity reducesrodent cliversity.
effect.It reducesheterogeneityof micronutrient combina-
lsracl'ssand-dwclling rodents :rlsoseemto fit. Arcls of
tions and hairitats. Productivity ter"rdsto be spread morc interrnediate procluctivity have patchy ground cover ex-
evenlywithin and betwccn years, reducing fhe variety of ploitirble both by speciesusually living in denser cover
viabletenrporal specializations.Diversity declines.
arrd by those often found where there is little cover (Ro-
Notice tlrat this hypothesis explains both the rncrease s e n z w e i g ,A b r a m s k y , a n d B r a n d 1 9 8 4 ; A b r a m s k y , B r a n d ,
and decreirsephases of the producfiviry pattern. It does a n d R o s e n z w e i g1 9 8 - ! ;R o s e n z w e i ga r t d A b r a m s k y l 9 t t - 5 ;
not require low producrivity to cause r:rrity and thus Rosenzweigand Abramsky 19li6).
higher extinction rates. So it predicts the entire hump-
Tilman (19871 reports the resultsof systematicerpcri-
shapedpattern with no help from any other hypothesis. ments to study the effects of nutrient enrichment in vari-
ous successionasl tagesfrom new field to woods. Increas-
RpesoNtnc; The variety of habitats in spaceand time un- ing the productivity drives diversity down. In three yeirrs,
derliesmuch of the specializirtionthat supports diversity more than 60"/" of the speciesdisappearedfrom plots tlrat
(Rcrsenzweig1987a\. Most ecologists would agree that receivedhigh nitrogen treatments. He believes "that nu-
relativelvbarren areasoffer only a few kinds of habitable trient addition makes plots more homogeneoussprrtially,
times and places: the mean habitat is inhospitable, al- forcing more speciesto compete for the same limitirrg re-
though some unusually favorable times or placescross the source."
CertainlS the natural history surveydata taken in the closelywith its cause,and concludethat therefore,the ef-
areaof Tilman's experimentssupport his hypothesis(ln fect is the cause.
ouyeet al. 1987).The older a field,rhe more nitrogenand the fewer speciesit has.
Dynamical Instability
The literature repeatedlytells us that nutrient enrich- HypornEsls: More productivity reducesdynamical sta-
ment will depresspiant speciesdiversity.Huston (I979) bility.The lossof dynamicalstabilityincreaseesxrinction
tracedsuchexperimentsbackto 1882.He citesnumerous rates,reducingdiversity.
otherexamplesin plants.
Coldbergand Miller (1.990b) ringus up to thisdecade. RrasoNrNc: One of us (MLR) inventedthis hypothesis
Their experimentsalso supporrTilman'sinterpretation. as a theory of eutrophication(Rosenzweig1971).Higher
Adding water greatlyincreasedproductivity,but did not productivity reducesthe negativefeedbackof competirion
change diversity.\Uater amounts do not usually vary within speciesa, nd increasesthe positivefeedbackthat
rnuchoverthe smallspatialscaleof suchexperimentsO. n predatorycontrol produces(Rosenzweig1, 977a\.There
the other hand, addingnitrogencausesonly a small in- is a netlossof dynamicasl tabiliryat higherproductivities. creasein productivity,but greatlydepressetdhe diversity. Wollkind (19761extendedthe theory to three-level Nitrogen,like otherchemicapl r()perrieosf soil,ofrenvar- fbod chains.Riebesel(l1974)extendedit to competition iesgreatlyat smallspatialscalesa; ddinga uniformly ap- betweenspecies.
p l i e d e x t r a a m o u n t m a s k ss u c hv a r i a t i o n .
The heterogeneityhypothesisdoes leave a nagging EvRLuarrov:This hypothesisbreaksdown when natural question:Is it tautology?Habitat and resourceheteroge- selectionhasenoughtime to do its job. Rosenzweigand neitypartitioningareevolvedresponseosf organismsW. e Schaffer,1978\showedthat naturalselectiontendsto reknow no a priori reasonto expectthat life will subdivide storethe dynamicalstabilityof enrichedsystemsT. hat is
any particularvarianceinto more lrichesthan it will a how someextremelyproductivebiorneslike Coral Reefs
smallervaritrnce.Speciesdiscriminatehabitatsbecause and tropicalforestscan be extraordinarilydiverse.
nafuralselectionforcesthemto. More sDeciems eanmore
Yet, many have reportedthat diversitydoes decline
selectionfor finerhabitatdiscriminationR. emembewr hat after nutrientenrichmentI.n additionto the plant exam- Janzen(19671took for grantedin the title of his classic plesthat we mentionedin connectionwith the previous pirper:Mountain passesare higher in the tropics.Thc hypothesist,hereare a numberof aquaticexamples(e.g.,
samedegreeof physicalchangeplacesmore oi a rcsrric- S w i n g l e1 9 4 6 ;Y o u n t 1 9 . 5 6S; c h i n d l e r1 9 9 0 ) .W e c a n n o t
tion on tropicalspeciesthan on temperateones.Physical easilyexplain all thesecasesas the result of increasing
environmentahl eterogeneityis continlrousand, in princi- c()mpetitionfor Iight, or homogenizationof habitat.So
ple,it shouldbe infinitelysLrbdivisiblbey tife.
perhaps the hypothesisof dynarnical instability does
That is why birdsin PuertoRicanrainforesr ecognize apply to somerecentlyenrichedenvironmentse, specially
only two foliagelevelswhile birdsin Panamanianrainfor- acluaticoneslike eutrophiclakes.At leastthe stabilityhyest recognizefour (MacArthur,Rechera, nd Cody 1966). p()thesismakes a field-testableprediction that could Both forestspresent he samephysiognomiccomplexity, somedayforceus to acceptit: Enrichmentshouldbe acbr,rtthe one in PuertoRico housesa depauperateisland companiedby an increarsien oscillatorydynamicsanda fauna.lJnderno pressureto subdividethe forestasfinely, d e c l i n ei n r e t u r nt i m e s( P i m m1 9 8 2 ) .
