Download ROLE OF KEYSTONE SPECIES IN AQUATIC ECOSYSTEM

ROLE OF KEYSTONE SPECIES
IN AQUATIC ECOSYSTEM
M.Feroz K h a n ’ , P re e th a P a n ik k a r ', A .P .S h a rm a *', B . C J h a ’ *a n d M .E .V ija ya ku m a r*
" C e n t r a l I n l a n d F is h e r ie s R e s e a r c h I n s titu te ,B a r ra c k p o r e , H/es( B e n g a l - 7 0 0 1 2 0
" R e s e a r c h C e n t r e C IF R I, B a n g a lo r e - 5 6 0 0 8 9
Introduction
“[Keystone sp ecie s] im p o rta n ce co n vin ce d m anagers and consen/ationists alike
that the ecological im p a c t o f sin gle s p e c ie s matters. T h at is, in o rd er to m anage,
understand, and re store e c o lo g ica l a s s e m b la g e s , th e roles o f individual species have to
be understood and c o n s id e re d .” - Dr. R o b e rt Pain
A keystone s p ecie s is a sp ecie s th a t has a d isproportionately large effect on its
environment relative to its abu n da nce . S uch species play a critical role in maintaining the
stru c tu re o f an e c o l o g i c a l c o m m u n i t y , a f f e c t in g m a n y o t h e r o r g a n is m s in
an ecosystem and help in g to d e te rm in e th e typ e s and num bers o f various other species
in the community.
The role that a k e y s to n e s p e c ie s plays in its ecosystem is analogous to the role o f a
keystone in an arch. W h ile th e k e ys to n e is u n d e r th e least p ressure o f any of the stones
in an arch, the arch still c o lia p s e s w ith o u t it. Similarly, an
e cosyste m may expenence
a dramatic shift if a ke y s to n e s p ecie s is re m o ve d , even though th a t species was a small
part of the ecosystem by m e a s u re s o f b io m a s s o r productivity. It has become a very
popular concept in c o n s e rv a tio n biology.
The keystone s p e c ie s co n c e p t w a s co ine d , in 1969. by the zoologist Robert T
Paine, professor e m e ritu s o f the U n ive rsity o f W a sh ing to n, to explain the relationship
between P is a s t e r o c h r a c e u s , a sp ecie s o f starfish, and M y tilu s californianus, a sp e a e s
of mussel. In his classic 1966 paper. Dr. R o b e rt Paine descnbe sue a
^ klw ctnnp
Bay in W ashington State, T h is led to his 1969 p ap e r w here he proposed the keystone
species concept. T he c o n c e p t has been v e ry p opular in conservation, deployed in a
range of contexts and m o b ilize d to e n g e n d e r su pp o rt fo r conservation.
Given that the re are m a n y historical d e fin itio h s o f the keystohe speoies oohcept^
and without a c o n se n s u s on its e x a ct defin itio n, a list o f exam ple s best illustrates the
concept of keystone species.
A c la s s ic k e y s to n e s p e c ie s is a s m a ll p r e d a t o r t h a t p r e v e n t s a
particular herbivorous species from elim inating d o m in a n t plant species. Since the prey
numbers are low, the keystone pre da tor num bers can be even lower and still be effective.
Yet without the predators, the herbivorous prey w o u ld e xplode in num bers, w ipe out the
dominant plants, and dramatically alter the character o f the ecosystem. T he exact scenario
changes in each example, but the central idea rem ains that through a chain o f interactions,
a non-abundant species has an out-sized im p a ct on e cosyste m functions.
As was described by Dr. Robert Paine in his classic 1966 paper, some sea stars may
prey on sea urchins, mussels, and oth e r shellfish th a t have no other natural predators. If
the sea s ta r is re m o ve d fro m th e e c o s y s te m , th e m u s s e l p o p u la tio n explodes
uncontrollably, driving out most o th e r species, w h ile th e urchin population annihilates
coral reefs.
Similarly, sea otters protect kelp forests fro m d a m a g e by sea urchins. Kelp “roots’',
called holdfasts, are merely anchors, and not the va s t nutrient gathering networks of land
plants. Thus the sea urchins only need to eat th e ro o ts o f th e kelp, a tiny fraction o f the
plant's biomass, to remove it fro m the ecosystem . T h e s e creatures need not be apex
predators. Sea stars are prey fo r sharks, rays, and se a a nem ones. S ea otters are prey
for Orca.
