Organism and Population

8/4/2019 Organism and Population

1/24

Diversity is not only a characteristic of living organisms but

also o f conte nt in b iolog y textbo oks. Biolog y is p resented either

as botany, zoology and microbiology or as classical and

mo dern. The la ter is a e uphem ism fo r mo lecular aspec ts of

biology. Luckily we have many threads which weave the

different areas of biological information into a unifying

princip le. Ec olog y is one suc h threa d which g ives us a holistic

pe rspec tive to b iology. The essenc e of biolog ica l understand ing

is to know how organisms, while remaining an individual,

interac t with othe r orga nisms and p hysica l hab itats as a g roup

and hence behave like organised wholes, i.e., population,

community, ecosystem or even as the whole biosphere.

Ec olog y explains to us a ll this. A p articu lar aspec t o f this is the

study of a nthrop og enic environmenta l de grada tion a nd the

soc io-politica l issues it ha s raised . This unit describes as well as

takes a c ritic al view of the a bo ve a spec ts.

Chap ter 13

Organisms and Pop ula tions

Chap ter 14

Ecosystem

Chap ter 15

Biod iversity and Co nserva tion

Chap ter 16

Environmental Issues


2/24

Ramd eo Misra is reve red as the Fa ther of Ec olog y in Ind ia. Born on 26 August

1908, Ramd eo Misra obta ined Ph.D in Ecology (1937) under Prof. W. H. Pea rsall,

FRS, from Lee ds University in UK. He esta b lished teaching a nd resea rch in

ecology at the Department of Botany of the Banaras Hindu University,Varanasi. His research laid the foundations for understanding of tropical

communit ies and their succession, environmental responses of plant

populations and productivity and nutrient cycling in tropical forest and

grassland ecosystems. Misra formulated the first postgraduate course in

ec olog y in Ind ia. Ove r 50 schola rs ob ta ined Ph. D degree und er his supervision

and m ove d o n to o ther universities and resea rc h institutes to initia te ec olog y

tea ching and resea rc h ac ross the c ountry.

He w as honoured with the Fellowships of the Ind ian Nationa l Sc ienc e

Acad emy a nd World Acad emy of Arts and Sc ienc e, and the p restigious Sanjay

Gandhi Award in Environment and Ecology. Due to his ef for ts, the

Go vernment o f Ind ia estab lished the Na tional Comm ittee for Environm enta lPlanning and Coordination (1972) which, in later years, paved the way

for the estab lishment of the Ministry of Environm ent and Forests (1984).

RAMDEO M ISRA(1908-1998)


3/24

Our livin g world is fas cinat ingly divers e an d a m azingly

complex. We can try to u nd erstan d i ts comp lexity by

investigating processes at various levels of biological

organ isa t ion ma cromolecules, ce lls , t i ssu es, organs ,

ind iv idua l o rgan i sms, popula t ion , communi t i e s and

ecosys tems and b iomes . A t any l eve l o f b io log ica l

organisation we can ask two types of questions for

exam ple, when we hear the b u lbu l singing early morn ing

in th e garden, we ma y ask How does th e bird sing ?

Or, Why does t h e bird s ing ? The h ow-type qu est ions

seek the mechanism beh ind th e process wh ile the why-

type ques tions seek th e significance of th e process . For

the first qu estion in ou r examp le, the an swer might be in

terms of th e operation of the voice box and the vibra ting

bone in the bird, whereas for the second question thean swer ma y lie in th e birds n eed to comm u nicate with its

mate du r ing breeding season. When you observe na ture

around you with a scientific frame of mind you will

certainly come u p with m an y in terestin g qu estions of both

types - Wh y are night-blooming flow ers gen erally w hite?

How does the bee k now w hich f lower has nectar? Why

does cactus have s o many thorns ? How does the chick

recognise her ow n m other? , and so on.

CHAPTER 13

ORGANISMS AND POPULATIONS

13.1 Organism and It s

Environment

13 .2 Popula t ions


4/24

220

BIOLOGY

You ha ve already lear nt in previous clas ses th at Ecology is a su bject

which s tud ies the in t e rac t ions among organ i sms and be tween the

organ i sm a n d i t s ph ys ica l (ab iot i c ) env i ronm ent .

Ecology i s bas ica l ly concerned wi th four l eve l s o f b io log ica lorganisation organism s, populations, comm u nities an d biomes . In th is

ch ap ter we explore ecology at organ ismic an d popu lation levels.

1 3 . 1 ORGANISMAND ITS ENVIRONMENT

Ecology at th e organ ism ic level is es sen tially ph ysiological ecology wh ich

t r i e s to unders t and how d i f fe ren t o rgan i sms a re adap ted to the i r

environm ents in t erm s of not on ly su rvival bu t also reprodu ction. You

ma y have learn t in ear lier clas ses h ow the rotat ion of ou r planet arou nd

the Su n an d th e t ilt of it s a xis cau se an nu al var ia t ions in the intensi ty

an d dura t ion of tempera tu re , resu lt ing in dist inct seasons . Thes e

variat ions together with annual variat ion in precipitat ion (remember

precipitat ion includes both rain a nd sn ow) accoun t for the form ation of

major biomes such as deser t , ra in forest and tundra (Figure 13.1) .

Figure 13 .1 Biome dist r ibu t ion wi th respect to an nu al temperatu re an d p recipi tat ion

Regiona l an d local var iations within ea ch b iom e lead t o the form ation of a

wide variety of h ab itats. Major biom es of In dia ar e sh own in Figu re 13 .2.

On plan et Eart h , life exists n ot ju st in a few favoura ble hab itats bu t even

in extreme and h ars h h abitats scorching Rajasth an desert , perpetually

rain-soaked Megha laya forests, deep ocean tr enches, torrential stream s,

perm afrost p olar regions, h igh m oun tain tops, boiling th erma l spr ings,

an d st inking comp ost pits, to nam e a few. Even our intest ine is a u nique

ha bitat for hu nd reds of species of microbes.


5/24

221


What are the key elements that lead to so much variation in the

physical and chemical condi t ions of d i f ferent habi tats? T h e m o s timp ortant ones a re temperatu re, water, light an d soil. We mu st remem ber

tha t th e ph ysico-chem ical (abiotic) componen ts a lone do n ot cha racterise

the habi ta t of an organism complete ly; the habi ta t inc ludes biot ic

componen ts also path ogens , paras ites, predators a nd competitors of

the organism with which th ey interacts cons tan tly. We ass u me th at over

a p eriod of t ime, the organism ha d th rough na tu ral selection, evolved

ada ptations to optimise i ts su rvival an d reprodu ction in i ts hab itat .