the birdsof Pr.rcrtRo ico don't. This point is worth a secondexample.One of the Changesin Ratio of Predatorsto Victims
world's richestflorasgrows on the impoverishedsoil of HypotHt.srs:As productivityincreasesp,redatorsabsorb
southwcsternAustralia.Another grows on the similarly much morethan a proportionalshare,and reducethedi-
nutrient-poorsoil of the fynbosin SouthAfrica.Tcra hu- versityof consumers.
man being,especiallyan untrainedone, the plant cover
in theseplaceslooksmonotorlousT. he botanistfindsfew Rp.ascrNrNcP:redatorscan absorb most or all of any growth forms (Adamson1927)-many heathlikeplants increasein stanclingcrop causedby higher productiv-
in Australia,for instance.And most of the plantscome ities(Rosenzweig1971, 1972).Moreover,Oksanenet al.
from f:rirly closely relaredspecies.Ncverthelesst,hese (1981)determinedtheoreticallythat increasedproductivplantshavebrokenup theirtemporalworld into a succes- ity addscontrollingtrophic levelsin a food chain.A new
sion of floweringtimes;a continuumof narrow absolute highertrophiclevelshouldconsiderablyreducethestand-
rangehasevolvedinto a riot of distincthabitats.
ing cropsof the speciebs eneathit. Smallerstandingcrops
If speciesare forcedro recognizernorehabitatswhen shouldproducehigherextinctionratesand diminished i-
therearemorespeciest,henwe chaseour tailsto saythat verslty.
the reasonthereare so many speciesis that thereare so many habitats.Tcrmake a statenlenat bout the jntrinsic EvaluettoN: Predator-victimdiversityratios are almost
effectof l-rabitavtariancew. e needan indeoendenmt odel. constantfor a wide varietyof speciesdiversities(Mithen
It muststartwith a habirarconrinuum.It musrshowthat and Lawton 1985; Pimm, Lawton, and Cohen 1991).
speciationand extinctionratesvary with productivityso That fact contradictsthe prediction of the hypothesis. that we wind up with morespecies(andthusmorerecog- Evenif we did not know the predator^victimrariosw, e
nized habitats)at intermediateproductivity levels.Ve would distrustthe hypothesisT. he theory on whichit is
cannotstartwith an effect,noticethat the effectcorrelares baseddependson the presenceof Weak interactioncoef-
ficientasmongpredatorindividuals.\X/hilezooplankton higher productivities. lUhere are they? As Tilman and Pa-
maynot exhibit strong intraspecificinteractions,verte- cala point or.rt(chap. 2), we do not yet know of any group
bratems ust.So may many other life forms. Individuals whose diversity rises monotonically with productivity.
thatform dominancehierarchiesor steal one another's North American treesmay (Currie and Paquin 1987), but
territoriedson't fit the theory. But such life forms (e.g., data suggestingthey do are truncated by the southern poTexacsarnivoreds)o exhibitthe hump-shapedpattern. litical boundary of the United States. \Would extending
Furthermorea, lot of work has den.ronstratedthat rhesedata farther south reveal decreasesin their diversi-
predatocrsanaddto the sustainabldeiversityof theirvic- ties with even higher productivities? For the sake of this
tims(thekeystonepredator effect). Much of this work hypothesis,we hope not, becauseit is hard to imagine the
consisotsf incontrovertibl6eeld experimentsdone in all tirxon that would be responsible.IThich taxon can com-
sortosf biomeos n varioustaxa (e.g.,SummerhayeIs941; pete successfullywith trees at the highest productivities. Paine196H5;ay 1985).More recentlntheoreticasl tudies and drive down their diversities?
haveexplainedhow this can happenthrough apparent On the other hand, the tropical forest data that we used
competitioonr competitionfor predator-freespace(Holt 1 . 9 7 7 , 1 . 9J8e4f;f r i easn d L a w t o n1 9 8 4 ) . Thepredator-victimratio hypothesisfails.
to think exhibited the unimodal productivity relationship, may not. 'When Tilman (1982) suggestedthat the pattern existsamong tropic:rl trees(in two provinces,Malesia and
the Neotropics), he used a measure of soil nutrient con-
Hypotheseswith Unspecified Mechanism
centrations to stand irr for productivity. But no one hrrs
yet shown hclw tropicerIforest procluctivity can be predicted frorn any surrogatevariable.Although high precip-
Hypotsrsts:Once a critical productivityis reached,a itation leachestropical soil and makes it very poor, tropi-
multispeciteasxon (e.g.,a classor an order)cannotab- cal plants h:rveevolved a root mat that buffers the loss of
sorbanymore.All further productivityincreasegso to a nutrients from the soil itself (Stark and Jordan 1978; Jcompetintgaxon.In fact,the competingtaxon tirkeseven d a n 1 9 8 3 ) .T h i s m a k e si t e a s i e rf o r u s t o u r . r d e r s t a nhdo w
morethantheincreaseI.t actuallyreducesthe productiv- ultra-poor, sandy white tropical soils can support irn-
itygoingto thefirsttaxon.Sversity'.