T he c o n c e p t
The term keystone species has enjoyed an e n d u rin g popularity in th e ecological
literature since its introduction by Robert T. P aine in 1969: Paine (1969) w as cited in
more than 92 publications fro m 1970 to 1989; an e a rlie r p ap e r (P aine 1966), which
introduced the phenomenon o f keystone specie:- in intertidal system s but did not use the
term, was cited more than 850 tim es during the sa m e period.
Paine (1966, 1969) noted th a t e x p e rim e n ta l re m o v a l o f so m e ro c k y intertidal
carnivores (such as the starfish P is a s te r) led to n early co m p le te d o m in a n ce o f the
substrate by one or two sessile species (mussels), resulting in greatly decrea sed species
diversity. In this and other cases, the im portance o f the k e y sto n e p re d a to r derived from
two requisites (Paine 1969, P im m 1980): the p re d a to r preferentially ate and controlled
the density of a primary consumer, and the co nsu m e r w as ca pable o f e xcluding (through
competition or predation) other species from the com m unity. Essentially, then, the early
connotation was that keystone predators are im portant because they control the densities
of important com petitor or predator species. P redators have also been labeled keystone
when they control the densities o f o th e r types o f e c olo g ica lly significant p re y species.
For example, sea otters {E n h y d ra lutn's) have often been referred to as keystone predators
(e.g., Duggins 1980,
E s te s a n d P a lm is a n o 1 9 7 4 ) b e c a u s e t h e y lim it d e n s it y o f s e a u rc h in s
{S tro n g y lo c e n tro tu s s p p ^ . which in turn eat kelp and o th e r fle shy m a cro alga e th a t form
the basis of a different co m m u nity than is p resent in th e ir a bsence (V anB laricom and
Ml
Estes 1988). Thus, o tte r re m o va l has com m u nity-le vel influences, by releasing from
predation a primary c o n s u m e r th a t eats a plant th a t harbors o th e r organisms.
As used by Paine and o th e r ecologists, the re are two hallmarks o f keystone species.
First, their presence is cru cia l in m a in tain in g the organization and diversity o f their
ecological communities. S eco n d, it is im plicit that these species are exceptional, relative
to the rest of the com m unity, in th e ir im p o rtan ce . Given the assum ed importance of
keystone species, it is not surprising th a t biolog ists have advocated th a t key or keystone
species be special ta rg e ts in th e efforts to m a xim ize biodiversity protection (e.g., Burkey
1989, Frankel and Soule 1981, Soule and S im berloff 1986. Terborgh 1986) and as species
in need of priority p ro te ctio n (e.g , Cox e t al. 1991).
The keystone s p e c ie s play a central a nd critical role in m ain ten an ce of com m unity
structure and ecosystem functio ning . If an e c o s y s te m can be returned to a state in which
the keystone species flo un sh , the n all the o th e r species, which d ep e nd on them, will also
flourish. The im p o r ta n c e o f b io d iv e r s it y in e n v ir o n m e n ta l m a n a g e m e n t b e s id e
socioeconomic deve lo pm en t and well being o f hum an society, has led to the development
of various techniques fo r c o n se rva tio n o f e co lo g ic a l diversity. S om e simple w a ys of
managing the natural s ys te m s should be evolved so as to retain and conserve the identity
of a region for a better to m o rro w . O n e o f th e sim p le st w a ys o f doing so is by identifying
species, which play the key role o f h old in g to g e th e r the entire biological com m unity or
ecosystem. These sp ecie s a re know n as 'k e y s to n e species in ecological term.
The central core o f k e y s to n e ,c o n c e p t is th a t only a fe w species have uniquely
important effect on th e c o m m u n ity o r e c o s y s te m by virtue o f th e ir uniquely important
traits and attributes. O nly th o s e s p ecie s can be considered as keystone species that
had a significant e ffect on ‘tim e w in d o w ’ o f o th e r species. In m o s t o f the cases, it is
indeed groups of sp ecie s ra th e r th a n individual species th a t a ssum e im portance and
these species groups could be referred to as th e ‘keystone g ro u p s ’ o r ‘functional groups .
Keystone species or 'k e y s to n e species g ro u p s ' play a vital role in m aintaining ecosys em
and regulating the biodiversity. Loss o f vital fu n c tio n , and ch an g es within the ecosystem
or community would fo llo w if su c h s p ecie s g ro u p s are rem oved fro m the system. These
species are Responsible' fo r th e e x isten ce o f an ecosystem o f certain type and create
possibilities for the d e v e lo p m e n t o f o th e r ty p e s o f com m unities.