1 3 .1 .1 Major Abiotic Fact ors

T e m p e r a t u r e : T e m p e r a t u r e i s t h e m o s t e c o l o g i c a l l y r e l e v a n t

environm enta l factor. You ar e aware that th e average temp eratu re onlan d varies s eas ona lly, decreases progressively from th e equa tor toward s

the poles an d from plains to the m oun tain tops. It ran ges from su bzero

levels in p olar a reas an d h igh alt itu des to >50 0C in tropical deserts in

su mm er. There are , however, un ique h abi ta ts su ch a s th ermal spr ings

and deep-sea hydrothermal vents where average temperatures exceed

100 0 C. It is general knowledge th at m an go trees do not an d can not grow

in temperate coun tries l ike Cana da a nd Germany, sn ow leopards are n ot

found in Kerala forests and tuna fish are rarely caught beyond tropical

Figure 13 .2 Major biomes of India : (a) Tropical rain forest; (b) Deciduous forest;

(c) Desert; (d) Sea coast

(a) (b)

(c) (d)


6/24

222

BIOLOGY

latitu des in th e ocean. You can readily app reciate th e significan ce of

temp eratu re to living organ ism s wh en you realise th at it affects th e kinetics

of enzymes and through i t the basal metabolism, activity and other

ph ysiological fu n ction s of th e organ ism. A few organ isms can tolerat e an dth rive in a wide ran ge of temp eratu res (they are called eurythermal), bu t, a

vast m ajority of th em a re restricted to a n arrow ran ge of tempera tu res (su ch

organisms are called stenothermal ). The levels of thermal tolerance of

different sp ecies d etermine to a large extent th eir geograp hical distribution.

Can you think of a few eurythermal and stenothermal animals and

plants?

In recent years, th ere has been a growing concern ab out th e grad u ally

increasing average global temperatures (Chapter 16) . If this trend

continues , w ould y ou expect the d is tributional ran ge of some s pecies to

be affected?

Water: Next to temp eratu re, water is the m ost importa n t factor in fluen cin gthe life of organisms. In fact, life on earth originated in water and is

u ns u sta ina ble with out water. Its availability is so l imited in des erts th at

only special ad ap ta tion s m ak e it poss ible to live ther e. Th e produ ctivity

an d distribu tion of plan ts is also heavily depen den t on water. You m igh t

th ink tha t organ ism s l iving in ocean s, lakes an d rivers sh ould not face

an y water-related problems , but i t is n ot true. For aqu atic organ ism s th e

qu ality (chem ical composition , pH) of water b ecomes imp ortan t. Th e sa lt

concentra t ion (measu red as sa l ini ty in par ts per thou san d), is less th an

5 in inland waters, 30-35 the sea a nd > 100 per cent in s ome hypersaline

lagoons. Some organism s a re toleran t of a wide ran ge of sa linit ies

(euryh aline) but others are res tricted to a na rrow ran ge (sten oha line).Many fresh water an imals can n ot live for long in sea wa ter an d vice versa

becau se of the osm otic problems, th ey would face.

Light: Since plants produ ce food th rough photosynthesis, a process which

is only pos sible wh en s u n light is availab le as a s ou rce of ener gy, we can

quick ly unders t and the impor tance of l igh t fo r l iv ing organ i sms,

par ticu larly au totrophs . Man y sp ecies of sm all plants (herb s an d sh ru bs)

growing in forests a re ada pted to ph otosynt hes ise optimally un der very

low light cond it ions becau se th ey are cons tan tly oversh adowed by tal l,

canopied trees. Many plan ts a re also dependent on su nlight to meet th eir

ph otoperiodic requ iremen t for flowerin g. For m an y an imals too, light is

importan t in th at th ey use th e diu rna l and seasona l var ia t ions in lightintens ity an d d u ration (ph otoperiod) as cues for t iming their foraging,

repr odu ctive an d m igrat ory activities. The a vailability of ligh t on land is

closely lin ked with tha t of tempera tu re since the s u n is th e sou rce for

both. Bu t, deep (>500m ) in t he oceans , the environm ent is perpetu ally

dark an d its inha bitants are n ot aware of the existence of a celestial sou rce

of ener gy called Su n. Wh at, then is their s ource of energy? ). Th e sp ectral

qu ality of solar ra diation is also imp ortan t for life. Th e UV compon ent of

the spectrum is harmful to many organisms while not al l the colour

compon ents of th e visible spectru m are a vailab le for ma rine plan ts living


7/24

223


at d ifferent d epths of the ocean . Am ong the red , green a nd brow n algae

that inh abit the se a, w hich is lik ely to be found in the deep es t w aters?

W h y ?

Soil: The nature and properties of soil in different places vary; it isdependent on the c l imate , the weather ing process, whether soi l i s

tra ns ported or sedimen tar y an d h ow soil developm ent occur red. Various

characterist ics of the soil such as soil composit ion, grain size and

aggregation d etermine th e percolation a nd water h olding capa city of the

soils. Thes e chara cterist ics along with p ara meters s u ch as pH, mineral

composition a n d topograph y determine to a large extent th e vegetation in

an y area. This is in tu rn dictates the type of an ima ls th at can be su pported.

Similarly, in th e aqu atic environm ent, th e sedimen t-char acteristics often

determine the type of benth ic an ima ls th at can thrive there.

13 .1.2 Response s to Abiot ic Factors

Having realised th at t he a biotic condit ions of ma ny h abitats ma y vary

drastically in time, we now askhow do the organ is m s living in su ch

habitats cope or man age w ith s tress ful conditions ? Bu t before at tempting

to ans wer th is qu estion, we shou ld perh aps ask first wh y a highly variable

external environm ent sh ould bother organism s after all. One wou ld expect

th at d u ring the cou rs e of million s of years of th eir exist ence, ma n y species

would have evolved a relat ively constant internal (within the body)

environm ent tha t perm its al l biochem ical reactions an d p hysiological

f u n c t i o n s t o p r o c e e d w i t h m a x i m a l

efficiency and thus, enhance the overall

fitnes s of th e sp ecies. Th is cons ta n cy, forexample , could be in terms of opt imal

temperatu re an d osm otic concentra tion of

body fluids. Ideally then, the organism

sh ould try to ma intain th e cons tan cy of its

internal environment (a process ca l led

homeos tas i s ) desp i t e va ry ing ex te rna l

envi ronmenta l condi t ions tha t t end to

u pset it s homeostasis . Let u s take an

an alogy to clar ify this im porta n t concept.

Su ppose a p erson is able to perform h is/

her best when th e temperature is 25 0C and

wish es to m aintain i t so, even wh en i t is

scorch ingly hot or freezingly cold ou tside.

It could be ach ieved a t h ome, in th e car wh ile travelling, an d a t workplace

by using an a ir condit ioner in s u mm er an d hea ter in winter. Then his/

her performance would be always maximal regardless of the weather

aroun d h im/ her . Here the persons h omeostas is i s a ccomplish ed, not

th rough phys io log ica l , bu t a r t i f i c i a l means . How d o other living

organism s cope w ith the s ituation ? Let us look at various possibilities

(Figu re 13 .3).

Figure 13 .3 D i a g r a m m a t i c r e p r e s e n t a t i o n o f

organi smic r esponse


8/24

224

BIOLOGY

(i) R eg u l a t e : Some organisms are able to mainta in homeostasis by

ph ysiological (sometimes beha viou ral also) mea ns which ens u res

cons tan t body temperatu re , cons tan t osm ot ic concentra t ion, e tc .