Also, controversy surrounds the correlation Noticethatthishypothesicslaimsthat thereis only one iorest diversity to soil fertility in Neotropical and in Afri-
relationshipbetween diversity and productivity: The can rainforests.In those forests,rrnnual precipitation cor-
moreproductivityt,he more diversity.The twist is that, relateswell with plant diversity,whereas soil fertility does
accordintgo thishypothesish, ighergenerapl roductivities not (Hall arrclSwaine 1976; Gentry and Enrmons 19ti7;
meanlowerproductivitiesto sometaxa. (The predator- G e n t r y 1 9 8 8 a , 1 9 l l 8 b ) . T h e m o r e p r e c i p i t a t i r > nt,h e m o r e
victimratiohypothesis haresthis twist, but r>therwisiet species.Now, if we only knew how productivity relatestistooweakto considerfurther.)
precipitation, we would have the tropical forest pattern.
But, we do not even know if productivity does relate tRrasoNtttc:Intertax<>nonrcicompetitionexists.For in- p r c c i p i t a t i o nr n t h c t r o P r c s .
stance,granivorous rodents do compete with ants (Brown,Davidson,and Reichman1979; Davidson,In- Change in Competitive Structure
o u y ea, n dB r o w n 1 9 8 4 ) .B r o w na n d D a v i d s o n( 1 9 7 7 \d e - Hvpotrresrs: Interfererrcecompetition prevails at tnter-
tecteda likely suppressionof rodent diversityby rnts in rnediateproductivities and adds ct>nsiderablyt() the diver-
theireast-wesptroductivitytransect.Possiblye, achtype sity maint:rinable at tlrem.
(bauplan?o)f organismcompetesbestat a restrictedset
Notice that this hypothesis also requires no other to
ofproductivitieas nd is largelydefeatedat others.
predict the entire hump-shaped pilttern.
EvaLuRttott:Competitiveabilitiesamong speciesoften do differalong a richnessgradient(Rosenzweig1987a, 1 9 9 1 ;K e d d y 1 9 9 0 ) .W h y s h o u l d n ' tt h e s a m eb e t r u e amonghighertaxa?Also,taxa differ in the productivities atwhichtheirdiversitiespeak,justaswe expectfrom this hypothesisC.ompareTexasrodentsto Texascarnivores, for example.Rodentspeak west of El Pasoin Arizona; carnivorepseakin EastTexasat far higherproductiviries. Or comparethe oceandepthsat which the varioustaxa citedby Rex (19ti1)reachtheir peak diversitiesE. achis differentfrom the others. Neverthelessth, is hypothesisneedsmuch more work. It needsa coherenttheoreticaltreatrnent,which would multiply its predictionsand make it easierto test. One untestedprediction:The brozlderthe t:rxonomicgrouping,the weakerthe decreaspehase. Finallv.sometaxa shouldexist that do not declineat
REesoNtNc: Brown (19711 noted that territoriality in chipmunks first increases,then declines as productivity increases.T'erritoriality commonly appears in asymmetrical competitive systemsand can help to promote conrpetitive coexistence in them (e.g., Pimm, Rosenzweig, and Mitchell 1985). Brown arguesstrongly that productivity should influence territoriality: poor situations are not worth defending; rich ones are too costly to defend because they support so many individuals that excluding them would take too much time. Perhaps speciesare less diverse in richer placesbecar.rstehey cannot therein ameliorate the effects of competition by being territorial. In the termir.rologyof Nicholson (1954), perhaps they are forced from contest competition into scramble competition. We may be able to extend this reasoningto plants. Perhaps plants in poorer places cannot afford the costs of
M I C ] H A F , L I - . R O S E N Z T f l E I C ;A N D Z V I K A A B R A M S K Y
chemical defense (allelopathy). And perhaps plants in very rich placescannot preservethe areas around themselvesfor their own seedlingsbecauseif thev did, they u'ould encourage foraging by too nrany herbivores. We merely mean to suggestthat such an argument is possible, not that we believe this one. We would welcome its improvemcnr. Tlre plant argunrent usesherbivorl'to trnderstanclhow interferencecompetition coulcl wane at high productivity. T'heanirnal argument usesthe time cost of activc territoriality. Nevertheless,we join thern into onc hypothesis bec.rusethcy nrake the same lrroad prediction: Interference competition shor.rlcpl eak at ir"rtennediatclevclsof procluctivity.
L,veLuerrot'r: This hl.pothesis has thrce problerrrs.Tlrey
probirbll,all stenrfronr thc fact that rrrich phce hasplcnty
of lcvels of prodirctivity on which tr>spccia[iz-e.
First, interference(espcciallyas cxernplified by tcrrito-
r i a l i t y ) i s e rw e l l - d o c u m c n t e dp h c n o m e n o ni n s o n r eo f t h e
carth's nrost prodLrctiveplaccs (c.g.,coral rccfs and tropi-
c a l r a i n f o r e s t s )A. n y s u c c c s s f uhl y p o t h c s i sc o n c c r n i n gi n -
t c r f e r c n c cw i l l h a v e t o p r c d i c t t h e r tw h e r c p r o d u c t i v i t y i s
h i g h , i n t e r f c r e n c es h o u l d c l c c l i n c ,b u t n o t v c r y l n u c h .