Biodiversity w ithin an are a can be ch a ra c te rize d by m e a s u r e s o f species richness
species diversity, ta x ic diversity, and fu n c tio n a l diversity, e ach highlighting d fferen
perspectives Functional diversity refers to th e varieties o f functions earned
hy d fle re n
species and groups o f sp e c ie s know n as fu n c tio n a l groups T he population ^ V ^ c s o
keystone species define the pattern o f su ccessio n
“ egetatio n ^T u rno ve rcyd es of
and energy flows in an e c o s y s te m are d o m in a te d by the life ac ivity o f l < 7 = ^ ®
and these activities d e te rm in e th e m a jo r s h ifts in ecosystem
‘ ‘ h®
tenporal scales P op u latio n m o sa ics o f ke y s to n e species have
dimensions, and pop u latio n m o sa ics o f su b o rd in a te species are th e re b y determined by
the keystone species. K eysto ne sp ecie s are re s p o n s ib le fo r th e e xiste n ce o f the
ecosystem and maintenance o f its species diversity. S o the biodiversity in any ecosystem
can be manipulated by perturbations in such u niq u ely im p o rtan t species.
A m a jo r research ch allen g e fo r e c o lo g is ts is to p re d ic t w h ic h s p e cie s in the
community would become keystone species.
The current level o f conceptual u nd e rsta n d in g o f th e effe cts o f biodiversity on
ecosystem processes is so prim itive that at this sta g e it is possible to recognize the
linkages at the level of functional g ro up s only. In a n y e c o s y s te m the re are diverse types
of functions performed by different species or sp ecie s g roups. However, no tw o species
or individuals are identical. It m a y be noted that sp ecie s diversity within the functional
groups or genetic diversity within the species has im p o rta n t ecosystem consequences.
Although certain species have much g re ate r in flue n ce than others on an ecosystem
structure, not all ecosystems include the sam e species th a t exert such pervasive influence
on them. In fact most ecosystem s are so m e w ha t se nsitive to the loss o f a fe w species,
though som e losses have g re a te r im pact on th e system than others. Nevertheless,
identification of such species, w hich w ould function a s keystone species in an ecosystem
can help in the conservation o f that ecosystem . T h e fa c t th a t so m e species m atter more
than the others, becomes especially clear in the ca se o f ‘keystone sp ecie s’ or ‘ecosystem
engineers’ or ‘organism s with high im portance va lu e fo r th e c o m m u n ity ’. These terms
may differ in usage, but all re fer to those sp ecie s w h o s e loss o r rem oval results in
disproportionately greater im pact on the com m u nity w h e n c o m p are d to th e loss o f other
species. M em bers o f the functional g ro up s m aintain and d ete rm in e th e resilience o f the
ecosystem by spreading a wide ra n ge o f ecological niche s exploited by th e component
species.
The contribution of individual species tow ard e c o s yste m d e ve lo p m e n t varies in time
and space, and accordingly, not all species are e q u a lly im p o rta n t w h e n w e look at the
community stability and functioning. T he co m m u n ity fu n c tio n m ay be m aintained by a
species or sum m ed effects o f a fe w species. S o m e s p e c ie s und o ub ted ly play more
significant role than others in ecosystem function. T he varieties o f functions that a species
can perform are limited and consequently, an increase in species richness also increases
functional diversity, producing an increase in e colo g ica l stability.
W ithin a com m unity it is not possible to su bstitu te s p ecie s fo r one another, rather
there are a good num ber o f co m b ina tio ns o f sp ecie s th a t ca n produce sim ila r ecological
roles. There is no intrinsically unique level at w h ich biotic diversity affects ecosystem
processes. Based upon their e colo g ica l roles and th e sp ecific ecological niches that
they exploit, species can be divided into 'functional groups'. A fu n c tio n a l g ro up refers to a
group of species, which perform ecologically sim ila r roles in ecosystem processes.
For heuristic purposes, the u sages o f keystone species is divided into five types.
This categorization is not m eant to im ply m utually exclusive g ro up s or an exhaustive
review of the term s application, but rather to show the diversity of keystone effects referred
to in the literature
Table 1. C a te g o rie s o f p re s u m e d k e ysto n es and the effects of
th e ir e ffe c tiv e re m o va l fro m a system.