All birds and mammals, and a very few lower vertebrate andinver t ebra te spec ies a re indeed capable o f such regula t ion

(th erm oregulation an d osm oregulation). Evolu tionar y biologists

believe th at t h e su ccess of ma mm als is largely du e to th eir a bility

to mainta in a constan t body temperatu re and thr ive whether th ey

live in An tar ctica or in th e Sah ara des ert.

The m echa nisms u sed by most m amm als to regulate their body

temperatu re are similar to the ones tha t we hu ma ns u se. We maintain

a consta nt body temperatu re of 37 0C. In su mm er, when outs ide

temperatu re is m ore than our b ody temperatu re, we sweat profu sely.

Th e resu ltin g evapora tive cooling, sim ilar to wha t ha pp ens with a

desert cooler in operation, b rings down th e body temperatu re. In

winter when th e temperature i s mu ch lower tha n 37 0C, we star t to

sh iver, a k ind of exercise which produ ces hea t an d ra ises the b ody

t e m p e r a t u r e . P la n t s , on t h e o t h e r h a n d , d o n o t h a v e su c h

mecha nisms to maintain intern al temperatu res.

(ii) Conform : An overwhelmin g ma jority (99 per cen t) of an imals a n d

nearly all plan ts can not m aintain a consta nt intern al environmen t.

Their body temperatu re chan ges with the am bient temperatu re. In

aquatic animals, the osmotic concentration of the body fluids

cha nge with tha t of the a mb ient water osm otic concentration. These

an ima ls a nd plants are s imp ly conformers . Cons idering th e benefits

of a consta nt interna l environmen t to the organism , we mus t as k

why th ese conform ers h ad n ot evolved to b ecome regulators. Recall

the h um an an alogy we us ed above; mu ch as they like , how man y

people can really afford a n air cond itioner? Many s imply sweat it

out an d resign them selves to su bopt ima l performan ce in hot

su mm er m onth s. Therm oregu lation is en ergetically expensive for

ma ny organism s. This is pa rt icu larly true for sm all an ima ls l ike

sh rews and h u mm ing birds . Heat loss or heat gain is a fu nction of

su rface area. Since sm all an ima ls ha ve a larger su rface area relative

to th eir volu me, th ey tend to lose b ody heat very fas t when it is cold

outs ide; then th ey ha ve to expend m u ch en ergy to generate bodyh eat thr ough m etabolism . This is the ma in rea son why very sm all

animals are rarely found in polar regions. During the course of

evolu tion, th e costs an d ben efits of ma intaining a consta nt intern al

environm ent are ta ken int o consideration. Som e sp ecies h ave evolved

th e ability to regulate, bu t only over a limited ra nge of environm enta l

condit ions , beyond which they s imply conform.

If th e stres sful extern al conditions are localised or rem ain on ly

for a sh ort du ration, the organ ism ha s two other a lterna tives.


9/24

225


(iii) Migra te : The organism can move away temporari ly from the

stressful habitat to a m ore hospitable area an d return when stress ful

period is over. In human analogy, this strategy is l ike a person

moving from Delhi to Sh imla for th e du ration of sum mer. Man yan imals, particularly birds, du ring win ter un derta ke long-dista n ce

migrations to more hospitable areas. Every winter the famous

Keolado Nationa l Park (Bha rtpu r) in Rajasth an hos t thou sa nd s of

migratory birds coming from Siberia and other extremely cold

north ern regions.

(iv) Suspend:In bacteria, fu n gi an d lower plant s, variou s k inds of th ick-

walled sp ores are form ed which h elp th em to s u rvive un favoura ble

condition s thes e germinat e on a vailab ility of su itable environm ent.

In higher plants, seeds and some other vegetative reproductive

stru ctures serve as mean s to tide over periods of stress besides helping

in d isp ersa l th ey germinate to form n ew plant s u n der favoura blemoistu re an d temp eratu re cond itions . They do so by reducing their

m etab olic activity an d going into a da te of dorm an cy.

In an ima ls, the organism , if u na ble to migrate, might avoid th e

stress by escaping in time. The familiar case of bears going into

hibernation du ring wint er is an example of escape in t ime. Some

sna i l s and f i sh go in to aestivation t o avo id summerre la t ed

problems-heat and desiccation. Under unfavourable condit ions

man y zooplan kton sp ecies in lakes a nd ponds are kn own to enter

diapause , a sta ge of su spen ded developm ent.

13 .1 .3 Adaptat ionsWhile considering the various alternatives available to organisms for

copin g with extrem es in their environm ent, we have seen tha t some ar e

able to respond through certain physiological adjustments while others

do so behaviourally (migrating temporarily to a less stressful habitat).

Thes e respons es are a lso actu ally, their adap tations . So, we can say th at

adaptation is any attribute of the organism (morphological, physiological,

behavioural) that enables the organism to survive and reproduce in its

habitat. Many adaptations have evolved over a long evolutionary time

an d a re genetically fixed. In th e ab sen ce of an externa l sour ce of water,

th e kan garoo rat in North American deserts is capa ble of meeting all its

water requiremen ts th rou gh its in terna l fat oxida tion (in which water isa by produ ct). It also ha s th e ability to concent rate its u rine so th at

minimal volume of water is used to remove excretory products.

Man y desert plants ha ve a th ick cu ticle on th eir leaf sur faces an d

ha ve their stomata arra nged in d eep pits to m inimise water loss thr ough

tran spiration. They also ha ve a s pecial photosynth etic path way (CAM)

tha t enab les their stoma ta to rema in closed du ring day time. Some desert

plants like Opuntia , have no leaves they are reduced to spines an d th e

ph otosynth etic fu nction is taken over by the flat ten ed stem s.


10/24

226

BIOLOGY

Mamm als from colder climates gener ally have shorter ear s an d limb s

to min imise hea t loss . (Th is is called the Allens Ru le .) In the polar sea s

aqu atic ma mm als like s eals h ave a th ick layer of fat (blub ber) below their

skin tha t ac ts as a n insu la tor and redu ces loss of body heat .Some organisms possess adapta t ions that are physiological which

allow them torespon d qu ickly to a st ressful situ ation. If you ha d ever

been to an y high a lt itu de place (>3,500m Rohta ng Pass nea r Manali an d

Mans arovar, in China occu pied Tibet) you mu st h ave experien ced what

is called altitude sickness . It s symptoms includ e nau sea , fa t igu e and

hea rt palpitat ions . This is becau se in th e low atm osph eric pressu re of

high a lt itu des, th e body does n ot get en ough oxygen. But, grad u ally you

get acclima tised a nd stop experiencing alt itu de sicknes s. How did your

body s olve this problem? The body comp en sa tes low oxygen a vailab ility

by in creas ing red blood cell prod u ction , decreas ing the bind ing affinity

of hem oglobin an d by increasing breath ing ra te. Man y tribes live in th ehigh altitud e of Him alaya s. Find out if they norma lly ha ve a higher red

blood cell count (or total hemoglobin) than people living in the plains.

I n m o s t a n i m a l s , t h e m e t a b o l i c r e a c t i o n s a n d h e n c e a l l t h e

ph ysiological fu nctions proceed optima lly in a n arr ow temp eratu re ran ge

( in humans, i t i s 37 0C). But there are microbes (archaebacteria) that

f l o u r i sh i n h o t sp r i n g s a n d d e e p se a h y d r o t h e r m a l v e n t s w h e r e

temp eratu res far exceed 100 0C. How is th is poss ible?