Secorrd,cvcri ilr a region of high prodrrctir,'ity,spccics
nray specirrlizcon nrodcratc habitlts rrnd so escrrpcthe
pressureto frlrgo tcrritorialitv. All spcciesin an ecosystcnr
do trot cxpcricncc its proclr.rctivityto tlrc srlme cxtent. ln
ir richer orrc, therc tencl to bc s1-rccictshat spccirrlizcon
r.vealthirrrclothers thrrt crllr rrlso usc the sprtrscrtinres rrncl
placesthirt rrn cnvironmcnt offcrs. A gooclcxample is thc b c ec o n r n r u n i t yo f t h e S a n t a( , a t a l i n r rM o u n r r r i n s( S c h a f f c r
ct al. 1979). During each clay,honcvbccsforarc firsr,
rvhcrrncctrr .sLrpplicisrrc nr()stplcntifLrl.The rr conrc brrnr-
b l e b c e s .a n c l f i n a l l y c r r r p e n t c rl r c c s . A r i z o n a h r r r l n r i n g -
b i r d s g i v c u s r l n ( ) t h e rc x l m p l c ( P i n r n r ,l t o s c n z w e i g ,a, n c l
M i t c h c l l l 9 l t . 5 ) .O n c s p c c i e sl i v c s c x c l u s i v r l y i r r t h e r i c i r
riparian fprcfcr that habitat, but usuirllyforrrgcin p(x)rer woods,
r r r i d s l o p e s ,o r h i g h e r c l e v a t i o n s .T h e r e a r e h u n c l r e d so f
o t h e r c x a n l p l e s ;t h c y c < l r n b i n et < l f o r m a c o n l n o n s < l r to f
c o n r r n u r r i t yo r g a n i z a t i o n :s h a r e d; r r e f e r e n c e( l { o s c n z w c i g
87t -fhi
t r
d. l, 9
u,e ccrtninl)' rrgreethrrt spccicsnrrry stirkc out a
r r r r r g co f h a b i t r r tr i c h n c s s e sf o r t h c i r n i c h c . B u t , a l t h o u g h
thc1, nray do so hy interferencc, thcy necd not (Browrr
1 9 8 6 , 1 9 8 9 a , 1 9 8 9 b ) .V a r i o u s s ( ) r t st i f r n r > r p h o l o g i c aolr
phvsiologicrl adaptationscan trrke the place of interfer-
ence if thar gets to be an impractical srraregy.Replacing
interferencc as the ntearlsof subc{ividinga proclucrivity
irxis does nothing to rcducc the number of speciesthat can
For example, some speciesof clesertrodents foragc dur-
ing the poorer months oi the year or in poorer patchesof
seeds.Others actually require the richcr tinres irrrd places
that all prefer. But rlnlong heteronryid species,only the
lirrgestkangaroo rats useinterspecificterritorir.rlitl'ro pro-
tect their end of the habitat spectrum (the richest) (Frye
1 9 8 3 ; B r o w n a n d M u n g c r 1 9 8 5 ) .T h e n i c h e so f t h e o t h e r s
are determined by their positions in a trade-off contin-
ruum:the nrore efficientlv a rodent dealswith food (once
found), the lessetficiently it travels in spaceand/or time to find it. (Hibernation [and torpor in general] provides the efficient way of travelirrg between times of abundance.)Efficient travelerstend to discoverthe rich patches first and so tend to monopolize thenl. But, usually because they are larger,efficienttravelersare too inefficient at food use to exploit the poorer patches at all (Brown 7986, 1 9 8 9 b ;K o t l e r a n d B r o w n 1 9 8 8 ) . Someone needs to produce a quantitative model predicting the predorninanceof interfcrenceand its effectson diversity.Otherwise, we cannot believerhat this hypothesis lvill help us to understanclthe diversity-prociuctivit,v Pattern. Hypotheses That Reduce the Pattern to Another Time Hvporrrp.srs: Richer patches have been around for a shorter tirne than po()rcr patches have. They have not reachcdequilibriurn irnd are prodLrcingnew specicsfaster than they are losing thcrr. Thc decreasc phase rs temporary.
l{E,tsoNtNc;:Speciation trrkestirne. Oonsicleringthe uphetrvalsof thc Pleistr)ccne,rnany h:ibitats ancl trrxa may n ( ) t h a v e r c a c h e de q u i l i b r i u n r .l f t h c m o s t p r o d u c t i v eh a b itlrts are also the newest ()r werc the hardest hit, they s h o u l d b e t h e r n o s tc i c p r c s s e db c k r w c c l u i l i b r i u m .
L,vrrr.unlroN: We know of no evicicnccto support this hypothesis.It dcpendson richer patchcs bcing considerably y o u l r g c r t h a n t h c t i n r c i t t r r k e se v o l u t i o n ( a n d c o l o n i z a t i o r r ) t o f i l l t h c m . l t a l s o d c p e n d so n r i c h e r p a t c h c sb e i n g c o n s i d c r r b l y ) ' o u u g c r t h r n p o o r c r p a t c h e s .D o e s a n v o n e t h i n k t h i s i s g e n c r a l l yt r u c ? H r i w c i l n t h e t i m c h v p o t h e s i s cxplain the prrttcrn'sexistcnccand persistencefor seventy6 v e r l i l l i o n y c a r sd u r i n g t h c O r c f t r v i c i a na n d S i l u r i a n ?W e r-llcnti()ntlris hypothesis becarrscsomc()nc wirs bounclto, But we also rcject it.
H ' r ' p Ot ' procluc
un tiv
Sts: ity-c
Varying livcrsity p
distur ttcrn.
b -
ance fhe h
r:ttcs igher
ci he
tsethe roduc-
tivity, the more infrequent the disturbances. So, the
productivitv-divcrsity pirttern is the distur[rance-diversity
pattern: As disturbancc ratc falls, diversiry firsr rises,then
falls ((irime 1973, l97L)3LevinanclPainc 1974; Connell
1978; Lubchenco 1978; Sousa 1979a;Paine and Levin
1 9 8 1 ; P e t r a i t i s ,L a t h a m , a n d N i e s e n b a u m l 9 8 9 ) .
In a scnse, this hypothesis also predicts the entire
l.rurlp-shapedpattern. But the disturbance hyporhesisit-
self requirestwo mechanisms.We shall discussthem when
wc cvaluntcthis hypothesis.