K eystone
category
E f fe c t o f re m o v a l
Predator
In cre a se in one o r se veral p re da tors/ consum ers/com petitors,
w h ich s u b s e q u e n tly e x tirp a te s several p re y/co m p e tito r species
Prey
O th e r sp e c ie s m o re se n s itive to predation m ay becom e extinct;
p re d a to r p o p u la tio n s m a y crash
Plant
Extirpation o f d e p e nd en t anim als, potentially including pollinators
and se e d d is p e rse rs
Link
F ailure o f re p ro d u c tio n and recru itm e nt in certain plants, with
p ote ntia l s u b s e q u e n t lo sses
Modifier
Loss o f stru c tu re s /m a te ria ls th a t a ffect h abitat type and energy
Determining th e K e y s to n e S t a t u s o f S p e c ie s
All species are im p o rta n t fo r th e e xisten ce o f an ecosystem and fo r the m aintenance
of its various functions, b u t as m e n tio n e d earlier, all are not equally important. The
identification of species a nd g ro u p s o f sp ecie s, w h ich play key role in maintaining the
ecosystem stability a nd re s ilie n c e by in flu e n c in g th e s tru c tu re and fun ctio n o f an
ecosystem is a stup e nd o us task, and very fe w atte m p ts have been m a d e in this direction.
Since the importance o f s o m e sp ecie s m a y la rg ely be th e co n se q u e n ce of their rich
interaction structure, o ne p ossib le q u a n tita tive a p p ro ach to identify the most influential
species is to study th e ir p ositio n in the n e tw o rk o f interspecific interactions.
We have developed a n e tw o rk s tru c tu re o f th e reservoir e cosyste m is built using
Ecopath with Ecosim s o ftw a re fo r c h a ra c te riz in g th e in teraction structures o f each
species. This study w a s co n d u c te d at K a ra p u z h a reservoir, located at W ayanad Distnc
of Kerala In this p ap e r th e ke y s to n e n e s s o f th e functional g ro up s (species or gro up of
species) of food w eb m o d e l o f a re se rvo ir e c o s y s te m is exam ined. T he species in t^his
reservoir are assem bled into 15 fun ctio na l g ro u p s fro m D etritus to A qu a tic birds. The
total system throughput in th e re s e rvo ir is 3 0 0 3 9 t/km 2 with a connectance index of
0.277 The system o m n io v o ry in dex is e s tim a te d a t 0.109. T he sum o f all detritus flows
into detntus is 11268.45 t/km 2 . T h e a n a ly s is o f th e mixed tro p h ic impacts presented
allows ranking o f fun ctio na l g ro u p s by th e ir ke ysto n en e ss. T he keystone in ex
from 0.610 for P hyto plan kton to 2.839 fo r a q u a tic birds. T he im p o rtan t result is that
keystone species exert th e ir high im pact by m e a n s o f top-dow n effects, a feature initially
S'jggested being a defin in g c h a ra c te n s tic o f k e y sto n e species. C la n a s g a n e p in u s has a
very high key stone index at 2.168 which show s h o w m u ch influence an invasive species
has on the food web o f this rese rvoir e cosystem . T h e study sh ow s th a t lower biomass
species in this reservoir ecosystem are show ing v e ry high ke ysto n e indices.
Fig. Keystone indices o f K ara p u z h a R e se rvo ir ecosystem .
‘he Key Stone Species at
Reservoir
Aquatic Bird
2.8|9
t
0
Crustaceans
M
E
I
Major carps
®
I.34 fi
Minor cdr
Minno
„
1,371
Barbs 1,319
l.
ljl
1
African catfish
« 2.168
Eels 2.444
keheads
D
plankton
0.159
'h
O.mossamblcus'h, J \
^
0 978
^''''’’t o ^ l a n k t o n
0.1
' ‘W 469
0.2
0.3
0.530
0.4
0.5
0,6
0.7
0.8
0.9
1.0
Relative Total
C on clu sio n
n a t u r a lT v s t e m r w r t h
m hers7
ecoloqicai functioninn nf
ml
ConL':vr;“
f
'^ ^
com p lexity o f interactions in
oause the loss of
species tinat are so im portant in determ ining the
rs,,;,; : ten^r^'^mofomr^
ecosystems or processes If o u r nn;=??rtn
b e tw e e n pro te ctin g species, areas,
a reliable supply of ecosystem serw'ces th e n ^ r iP ^ r ^ ^ e co s ys te m structure and maintain
will be the only real long-term s o lu tio n ’fn r
consenting im portant species
only the rarest sp e c e s is only a sym ptom atic S m e n f p ^ ' * ^ loss u ltim a tely protecting
inspire theoreticians to e x o L p thp
.
x
® tantalizing results should
weakly interacting specres a^d o n l ?
a s s e m b la g e s stru cture d with many
useful^'in d em on sir U g h a r u n d e 'o e rt
strong interaotors.. The co ncept has been
_conditions so m e species have particularly
strong interactions, a nd w e recognize th a t in recom m ending tiie abandonm ent o f a
popular and e vocative co n ce p t the re is a d a n g e r of making it more .
For fu r th e r re a d in g .
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