Many fish th rive in Ant arctic waters where th e temper atu re is a lways

below zero. How do they m an age to keep their body fluids from freez ing ?

A large variety of m arine in vertebra tes a n d fish live at great d epth s in

the ocean where the pressu re cou ld be >100 times th e normal atmosph ericpressure that we exper ience . How do they live un der su ch crus hing

pressures and do they have any special enzymes ? Organ isms livin g in

such extreme environments show a fascinating array of biochemical

adapta t ions.

Some organisms sh ow behavioura l responses to cope with variat ions

in their environment. Desert lizards lack the physiological ability that

ma mm als have to deal with th e high temperatu res of their h abi ta t , but

ma na ge to keep their body temperatu re fairly cons tan t by behavioura l

m e a n s . Th e y b a sk i n t h e su n a n d a b so r b h e a t w h e n t h e ir b od y

temp eratu re drops below the comfort zone, bu t move into sh ade when

th e ambient tempera tu re starts increas ing. Some species are capab le ofbu rrowing into the soil to hide and escape from th e above-groun d h eat .

1 3 . 2 POPULATIONS

1 3.2 .1 Populat ion Attributes

In na tu re, we ra rely find isolated, sin gle in dividu als of an y species; majority

of th em live in groups in a well defined geograp hical area, s ha re or comp ete

for s imi lar resources, potent ia l ly interbreed and thus const i tute a


11/24

227


popu lation. Alth ough the term interb reeding implies s exual reprodu ction,

a group of ind ividu als resu lt ing from even a sexua l reproduction is also

genera lly cons idered a popu lation for the pu rpos e of ecological stu dies.

All the cormoran ts in a wetland, ra ts in an aba nd oned d welling, teakwoodtrees in a forest tract , bacteria in a cultu re plate an d lotu s plan ts in a

pond , are some exam ples of a popu lation. In earl ier chapt ers you h ave

learnt th at a l though a n individu al organism is the one th at h as to cope

with a changed environment, i t is at the population level that natural

selection operates to evolve the desired traits. Population ecology is,

therefore, an important area of ecology because i t l inks ecology to

popu lation genetics an d evolu tion.

A population ha s certain attribu tes th at a n individu al organism d oes

not. An individu al may have births an d death s, bu t a popu lation h as birth

rates an d death rates . In a p opulation th ese rates refer toper cap ita births

an d death s, respectively. The ra tes, hen ce, are expressed is chan ge in nu mbers(increas e or decrease) with respect to m embers of the popu lation. Here is an

example. If in a pond there are 20 lotus plants last year and through

reproduction 8 n ew plants a re ad ded, taking the cur rent popu lation to 28,

we calculate the b irth rat e as 8 / 20 = 0.4 offsp ring per lotus per year. If 4

individu als in a labora tory popu lation of 40 fru itflies d ied du ring a s pecified

time interval, say a week, the death rate in th e popu lation du ring that p eriod

is 4 / 40 = 0.1 ind ividu als per fruitfly per week.

Another at tribute characterist ic of a population is sex ratio. An

ind ividu al is ei ther a m ale or a fema le bu t a popu lation ha s a sex rat io

(e.g. , 60 p er cent of the popu lation ar e fema les a nd 40 per cent ma les).

A popu lation at an y given time is com posed of individua ls of different

ages. If the age distribution (per cent individuals of a given age or age

grou p) is plotted for th e popu lation, the resu lt ing stru ctur e is called an

age pyramid (Figure 13.4). For human population, the age pyramids

generally show age distribution of males and females in a combined

diagram. The shape of the pyramids reflects the growth status of the

pop u lation - (a) wh eth er it is growin g, (b) sta ble or (c) declinin g.

Figure 13.4 Representat ion of age pyramids for human populat ion

The s ize of the p opulation tells u s a lot abou t i ts statu s in th e ha bitat .

Wh at ever ecological process es we wish to in vestigate in a popu lation , be

it the outcome of competi t ion with another species, the impact of a


12/24

228

BIOLOGY

pred ator or th e effect of a p esticide a pplication, we always evalua te th em

in terms of an y chan ge in the p opu lation size. The s ize, in n atu re, cou ld

be as low as


13/24

229


So, if N is th e popu lation den sity at time t, th en its d ens ity at t ime t +1 is

Nt+1

= Nt+ [(B + I) (D + E )]

You can see from th e above equation th at popu lation d ens ity will

increas e if the n u mb er of births plus th e nu mb er of imm igran ts (B + I) is

more than the n u mber of deaths plu s th e nu mber of emigrants (D + E),

otherwise i t will decrease. Under n orma l condit ions , birth s a nd death s

are th e most importa nt factors influ encing population den sity, the other

two factors as su ming imp ortan ce only u nd er special condit ions . For

instance, if a new habitat is just being colonised, immigration may

contribute m ore significan tly to popu lation growth tha n birth ra tes.

Growth Mode ls : Does th e growth of a p opu lation with t ime s how an y

specific and p redictable pa ttern ? We have been concern ed abou t un bridled

hu ma n p opula t ion growth an d problems created by it in our coun try

and i t i s therefore natura l for us to be cur ious i f di f ferent animal

popula t ions in n atu re behave the sam e way or show some rest ra ints on

growth. Perhaps we can learn a lesson or two from nature on how tocontrol population growth .

(i) Exponent ia l g rowt h : Resource (food and space) availability is

obviously essential for the unimpeded growth of a population.

Ideally, when resou rces in th e ha bitat are u nlimited, each s pecies

h as th e ability to realise fu lly its inn ate p otent ial to grow in nu m ber,

as Darwin obs erved while developing h is th eory of n atu ral selection.

Th en th e popu lation grows in a n expon ent ial or geometric fas h ion.

If in a p opu lation of size N, the b irth rates (n ot total nu mb er bu t


14/24

230

BIOLOGY

per capita births) are represen ted as b an d death ra tes (again, per

capita death rates) as d, then t he increas e or decrease in N du ring a

u n it time period t (dN/ dt) will be

dN/ dt = (b d) N

Let (bd) = r, th en

dN/ d t = rN

Th e r in th is equ ation is called th e intr insic rate of na tu ral increase

an d is a very imp ortant p ara meter chosen for ass essing imp acts of

an y biotic or ab iotic factor on popu lation growth .

To give you some idea abou t th e ma gnitude of r valu es, for th e

Norway rat th e r is 0.0 15 , an d for the flou r beetle it is 0.12. In

1981, th e r valu e for hu man popula t ion in India was 0.0205. Find

out w ha t the current r value is. For calculating it, y ou need to

kn ow the birth rates a nd death rates.