RresoNtr.ic: As rainfrrll increases.its coefficient of variation ccrtainly does decline. Rainfall, necessaryfor actual evapotranspiration, helps set productivity (Rosenzweig 1968). Ntoreover, some ecologists believe that evenin tropical biomes, the r','ettest,wannest habitats are the most stable.Whl, usetwo variables(disturbanceandproductivity) to do the work of one?Let'skeep disturbance-
we have experimental proof of its role-and iorget aboutproductivity'. EvetuetroN:Part of the reasoning is suspect.How well doproductivityand disturbance correlate? After all, disturbanceis the converse of stability, and few ecologists acceputncritical,undefinedstatementsabout stability any more.Few believe they know how to rank biomes, let alonehabitats,as to their stability or freedom from disturb a n c eT. a k e a w a l k i n a N e o t r o p i c a l l o w l a n d r a i n f o r e s t . Thatpoppingand crashing .vouhear all around you is the soundof falling vegetation creating new Open spaces. Howisthat open spaceso different from a patch of intertidalsubstratenewly opened by a violent wave? lvlaybeit is far too soon to be sure that proc{uctivity anddisturbancecorrelatc well from middle to higher productivitiesB. ut there is no question that they do from lowerto middle productivities. From tlre most cxtreme deserttso semiaridgrasslandsto rlesic forests,procluctivity and disturbance surely correlate inversely.Isn't that enoughto prove the point? In fact, it is not. The disturbance-diversitypattcrn conres from a wellunderstoodfheory that rnakcs sever,rlcollateral prcdictions.As diversity increasesfrom extrenredesertsttributes of a disturbance pattern. But we know er-roughof the details of the mammal patterns to believethat tl.reydrr not frt the disturbance hypothesis. First, low diversity-high productivity points should corrtain a small, predictable subset of the speciesfrom peak diversity points. But the specieslists from East Texas (high productivity) and from lowlarrd rainforestsin tropical Australia (high productivity) contain many speciesnot for,rndat all in peak-diversity places.This is glirringly so in Texas:only ir few speciesof the rich Trans-Pecosrodent fauna also live in east Texas, and there thcy make up ir small proportion of its irnpoverishedrodent fauna. Finally, at least for the sites of Southern Arizona, we can saythat there is no apParenthistoricirl pattern. Occasionally,wc have seenthc nrost species-richpatchesdevastrrted by a predator (such as a bardger);but soon (in less than a year), the samc specicsthat were wiped out recoloniz.c.And no one has evcr seena short grasspatch (slightly h i g h p r o d r r c t i v i t y )a t a n " i n t e r m c d i a t e s t a g e o f h i s t o r v " with lots of the speciesgenerally found in rnixed desert scrub patches (of highcst diversity and lower productivity). An army of studentsof manrrnalshas sarnplcdso r n a n y s h o r t g r a s s p a t c h e sf o r s o m a r r y d e c a d e st h a t b y n()w someone shor,rldh.rve reporteclsuch a high-diversit,v a s s e r n b l a g ien a s h o r t g r a s sp a t c h . R e p o r t e d o n e , t h a t i s , if they cxist.No onc hat. We now coniront a m()st rt'tOnlalousfcatLrreof the clisturbance hypothesis.According to the disturbrncc h,vpothesis,productivity and disturbancc .lre inversely correlated. But that is only if you are rr terrcstrialecologist. Marine ccologistsagrin ancl again claim thc opposite ( c . g . ,S a n d e r s1 9 6 8 ,a n d m a n y , m r u r yo t h e r s ) .T l ' r c ya s s t r r e us that thc rnost productivc, shallowest wilters are the most clisturbcd.Thc mrlrine pirlcoecologistsccsthe sirme p i r t t er n i n t h e i o s s i ld a t a . M i k u l i c r r n dW a t k i n s ( l 9 1 31 ) , f o r i r r s t a n c cn, r a k c i t c l e l r t h a t t h e s h a l l o w ,p r o c l t t c t i v ec o t t t rnunity is thc one that is most stortn-influencccland tttrstr-tble. Thus, if we believc the disturbance hypothcsis, wc must admit thrrt the got>dcorrelirtior-rof distLrrbirnceratc with procluctivity is p62
turbances, the fewer the species. So, high-productivity, low-disturbance terrestrial localesought to have the most species.They do not. Reduction in the Covariance of Population Densities Hvporur,sts: High temporal covariance of population sizes among speciesleads to higher diversity. As productivity increases,the covariancediminishes.So the decrease phase of the productivity pattern is due to decreasrngcovariance, not increasingproductivity. RlasoNtNc: Population sizes fluctuate. The covarlance statistic of this hypothesis summarizes the tendency for different species to simultaneously experience aboveaverage populations and, at other times, to simultaneously experiencebelow-averagepopulations. Some density-dependentOptimal foraging theory predicts that individuals of competing speciesshould restrict themselvesto their specialhabitat(s) only if they and their competitors (of other species)are similarly common relative to their respective averages (Rosenzweig, 1979, 1987b\. \When speciesdo restrict themselvesin that way, they compete minimally and havethe bestchanceto coexist. On the other hand, when one speciesis common and another is very rare, then the common one must use the rare one'sspecialhabitat, and they compete intensely(Ros e n z w e i g1 9 8 1 ; P i m m a n d R o s e n z w e i g1 9 8 1 ; B r o w n a n d Rosenzweig1986).In sum, accordingto optimal habitat selection theory, high temporal covariance leads to dissimilar behavior, reduced competition, :rnd lowcr extinction rates; low covariance to more similar behavior, incrcasedcompetition, and higher cxtinction rates. Covariance and productivity probably do correlate negatively (Rosenzweig 19791.In a truly harsh, unproductive place, some master variable (like wirter availability in deserts)should govern the fate of all (or most) species.A good year for one is likely to be good for all. Once productivities increase, other, m()re diverse variables should exert more contrcl, thus reducing covariances. EveluetloN: This mechanism seemstoo restricted.It applies only to unstable environments whose specieshave habitat specialties and whose individuals cannot sensea patch's quality except by taking resourcesfrom it (Brown and Rosenzweig 1986).Desertrodents,for example, do not conform to theserequirements.They can sense (probably by olfaction) seedquantities in their immediate vicinity (Brown 1989b). And they do not have true habitat specializations:some speciesmerely get to the best placesfirst (Brown 1989b) or are capableof defendingthe best places(Frye 1983); but all prefer areaswith the highest seedabundances.Such a relationship among the niches in a guild is called "shared preference." Field ecologists discover shared preference relationships more than any other kind of Community Organization (Rosenzweig 1991), and the hypothesisofcovariance reduction applies to none of them. A second problem: this hypothesis is 90% theory. No one has good evidence for the connection between productivity and covariance. Several field experiments do support the isolegmodels that make the optimal foraging