The ab ove equ ation des cribes the exponen tial or geometric growth

pattern of a popu lation (Figure 13.5) an d resu lts in a J -sha ped cu rve

when we plot N in relation to time. If you are familiar with basic

calculus, you can derive the integral form of the

exponential growth equa tion a s

Nt= N

0e rt

where

Nt= Popu lation d en sity after time t

N0

= Popu lation d ens ity at time zero

r = intrins ic rate of na tu ral increas e

e = the bas e of na tu ral logari thms (2.71828 )

Any s pecies growin g expon ent ially u n der u nlimited

resour ce conditions can reach enorm ous p opulation

den sities in a sh ort time. Darwin sh owed h ow even

a slow growing animal like elephant could reach

enormous nu mbers in th e absence of checks. The

following is an anecdote popularly narrated to

d e m o n s t r a t e d r a m a t i c a l l y h o w f a s t a h u g e

p o p u l a t i o n c o u l d b u i l d u p w h e n g r o w i n g

exponentially.

The king and the minister sat for a chess game. The king, confident

of w inning the gam e, w as ready to accept any bet proposed by the

m inister. The m inister hu m bly sa id tha t if he w on, he w an ted only

som e w heat grains , the quan tity of w hich is to be calculated by placing

on the chess board one grain in S quare 1, then tw o in S quare 2,

then four in Square 3, and eight in Square 4, and so on, doubling each

time the previous quan tity of w hea t on the next s quare un til all the 64

squares w ere filled. The king accepted the s eem ingly silly bet and s tarted

the gam e, but un luckily for him, the m inisterw on. The k ing felt that fulfilling

Figure 13 .5 Populat ion growth curve

a w h e n r e s p o n s e s a r e n o t

l imi t ing the growth , p lo t i s

exponent i a l ,

b when respons es a re limi t ing

the growth, plot is logistic,

K is carrying capacity


15/24

231


the ministers bet w as so eas y. He s tarted w ith a s ingle grain on

the firs t squ are an d p roceeded to fill the other squ ares follow ing

ministers suggested procedure, but by the time he covered half the

chess board, the k ing real ised to his dism ay that al l the w heat produced in h is ent ire k ingd om pooled together w ould s t il l be

in a d e q u a t e t o co v e r a l l t h e 6 4 s q u a re s . N o w t h i n k o f a t in y

Param ecium s tarting w ith jus t one individu al and th rough bina ry

f iss ion, doubling in n um bers every da y, an d ima gine w hat a mind-

boggling population s iz e it w ould reach in 64 d ay s . (provide d food

and spa ce rema in unlimited)

(ii) Logi s t i c g rowth: No popu lation of an y species in n atu re ha s i ts

disposal u nlimited resou rces to p ermit exponential growth. This

leads to competition between individuals for limited resources.

Eventu ally, the fittest individu al will su rvive and repr odu ce. Th e

governments of many countries have also realised this fact and

introdu ced variou s r estraints with a view to limit hu ma n popu lation

growth. In na ture, a given h abitat has enough resou rces to sup port

a ma ximu m poss ible nu mb er, beyond wh ich n o fu rth er growth is

poss ible. Let u s ca ll th is limit as n at u res carry ing cap acity (K) for

tha t species in th at ha bitat .

A popu lation growing in a h abitat with l imited resou rces s how

init ial ly a lag phase, followed by phases of acceleration and

deceleration a nd finally an a sympt ote, when t he popu lation den sity

reaches the carrying capacity. A plot of N in relation to time (t)

resu lts in a s igmoid cu rve. Th is type of popu lation growth is calledVerhu lst-Pea rl Logis tic Grow th (Figure 13.5) and is described by

th e followin g equ at ion:

d N/ d t =K N

rNK

Where N = Popula t ion dens ity a t t ime t

r = Intrins ic rate of na tu ral increas e

K = Carr yin g capa city

Since resou rces for growth for mos t an imal popu lations are finite

an d becom e limitin g sooner or later, th e logistic growth m odel is

cons idered a more rea listic one.Gather from Government Census data the population figures

for Ind ia for the last 1 00 ye ars, plot them an d check w hich grow th

pattern is evident.

1 3 .2 .3 Life History Variation

Popu lation s evolve to m aximise their reprod u ctive fitnes s, also called

Darwinian fitn ess (high r value), in th e ha bitat in which th ey live. Under

a p articular set of selection pr essu res, organisms evolve toward s th e most


16/24

232

BIOLOGY

efficien t repr odu ctive stra tegy. Some organ isms breed on ly once in th eir

lifetime (Pacific salmon fish, bamboo) while others breed many times

du ring their lifet ime (most birds a nd ma mm als). Some produ ce a large

number of small-sized offspring (Oysters, pelagic fishes) while othersprod u ce a sm all n u m ber of large-sized offsp ring (birds , ma m m als). So,

which is desirab le for ma ximisin g fitnes s? Ecologist s su ggest th at life

history trai ts of organism s h ave evolved in r elat ion to th e constr aints

imposed by th e abiotic an d biotic comp onen ts of the h abitat in which

th ey live. Evolu tion of life h istory tra its in differen t s pecies is cu rren tly an

importan t ar ea of research bein g cond u cted by ecologists.

1 3.2 .4 Populat ion Inte ract ions

Can you th ink of an y natu ra l habi ta t on ear th th at i s inh abi ted just by a

single species? There is no su ch ha bitat and s u ch a si tua tion is even

inconceivable. For any species, the minimal requirement is one more

species on which i t can feed. Even a plant s pecies, which m akes its own

food, cann ot su rvive alone; it n eeds s oil microbes t o break down th e organic

ma tter in s oil an d retu rn th e inorgan ic nu trients for absorption. And then ,

how will th e plan t ma na ge pollination with out a n a nima l agent? It is

obvious th at in na ture , animals , plants and microbes do not an d cann ot

l ive in i sola t ion but interact in var ious ways to form a biological

commu nity. Even in minima l commu nities, man y interactive link ages

exist , al thou gh all ma y not be readily appar ent.

In ters pecific intera ction s a rise from th e int eraction of popu lation s of

two different species. They could be beneficial, detrimental or neutral

(n eith er h ar m n or ben efit) to one of th e sp ecies or both . Ass ign ing a +

sign for ben eficial intera ction , - sign for detrimenta l an d 0 for n eutr al

interaction, let us look at al l the possible outcomes of interspecific

interactions (Table13.1).

Both th e sp ecies b enefit in mutual i sm an d both lose in competi t ion in

th eir interactions with each oth er. In b oth parasi t i sm an dPredation only

one species ben efits (pa ra site an d pr edat or, resp ectively) an d th e in teraction

Spe c ie s A Spe c ie s B Nam e of In te rac t ion

+ + Mu tu a lis m

Com petition

+ Pred a tion

+ Pa ra s itis m

+ 0 Com m en s a lis m

0 Am en s a lis m

Table 13.1 : Population Interactions


17/24

233


is d e t r im e n t a l t o t h e ot h e r s p e c ie s (h o s t a n d p r e y , r e s p e c t i ve l y).

Th e int eraction wh ere one s pecies is benefi t ted an d th e oth er is n either

benefi t ted nor harmed is called commensa l i sm . In amensa l i sm on

t h e o t h e r h a n d o n e s p e c i e s i s h a r m e d w h e r e a s t h e o t h e r i su na ffected. Predation, paras it ism a nd comm ens alism s sh are a comm on

ch ar act eristic th e in tera cting species live closely togeth er.

(i) Pre d a t io n : W h a t w o u ld h a p p e n t o a ll t h e e n e rg y f ix e d b y

au totrophic organism s if the com m un ity h as no anima ls to eat the

plants? You can th ink of pred ation as n atu res way of tra n sferrin g

to higher troph ic levels th e energy fixed by plant s. Wh en we thin k

of predator an d pr ey, most p robably it is the t iger an d th e deer tha t

readily come to our mind , bu t a s par row eating an y seed is n o less

a predator . Al thou gh an imals ea t ing plan ts are ca tegorised

separately as herbivores , they are, in a broad ecological context,

n ot very different from pred ators .

Besides acting as condu its for energy tran sfer across troph ic

levels, pred ators play other importa nt roles. Th ey keep prey

popu lations u nd er control. But for preda tors, prey species could

achieve very high popula t ion densi t ies and cause ecosystem

instabil i ty. When certain exotic species are introduced into a

geograp hical area, they become invas ive an d s tart s pread ing fas t

becau se the invaded land does n ot have its n atu ral predators. The

prickly pear ca ctus int rodu ced into Au stra l ia in the early 1920s

caused havoc by spreading rapidly into mill ions of hectares of

ran gelan d. Fina lly, the invasive cactus was brought u nd er controlonly after a cactu s-feeding predat or (a m oth) from its n atu ral ha bitat

was introduced into the coun try.Biological con trol methods a dopted

in agricultur al pest contr ol are b as ed on the ability of the p redator

to regulate prey population. Predators also help in maintaining

species diversi ty in a community, by reducing the intensity of

competition am ong comp eting prey sp ecies. In th e rocky intert idal

comm u n ities of th e Am erican Pacific Coas t th e sta rfish Pisasteris

an imp ortan t preda tor. In a field experiment, when all th e sta rfish

were removed from a n en closed intertidal area, m ore tha n 1 0 sp ecies

of invertebrates becam e extinct with in a year, becau se of inter -

sp ecific comp etition.If a p reda tor is too efficien t an d overexploits its prey, th en t h e

prey m igh t b ecome extinct a n d followin g it, th e pr eda tor will also

become extinct for lack of food. Th is is th e reas on wh y preda tors in

na tu re ar e pru den t. Prey sp ecies h ave evolved various defens es to

lessen th e imp act of preda tion. Some sp ecies of ins ects an d frogs

ar e cryptically-colou red (camouflaged) to avoid being det ected ea sily

by the preda tor. Some are poisonou s a nd therefore avoided by the

preda tors. The Monarch bu tterfly is h ighly distas tefu l to its pr edator


18/24

234

BIOLOGY

( b i r d ) b e c a u se o f a sp e c i a l c h e m i c a l p r e se n t i n i t s b o d y .

Interest ingly, the but terf ly acquires this chemical dur ing i t s

cater pillar sta ge by feeding on a poison ou s weed.

For plan ts, h erbivores are th e predat ors. Nearly 25 p er cent ofal l insects are known to be phytophagous (feeding on plant sap

an d oth er par ts of plan ts). The p roblem is p art icularly severe for

plants b ecau se , un like an ima ls , they cann ot run away from th eir

preda tors. Plan ts therefore ha ve evolved a n as tonishing variety of

morp hological an d ch emical defences against herb ivores. Thorn s

(Acacia , Cactus ) are the most common morphological means of

defence . Man y plan ts produ ce and store chemicals th at m ake th e

herbivore sick when they are eaten, inhibit feeding or digestion,

disru pt i ts reprodu ction or even kill it . You m u st h ave seen th e

weed Calotropis growin g in aba n doned fields. The plan t produ ces

highly poison ous cardiac glycosides an d th at is wh y you never seean y cattle or goats b rowsin g on th is p lant. A wide variety of chem ical

substances that we ext ract f rom plants on a commercia l sca le

(n icotin e, caffeine, qu inine, str ychn ine, opium , etc.,) are p rodu ced

by them a ctua lly as defences against grazers an d browsers.

(ii) Compet i t ion :Wh en Da rwin sp oke of th e stru ggle for existen ce an d

su rvival of th e fittest in n atu re, he was convinced t ha t inters pecific

competition is a potent force in organic evolution. It is generally

believed that competi t ion occurs when closely related species

compete for the sa me resou rces tha t are l imiting, bu t this is not

en tirely tru e. Firs tly, totally un related s pecies could also compete

for the same resource . For instance , in some shal low SouthAmerican lakes visiting flam ingoes a nd resident fishes compete for

the i r common food , the zooplankton in the l ake . Secondly ,

re sources need no t be l imi t ing for compe t i t ion to occur ; in

interference com petition , th e feeding efficiency of one sp ecies m igh t

be redu ced du e to the interfering an d inh ibitory presence of th e

other species, even if resources (food and space) are abundant.

Therefore, competit ion is best defined as a p rocess in which the

fitness of one s pecies (meas u red in t erm s of its r th e in trins ic ra te

of increa se) is significant ly lower in th e pres ence of an other sp ecies.

It is relat ively easy to dem ons trate in labora tory experiments , as

Gau se an d other experimen tal ecologists d id, when resou rces arelim ited th e comp etitively su per ior sp ecies will eventu ally elim ina te

the other species, but evidence for such competitive exclusion

occu rring in na tu re is n ot always conclusive. Strong and persu asive

circumstantial evidence does exist however in some cases. The

Abingdon tortoise in Galapagos Islan ds becam e extinct with in a

decade after goats were int rodu ced on the islan d, appa rently du e

to th e greater brows ing efficien cy of th e goats . An oth er eviden ce for

th e occu rrence of competit ion in na tu re comes from what is called


19/24

235


comp etitive releas e. A sp ecies whos e distr ibu tion is res tricted t o a

sm all geograph ical area b ecau se of th e pres ence of a comp etitively

super ior species, i s found to expand i t s dis t r ibut ional range

dra ma tically when th e competing sp ecies is experimen tally removed.Conn ells elegan t field experimen ts s h owed tha t on th e rocky sea

coasts of Scotlan d, th e larger an d comp etit ively su perior b arn acle

Balanus domina tes th e int ert idal area, an d exclu des th e sma ller

barnacle Chathamalus from th at zone. In general , herbivores an d

plants app ear to b e m ore adversely affected by competi t ion th an

carnivores.