predictions(Pimm,Rosenzweiga, nd Mitchell 1985;Rosenzweig1985;Abramskyet al. 1990;Abramsky,Rosenzweig, and Pinshow 1991). But only theory saysthat hig'She7reicnocvluadrieatnhciselheyapdostthoelsoiwsbeerceaxutsinewcteiownarnattetos.emphasizecovariancerelationshipsT. his particularhypothesis may need so much revision that it becomesunrecognizable beforeit becomesvery useful.But we will probably needto test a better hypothesisbasedon covariancebefore the problemgetssettled.The modelsof Chessonmay supplythe improvements(Chessonand Huntly 1988). Area Hvpotnnsrs: High-productivityhabitat is scarcecompared with intermediatehabitat. Smallerareasusually harbor fewerspeciesT. he productivitypatternis iustthe species-arecaurveon differentcoordinateaxes. If the productivityof very largepatchesis distributed with a centraltendency(say,normally or log-normally), thenthis hypothesisalsopredictsthe entirehump-shaped pattern. Both rich and poor patcheswill be relatively scarceand relativelydepauperate. REesoNrrucL:ike all variablesp, roductivityshouldhave a centraltendencyD. eviationsin eachdirectionshouldbe lessand lessprobableasthey departfrom the meanT. he r e s ti s j u s tt h e s p e c i e s - a rc uar v ei n e c t i o n . EveluRttoN:Much suggesttshat the areahypothesidsoes not accountfor the productivitypattern.First,if areais the answera, ll taxa shouldpeakoverthe sameproductivity (sincethat representsthe most common state)B. ut theydon't.Comparedwith plants,desertrodentsin Israel peakat half the productivity.Comparedwith carnivores, rodentsin Texaspeakat aboutone-tenththe productivity ( O w e n1 9 8 8 ) . Second,the area hypothcsiscannot apply to United Statesrodents.They peak in the semideserat nd fallto muchlowerdiversitiesin grasslandsW. ho will defendthe ideathatgrasslandasrerelativelyscarcein North America comparedwith semideserts? Last, Abramsky and Rosenzweig(1984) discovered that the productivity-diversitypattern remainsin their data evenafter they take out the unequal-areaeffectsG. oing a step further, Rahbek shows that the productivity pattern doesn'tevenappearin tropical bird data untilhe actuallyremovesthe effectof area.
CoNcluorNc OssnnvATroNs
\tfiofeneocfowlohgaist ctsroenttarionlsatnhaebdiidvienrgsfiatysocfinsapteiocniewsi't.Wh eth'veemquaedse-
considerableprogressin understandingsome aspectosf this control. Preston (1962), MacArthur And lWilson
(1963,19671,andSimberloffand Wilson (1969b)taught
us the fundamentalsof island diversity patterns.Three
overviewsof the species-arecaurve (Connor and McCoy
1.979;Colemanet al. 1982;McGuinness1984a)empha-
sizehow much we now know about that. The puzzleof
t1h9e7l0ast i(tTu deirnbaolrdgihv1e9r s7i3t y; Rg roasdei ennz twweai gs' 1s,o9l7v5e1,d9i n7
the 7 c l(
aboutthis below). And the relationship between disturbanceand diversity (e.g., Grime 1973; Levin and Paine 1974; Lubchenco 1978)-at least for some associations-is alsowell modeled and documented. Meanwhile,the connection between productivity and diversity-long thought to be intimate-remains murky. In this chapter,we hope we have convinced you of the patternproduced by that relationship within large regions:Diversity rises ove'Wr elow productivities and falls overhigh productivities. also hope you have decided thatno one knows why. Not a singlehypothesis successfullyexplains the p:rttern.The rising phaseof the pattern is most likely caused byhigherproductivities being able to support larger populationsizes.Larger population sizes should experience lowerextinction rates. However, all of the nine hypothesesto explainthe decreasephase have noteworthy weaknessesT. hree of them-the time, area, and predatorvictimratio hypotheses-are so weak we need not discuss themfurther. Two others-the dynamical stability and disturbance hypotheses-addresscalesof tirne or spacetoo small to allowfor evolution. Given enough time, evolution tends to restoredynamical stability. And in evolutionary time, the relationshipbetween diversity and disturbance rate shouldbe monotonically negative, not unimodal. Wealsodoubt the hypothesis that interferencecompetition peaks over intermediate productivities, thereby causintghe pattern. This hypothesis seemsfar too poorly workedout and has far too little ernpirical support. The covirriancehypothesis is nrore promising. Howevera, t present,too many
  • decide,we need a better theory of theevolution of habit:rt heterogeneity. The other strong hypothesis, intertaxon comperirion, fitswhat few facts we have. It predicts great variety from taxonto taxon in the place of the peak diversity. Certainly,we seethis. But it lacks a mechanistic model and is thereforepoor in predictrons. The intertaxon hypothesis needs fuller explanation. Whyshouldtaxa have optima along a productivity gradient?Why can't they at least defend the share of the productivitythey have already acquired when that optimum i s e x c e e d e d? Experiments Correlationsare among the best ways ecologistshave of searchingfor patterns. Once found, however, patterns must be investigated further, preferably by experiment. That is particularly true of productivity patterns because productivity itself correlates with so many other importantecological variables. Proper control of ecological experimentsdoes require a smaller than regional scale.
    But we can usethe resultsif we are cautious about designing the experiments. Ecologists have done many experiments on the relationship between productivity and diversity (Swingle 1946; Yount 1955; Patrick 1963; Kirchner 1977; Sllvertown 1980; Tilman 1987; Schindler 1990; Goldberg and Miller 1990, and many others reviewed in the latter and in Huston 1979). All these experiments involved increasingproductivity. Occasionally diversity failed to respond to the change, but far more often it decreaseddramatically. Only one experiment (Abramsky 1978\ reported an increase.Does that confirm the existenceof the decreasephase?Does it mean we should distrust the i n c r e a s ep h a s e ? Ve think the set of experimental resultsnow available casts little or no light on the regional pattern. None of these experiments take evolution into account, although the evolution of speciesto take advantage of higher productivity must constitufe an important part of the response of a region to higher productivity. Some experiments fail to allow even enoutrihtime for succession, although we know how irlportant that can be. \Vhittaker ( 1 9 7 s i t ,u s i n gt h e d a t a o f ! 7 < r o d w e l la n d H o l t , s h o w s t h a t diversity and productivity both increaseduring the first fcw yearsof succession. Experiments to test the increasephaseshould compensate for a lack of evolutionary time. For example, they might be sited near enough t() potential sourcesof "new" speciesso that immigration could supply them. And irt least some experiments shoLrldtake place at sites where the regiorrirlpattern predicts a diversity increase.Before we develop enrichment experiments that cl ) r o d u c t i v i t yv e r s u sB i o m a s s
    M e a s u r e m e n t so f s t a n d i n gc r o p ( i . c . , b i o m a s s )a r e e a s i e r
    to find than mcasurementsof energy flow (i.e.,productiv-
    ity). The two ought to correlate well, although not per-
    fectly. Consequently, some ecologisfs use either one tr-r
    study the relationship of productivity to diversity. We
    hope this does not turn out one day to have createdcon-
    The two variables do differ. One of them may be
    causal. If we are to discover which one, keeping them
    sor'Wteed out seemsa minimal strategy. found one data set (Haedrich, Rowe, and Polloni
    1980) that offers measuresof both bio per lrour of sample) and productivity
    mass (g (ocean
    rams ca depth).
    utrih 'fhe
    .ruthors separatetheir data ir.rtofishes,echinoderms, and
    decapods. Both fishesurndechinoderms clearly show the
    unimodal pattern for depth. Decapods prohably also
    show it, but their peak occurs in relatively shallow water,
    so only the shallowest sanrples(40-264 m) attest to the
    decreasephase.None of the three taxa's diversitiesfit bio-
    mass as well as they do depth. Low biomassesare associ-
    ated with almost any diversitS although higher biomasses
    always seem to 'bWee associirtedwith at least moderately high diversities. hope to seemore such comparisons
    ln luture reports.
    Productivity versus Energy Wright (1983), realizingthat a barren hectarehas no spe-
    area component should be so important as to overwhelm and mask the influence of productivity. A caveat: Although area's influence has to be teased
    I I 1
    cies, extended the concepr of species-areastudies. He apart from that of productivity, rnultiple regressloncan-
    created a new variable by multiplying area times productivity. The result is grarifying. Adjusting area for productivity irnprovesthe fit of data to the independentvari-
    not do it beyond the scaleof the region. Multiple regression eff'ectivelyreduces each sample to a common area and then looks for the influenceof energyflow on there-
    t t s
    able. More productivity always seems ro increase an siduals.That takes care of the regional species-are:crurve,
    area'scapacity for species.But doesn't that monotonic re- but not Terborgh's latitudinal gradient of area (which
    lationship contradict the uninrodal relationship we've works in evolutionary time at the level of speciaticlnand
    c h a m p i o n e di n t h i s c h a p t e r ?N o , i t d o e sn o r . Pleaseread \Wright. Currie and Maurer's conrribution t o t h i s v o l u m e c a r e f u l l y ( c h a p . 6 ) . Y o u w i l l s e er h a t r h e i r
    extinction rates). Convincc yourself of the problem by imagining two bird lists, one from a square kilometerof Britain, the other from a square kilometer of Peru.Each
    p e
    chapter and ours ilgree on the patrern at the regiclnal scale:It is unimodal. The monotonic pattcrn appears ar the global scale. The Global Scale
    receivesimrtrigrants from its region. Since the Neotropical region contains so many more speciesthan the Eurasian regiorr does, r,vcexpect the Britislr area to har.efar fewer species.It is as if each square kilometer carriesthe cvolution:rry,rnark of its participation in thc larger regiT q P si pr
    So far we have limited ourselvcs to considcring regions. to which it belongs.ln fact, that is preciselythe case.Our But we do rrof want t() avoid the big picture forever. What sr>lutiorrto the problern? Stay inside regions to find out
    i s t h e p r o d u c t i v i t y - d i v e r s i t yr e l a t i o n s h i pa n l o n g b i o g e o - how productivity affects divcrsity. If we are correct,then
    grrphical province-s?ls it ntonot()rricallypositive?We :rre the tropics would be cven richer if we cor,rldmake them
    l e s st h a n s u r c .