Gau se s Competi t ive Exclusion Principle s t a t e s t h a t t w o

closely related s pecies comp eting for th e sa me res ources can not

co-exist indefinitely and the competitively inferior one will be

eliminated eventually. This may be true if resources are limiting,

bu t n ot otherwise . More recent s tu dies do n ot su pport su ch gross

genera lisa tions abou t competi t ion. While they do not ru le out th e

occu rrence of int erspecific comp etit ion in na tu re, th ey poin t ou t

that species facing competi t ion might evolve mechanisms that

promote co-existence rath er tha n exclu sion. One su ch m echan ism

is res ou rce pa rtitioning. If two species com pete for the s am e

resou rce, they cou ld avoid compet ition by choosing, for in sta n ce,

differen t times for feeding or differen t fora gin g pa ttern s. MacArth u r

sh owed th at five closely related sp ecies of war blers living on t h e

same tree were able to avoid competi t ion and co-exist due to

beh aviour al differen ces in th eir fora gin g activities.(iii) P a r a s i t i s m :Considering th at t he p ara sit ic mode of life ens u res

free lodging and meals, i t is not surprising that parasit ism has

evolved in so many taxonomic groups f rom plants to higher

vertebr ates . Many par as ites ha ve evolved to be host -sp ecific (th ey

can pa ras itise only a s in gle species of hos t) in s u ch a way th at both

h ost a n d th e par as ite ten d to co-evolve; tha t is, if th e hos t evolves

special mechanisms for rejecting or resist ing the parasite, the

paras ite ha s to evolve mechan ism s to cou nteract an d n eutra l ise

them, in order to be successful with the same host species. In

accordance wi th the i r l i f e s ty le s , pa ras i t e s evo lved spec ia l

adaptations su ch as the loss of un necessary sens e organs , presence

of ad hes ive organs or su ckers to cling on to th e hos t, loss of digestive

syst em a n d h igh repr odu ctive capa city. Th e life cycles of para sites

are often complex, in volving one or two in term ediate h osts or vectors

to facilitate pa ras itisa tion of its prima ry host . Th e hu m an liver fluke

(a trem atode pa ras ite) depend s on t wo intermediate host s (a s na il

an d a fish ) to complete its life cycle. Th e ma larial pa ras ite n eeds a

vector (m osqu ito) to spr ead to other h osts . Majority of th e para sites


20/24

236

BIOLOGY

h a r m t h e h o s t ; t h e y m a y r e d u c e t h e su r v i v a l , g r o w t h a n d

reprodu ction of th e host a nd redu ce its popu lation den sity. Th ey

might rend er the h ost m ore vu lnera ble to predation by mak ing it

physically weak. Do y ou believe tha t an idea l paras ite s hould be

able to thrive w ithin the hos t w ithout harm ing it? Then w hy didnt

natural selection lead to the evolution of such totally harmless

parasites ?

Paras ites th at feed on the externa l su rface of th e host organ ism

ar e called ectoparasites . Th e mos t fam iliar exam ples of th is grou p

are the l ice on hu man s a nd t icks on dogs. Man y marine fish are

infested with ectopara sitic copepods . Cuscuta , a paras itic plan t tha t

is common ly fou nd growing on h edge plan ts, h as lost its chloroph yll

an d leaves in th e cours e of evolu tion . It derives its n u trition from

the host plant which i t parasit ises. The female mosquito is notconsidered a pa ras ite, althou gh it needs our blood for reproduction.

Can you explain w hy?

In contras t , endoparasites are th ose tha t live ins ide th e host

body at different sites (liver, kidney, lungs, red blood cells, etc.).

Th e life cycles of en dopa ra sites a re m ore complex becau se of th eir

extreme s pecialisa tion . Their m orph ological an d a n atom ical featu res

ar e grea tly sim plified while emph as ising their reprodu ctive potent ial.

Brood parasitism in birds is a fas cina ting exam ple of par as itism

in which t he pa ras it ic bird lays i ts eggs in th e nest of its h ost an d

lets th e host incu bat e them . During the cour se of evolu tion, th e

eggs of th e pa ras itic bird h ave evolved t o resem ble the h osts egg in

size an d colour to redu ce the cha nces of the h ost bird detecting th e

foreign eggs and ejecting them from the nest. Try to follow the

movements of th e cuck oo (koel) an d th e crow in you r n eighborh ood

park du ring the b reeding season (spr ing to sum mer) an d watch

brood para sit ism in a ction.

(iv) Commensal i sm :Th is is th e in tera ction in wh ich on e species ben efits

an d th e other is n eith er ha rm ed nor ben efited. An orch id growing

a s a n epiphyte on a m an go bran ch, an d barn acles growing on the

back of a wh ale benefit while neither th e ma ngo tree nor th e whale

derives an y app aren t benefit. The cattle egret an d grazin g cattle in

close a ss ociation, a sight you ar e m ost likely to catch if you live in

farm ed rur al areas , is a clas sic example of commen sa lism . The

egrets always forage close to wh ere th e cattle are grazin g becau se

th e catt le, as they move, st ir u p a nd flu sh out from th e vegetation

ins ects th at oth erwise m igh t be difficu lt for th e egrets t o find a n d

catch . Anoth er example of comm ens alism is th e int eraction b etween

sea an emone tha t ha s s t inging tentacles an d th e c lown fish that


21/24

237


lives am ong them . The fish gets p rotection from p redators which

stay away from the st inging tentacles. The anemone does not

appear to derive any benefit by hosting the clown fish.

(v) Mutualism :This intera ction confers ben efits on both th e in teracting

species. Lichens represent an intimate mutualist ic relat ionship

between a fun gus a nd p hotosynth esising algae or cyanoba cteria.

Similarly, th e mycorrhizae are as sociations between fun gi an d the

roots of higher plan ts. Th e fu ngi help th e plan t in th e abs orption of

essen tial nu trients from th e soil while the plan t in tu rn provides th e

fu n gi with en ergy-yieldin g carb ohydr ates .

The most sp ectacular a nd evolu tiona ri ly fas cina ting exam ples

of mu tu alism are foun d in plant-animal relat ions hips. Plan ts n eed

th e help of an imals for pollina ting their flowers an d disp ers ing their

seed s. An imals obvious ly h ave to be paid fees for th e services th at

plant s expect from th em. Plan ts offer rewards or fees in th e form of

pollen an d n ectar for pollina tors a nd ju icy and nu trit ious fru its for

seed dispers ers. But th e mu tu ally beneficial system sh ould also

be s afegu arded against cheaters , for examp le, an ima ls tha t try to

steal n ectar with out aiding in poll ina tion. Now you can see why

p l a n t - a n i m a l i n t e r a c t i o n s o f t e n i n v o l v e co-evolution of themu tu alists , th at is, th e evolu tions of th e flower an d i ts p ollinator

sp ecies a re tigh tly lin ked with on e an oth er. In ma n y species of fig

trees, th ere is a t ight on e-to-one relat ionsh ip with th e pollinator

sp ecies of wasp (Figure 1 3.6). It m ean s th at a given fig species can

be pollinated only by its pa rtn er wasp sp ecies a nd n o other s pecies.

Th e fema le wasp u ses th e fru it n ot only as an oviposition (egg-laying)

si te but u ses t he developing seeds with in t he fruit for nou rishing

its lar vae. Th e wasp p ollin ates th e fig inflorescen ce wh ile sear ch ing

Figure 13 .6 Mutual relationship between fig tree and wasp: (a) Fig flower is pollinated

by wasp; (b) Wasp laying eggs in a fig fruit

(a ) (b )


22/24

238

BIOLOGY

for su itable egg-laying sites. In retu rn for th e favour of

pollinat ion th e fig offers th e was p s ome of its developing

seeds , as food for th e developing wasp larvae.