    somcwhat lcssproductive fB y c o n r b i n i n g t h c v a r i a b l e so i a r e i r a n d p r o d u c t i v i t y , $ T r i g h t( l 9 u . l ) l o s c st h e a b i l i t y t o d e r e r m i n ct h e i n f l u c n c e Prospects
    r>fcaclrvariahle scparately.Now, we arc not recluctionists, Maybe therc will never he a lrypotlresisthat fully cxplains
    but if a gotrvo variablcs hirs a scparatc irncl powerful influence on Iooking fr>r somcthing imaginary. Suppose the rncrease
    diversity, then we prcier to treat that variablc separately. phasccomes frorn the rclatisuizc (as nrost believc it does). Supposethar oncea
    i t i n t o a c c o u n t , w c c a n g o o n a n d c o n t b i r r ci t w i t h o t h c r systempassesa criticirl point, further increasesin popula-
    variables in scirrch r c c k o r rw i r h r h c k u o w r r . Thc known mcchanisnris alloparric spcciation.'l'hc
    ratcs.Thcn, beyl a r g c r a g e o g r a p h i c a lr a n g c , t h e m o r c l i k e l y t h a r a g c o - rrumbcr of cnvironnrcntal changes :rssociatedwith in-
    g r a p h i c a l b a r r i e r w i l l s p l i t i t i n t o i s o l a t e s( R o s c n z w e r g creirscdprocluctivity would deflate the diversity.For now,
    1975, 1977c). So. orher things bcir.rgequal, large ranges h o w c v e r ,w c w i l l p u r s u c( ) u r ( ) w n i n v e s t i g a t i o n as si f a s i n -
    p r o d u c em ( ) r cn e w s p e c i c sp c r u n i t t i n r c t h a n s m a l l r a n g e s gle successfuldeclinc phase hypothcsis exists.
    do. Quitc probably, lirrgcr ranges also redLrcecxrineritrrr
    The unirnodal curve that describesthe cffect of produc-
    r a t c s b y h a r b o r i n g n r o r e i n c l i v i d u r r l sa n d b y r e d u c i n gt h e tivity on diversity may fit vascul:rrplant divcrsities.It does
    risk that a pcrturbation will affect the entire rangc. Bur 6t the diversiticsof virrious taxa of marine invertebrates.
    those influcnces are mcrely hypothctical, whereas the in- It fits the diversitiesof mammals in tropical and subtropi-
    flucnce of irrea on irllopatric spcciatiortratc is inhcrent ilt cal Iltitudcs, :rnc[the divcrsities of birds along a tropical
    thc merchanisrnitself.
    clevation gradient. \Wehavc found it on five conrinents o
    T e r b o r g h( 1 9 7 3 i to b s c r v e dt h a t t h c r e i s a r n a r k c dg r a d i - far. Once you adjust for thc effects of area and latitude,
    cnt in area from tropical to polirr climates. Tropical re- the hump-shapeclpattern may pop out of your favorite
    gions covcr f:rr nrore territTerborgh's gradient coupled to alkrpatric speciation ac- good job conservingas many of the world's speciesaspos-
    counts for the latitLrdinalgradient in diversity (Roserrz- si[rle.we have tweig 1992). At any diversity.tropical regions should ex- this and other common diversity patterns.
    perience higher speciarion rarcs (and probably lower
    extinction rates too). Consequently,steady-statetropical diversitiescxceedthosc of any other laritudc. Only if trop,
    ical biotas were significanrly younger than biotas of other Thenumbeor f speciewsithina regionusuallvyarieusni-
    latitudesccruldthey be lessdiverse.
    modally with the rate of ecosystemenergyflow. This
    Now you can appreciatewhy we remain unsure of the hump-shapedpatternshowsup in many biogeographical
    global relationship of productiviry and diversity.Diversity provinces.Plant and animal taxa, includingvertebrates does increasemonotonically with \i7right's variable. But and invertebrates,follow it. \Wefind it in marineandin
    rve believetlrat stemsfrom its ereil component rather than terrestriabl iomes.
    from its energy flow componenr. On a global scale,that
    Most ecologistswill agreethat the increasein diversity
    thatoccursover low productivities comes about because thetotalabundanceof all speciestogether increasesover that range of productivities. \7e describe and evaluate ninehypothesesto explain the decreasephase of the pattern,i.e., why diversity declines as productivity grows pasta certain point. We reject five of these: dynamical instability;change in predator-victim ratio; extreme youth of the most productive places; disturbance; and area. Thesehypothesesoften explain other patterns of diversity, butnot this one. We have little confidence in a sixth hypothesisd:ecreasein interference competition at the highestoroductivities. The remaining three hypotheses all have problems. Twoarestill poorly modeled, and therefore have inadequateempiricalsupport. One, the hypothesis that a competingtaxon causesthe decrease,predicts that peak diversitiesshould vary among taxa, and they do. But it also predictsometaxa should not show the peak, but instead shouldincreasemonotonically with productivity. So far,
    we have found no such taxa. However, that may well be
    becausewe cannot adequately examine the pattern in the
    tropics sinceno one yet knows what environmental vari-
    ables control productivity among tropical ecosystems.
    The second hypothesis predicts that competition will be
    more intensewhen productivity is very high becausemore
    productive sitesgeneratelower covariancesbetween the
    competing species'population densitics. But no one has
    testedthis hypothesis.
    The third surviving hypothesis looks deceptively like
    the stror.rgestH: igher productivities reduceenvironmental
    heterogeneity.This one has been careftrlly modeled and
    has much empirical support. But it is probably a tautol-
    ogy. Heterogeneity, bcing an evolved rcsp()nseof lifc,
    sho'Wuled evolve to bc higher where discussthe relationship of
    thcre are more spccics. the regional pattcrn t()
    global patterns such as thc latitudinal gradicnt. We believc
    the effectof productivity on diversity is beststudicd :rt the

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