Orchids show a bewilder ing diversi ty of f lora lpattern s ma ny of which ha ve evolved to attr act th e right

poll inator insect (bees and bumblebees) and ensure

guaranteed poll ination by i t (Figure 13.7). Not al l

orchids offer rewards . The Mediterran ean orchid Ophrys

em ploys sexu al deceit to get pollina tion d on e by a

species of bee. One petal of its flower bears an u nca nn y

resem blance to th e fema le of th e bee in size, colour an d

ma rkings. The ma le bee is attr acted to wha t it perceives

as a fema le, ps eu docopu lates with th e flower, an d

during that process is dusted with pollen from the

flower. Wh en th is sa m e bee ps eu docopu lates withanother f lower , i t t ransfers pol len to i t and thus,

pollinat es th e flower. Here you can see how co-evolu tion

operates . If th e fema le bees colour pat tern s cha n ge even

sligh tly for an y rea son du ring evolu tion, pollina tion su ccess will be redu ced

unless the orchid flower co-evolves to maintain the resemblance of its

peta l to th e fema le bee.

Figure 13.7 Showing bee a poll inator

on orchid flower

SUMMARY

As a branch of biology, Ecology is the study of the relationships of

l ivin g organism s with th e a biotic (ph ysico-chemical factors ) an d b iotic

components (other species) of thei r environment . I t i s concerned

with four levels of biological organisation-organisms, populations,

communi t i es and b iomes .

T e m p e r a t u r e , l i g h t , w a t e r a n d s o i l a r e t h e m o s t i m p o r t a n t

phys ica l f ac tor s of the envi ronment to which the organi sms are

a d a p t e d i n v a r i o u s w a y s . M a i n t e n a n c e o f a c o n s t a n t i n t e r n a l

environment (homeostasis) by the organisms contr ibutes to opt imal

performance, but only some organisms ( regulators) are capable of

homeostasis in the face of changing external environment . Others

e i t h e r p a r t i a l l y r e g u l a t e t h e i r i n t e r n a l e n v i r o n m e n t o r s i m p l y

conform. A few other spec ies have evolved adapta t ions to avoid

unfavourable conditions in space (migration) or in t ime (aestivation,

h ibernat ion , and d iapause) .

Evolu t ionary changes through na tura l se l ec t ion t ake p lace a t

the population level and hence, population ecology is an important

area of ecology. A population is a group of individuals of a given

species shar ing or compet ing for s imi l ar r esources in a def ined

g e o g r a p h i c a l a r e a . P o p u l a t i o n s h a v e a t t r i b u t e s t h a t i n d i v i d u a l

organi sms do not - b i r th r a t es and dea th r a t es , sex r a t io and age


23/24

239


dist r ibut ion. The propor t ion of di f ferent age groups of males and

fema les in a popu lation is often p resen ted graph ically as age pyram id;

i t s shape indicates whether a populat ion is stat ionary, growing or

decl ining.

Ecological ef fects of any factors on a populat ion are general ly

reflected in i ts size (population density), which may be expressed in

dif ferent ways (numbers, biomass, per cent cover , etc. , ) depending

on the species.

Popula t ions grow through b i r ths and immigra t ion and dec l ine

throu gh deaths and emigrat ion. When resources are unl imi ted, the

g r o wt h i s u s u a l ly e x p o n e n t ia l b u t w h e n r e s o u r c e s b e c o m e

progressively l imi t ing, the growth pat tern turns logist ic . In ei ther

case, growth is ul t imately l imi ted by the car rying capaci ty of the

environment . The int r insic rate of natural increase (r) i s a measure

of the inherent potential of a population to grow.

In nature populations of different species in a habitat do not l ive

in i solat ion bu t interact in ma ny ways. Depending on th e ou tcome,

these interactions between two species are classif ied as competit ion

(both species suffer) , predation and parasit ism (one benefits and the

o t h e r s u f f e r s ) , c o m m e n s a l i s m ( o n e b e n e f i t s a n d t h e o t h e r i s

una f f ec t ed ) , am ensa l i sm ( one i s ha r m ed , o t he r una f f ec t ed ) and

mu tu al ism (both spec ies benef it ). Predat ion is a very impor tan t

process th rough which t rophic energy t ran sfer i s faci litated a nd some

predator s he lp in cont ro l l ing the i r prey popula t ions . P lant s have

e v o l v e d d i v e r s e m o r p h o l o g i c a l a n d c h e m i c a l d e f e n s e s a g a i n s t

h erbivory. In comp etit ion, it is presu med th at th e su perior competitor

elimina tes th e inferior on e (the Com petit ive Exclu sion Prin ciple), bu t

many closely related species have evolved various mechanisms whichfacil i tate their co-existence. Some of the most fascinating cases of

mutual ism in nature are seen in plant -pol l inator interact ions.

EXERCISES1. How is d iapau se d iffe rent from hiberna t ion?

2. If a m ar ine fish is p laced in a fresh water aqua r ium , will the fish be

ab le to su rvive? Why or why n ot?

3 . Define ph enotypic adapta t ion . Give one examp le .

4 . Most living organisms cannot su rvive a t t empera ture above 45 0C. How

are som e microbes able to l ive in h ab itats with tem pera tu res exceeding

100 0C?

5. Lis t the a t t r ibu tes tha t popula t ions but not individu a l s possess .

6. If a popu lat ion growing exponen t ial ly dou ble in s ize in 3 years , wha t is

th e intrins ic rate of increas e (r) of th e popu lation?

7 . Name impor tant defence mechan isms in p lant s aga ins t herb ivory.


24/24

240

BIOLOGY

8. An orchid p lan t is growing on the bran ch of man go t ree. How do you

descr ibe this interact ion between the orchid an d th e man go t ree?

9. Wha t is the ecological pr inciple behind th e biological control meth od of

ma na ging with p est ins ects?10. Dis t inguish be tween the following:

(a) Hibern ation an d Aestivation

(b) Ectotherms a nd En dotherm s

1 1. Wr it e a s h o r t n o te on

(a) Adap tat ions of deser t plan ts a nd an ima ls

(b) Ada pta tions of plan ts to wa ter sca rcity

(c) Behavioural adaptations in animals

(d) Imp ortan ce of ligh t to plan ts

(e) Effect of tempera tu re or water sca rcity an d th e ada pta tions of an imals.

12 . Lis t th e var ious ab iot ic environmenta l fac tors .

13 . Give an exam ple fo r:

(a) An endothermic animal

(b) An ectotherm ic anima l

(c) An organ ism of ben th ic zone

14 . Define popu l a tion and com m uni t y.

15. Define th e following terms an d give one exam ple for each:

(a) Comm ens alism

(b) Par as itism

(c) Cam ou flage

(d) Mut u alism

(e) In ters pecific com pet ition

1 6. Wit h t h e h e lp o f s u it a b l e d ia g r a m d e s c r ib e t h e l o gis t i c p op u la t i on

growth curve.

17 . Se lect the s ta tement which expla ins bes t paras i t ism.

(a) One organ ism is b enefited.

(b) Both th e organ isms a re ben efited.

(c) One organ ism is b enefited, oth er is n ot affected.

(d) One organ ism is ben efited, other is affected.

18 . Lis t an y three impor tant ch arac te r is t ics of a popu la t ion an d expla in .

Organism and Population

Documents