Chapter7

Whenever we walk into a forest or step by the side of a lake, we are in acommunity of populations interacting with one nother. In other words, in everyecosystem, organisms live together influencing each other's life directly Or

. indirectly.

If you observe various ecosystems, you will find that they canhave four types of species.

i. Native species: which normally live and thrive in a particularecosystem.

ii. Im.migrant or Alien species: which migrate into an ecosystem orwhich are accidentally. or deliberately introduced into anecosystem by humans.

iii. Indicator species: which serve as early warning that acommunity Dr an ecosystem is being degraded.

iv. Key-stone species: which play roles affecting many otherorganisms in an ecosystem. Generally key-stone species play animportant role in marine ecosystems than in terrestrialecosystems. The loss of key-stone specie can lead to sharppopulation drops and extinction of other species that depend on itfor certain services.

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An individual in nature, interacts not only with others of same species butwith individuals of other species. Again Ihe interactions may be between animalsthemselves, between plants and animals, as well as between plants themselves.

Most of the interactions between species involve food. Competing forfood, eating and avoiding from being eaten are the most common ways in whichorganisms affect one another. Often these interactions are of brief duration. Thevarious types of interactions may be classified into two main categories:

1. Symbiosis: in which one or both species are benefited and neitherspecies is harmed. .. .

2. Antagonism: in which at least one of the species is harmed.

Immediate effect of

Type of Interaction Detailed effects interaction on twopopulations A & BPop. A Pop. B

1 Neutralism Neither population affects the other 0 02 Commensalism One species is gained and the second is + 0

neither helped nor harmed \3 Protocooperation Interaction favourable to both but not + +

obligatory4 Mutualism Interaction favourable and obligatory to + +

both5- Competition Wherein each population affects - -

adversely the other in the struggle for -nutrients, food, space or other commonneeds

6 Predation Wherein one species (the predator) gains +while the second (the prey) is being killedand fed upon

7 Parasitism Wherein one species gains (the parasite) + -

while the second (the host) is harmed butnot killed by the association

8 Amensalism Wherein one species is harmed by a - 0second but the latter is neither harmednor gains

Note: In the above scheme (+) indicates a positive jnteraction, (-) indicates a negativeinteraction and (0) indicates neutral interaction.

I Q. What is the First Law of Ecology?

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Most of the ecologists, however, extend the meaning of symbiosis (whichliterally means living together) to embrace all types of inter-relationships includingharmful ones. Odum (1971) agreeing to the use of term symbiosis, preferred todivide various types of interactions into two major groups. These are:

A. Positive Interactions: where populations help one another, theinteractions being either one way or reciprocal. These includecommensalism, protocooperation, mutualism, etc.

B. Negative Interactions: where members of one population may eatmembers of other population, compete for food and space, excreteharmful wastes or otherwise interfere with other populations. Theseinclude competition, predation, parasitism, amensalism, etc.

Most of the interactions between any t\'\(o species are Neutral i.e.neither species directly affects the other (even through the two may be linkedindirectly through a series of interactions with other species).

For example, eagles certainly have a neutral effect on any given speciesof grass. Because rabbits eat grass and eagle eats rabbits, an eagle indirectlybenefits the grass by helping to control t~e rabbit population and the grassindirectly benefits the eagle by fattening up its prey. Still, by convention, theEagle-Grass interaction is regarded as neutral.

Wilen members of different species are associated in such a way thatonly one partner is benefited while the other one is neither benefited nor harmed,such an association is called Commensal. Some of the important examples ofcommensalism are as under:

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1. Lianas are vascular plants, which are common in dense forests oftropical countries. These plants maintain erectness by taking supportof other big trees. Thus these plants are exposed to sunlight by takingsupport of other plants hence one partneris benefited while the otherone is neither benefited nor harmed.

2. Many.kinds of birds roost or nest on tall trees. A tall tree afford themprotection from many kinds of predators whereas the tree gets nothingin return from the bird that do nothing else but roost or nest.

3. Epiphytes, such as Spanish moss (Til/andsia sp.) which live on thebranches of big trees can be considered commen~al because theydepend on the tree for aerial support and the proper environmentalconditions (such as light) that exist at this level. But tree neithersuffers nor benefits from such an association ..

4. A commonly cited example of commensalism is Remora-Sl:1arkrelationship. The dorsal fin of· remora fish, Echenis naucrates hasbecome modified in the course of evolution into a most effective ovoidsucking disc as shown in Fig. 7.1. With the help of "sucking disc"wh\ich can release its grip at will, the remora attaches itself to theunderside of some big shark, whale or other large animal. Sharkstransport the remora to a feeding area. The remora swims around,feeding on the scraps of food left behind' by the shark and thenreattaches itself to the belly of a shark to enjoy another free ride.Shark, however, is neither harmed nor benefited.

suctorial disc(modified interior dorsal fin)

~ _ suctorial disc~ (dorsal view)

Fig. 7.1: The remora fish and its attachment disc, amodified fin located on the dorsal surface of head.

17h

5. Another most extra ordinary instance of commensal living within itshost is that of a small tropical fish cRlled Fierasfer. This small fishfinds' shelter within thp c!c;;1';2,1 canal of a sea-cucumber (anholothurian). Th;:. .isi i occasionally emerges out t c:cd in ti j(j

neighbClurhood.

In this type of association, both the partners are benefited but theassociation is not obligatory i.e. not essential. Both partners can live without eachother, if separated (N.B. In mutualism also, both the partners are benefited, butassociation is obligatory i.e. one can not live without the other). Some of theexamples of protocooperation are:' .

1. A classic example of protocooperation is furnished by the seaanemone Adamsia which grows on the shell of the hermit crabEupagurus. The se'a anemone is carried by the crab to new feedingareas. The crab, in turn, is camouflaged by sea anemone and is thusprotected from its enemies (Fig. 7.2). Both can, however, live

independently, if separated.Fig 7.2: Diagrammatic sketch showing associationbetween sea-anemone and hermit crab.

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2. Another example is the relationship between snake (water moccasins)and large birds (such as, heron). The birds nest in the lower branchesof trees while the snakes congregate around the base of the tree.Thus, the bird is protected from the tree-climbing predators (e.g.raccoons) by the snakes. In turn, the snakes feed, in part .on fishdropped by birds and the occasional fall out of baby birds from tilenest.

3. Another good example of protocooperation is the relationship betweenan ox-tail bird and a rhinoceros. The bird sits on the back of rhinocerosand removes pests (i.e. blood sucking ticks and flies). The ox-tailbenefits by receiving food, warmth and protection from predators; therhinoceros benefits by receiving protection from parasites. Again therelationship is not obligatory; both are capable of surviving withouteach other.

When both the species gain from an association, and are unable tosurvive separately, such an association is called Mutualism. Following are fewexamples:

1. Lichens, which are composed, of a matrix formed by a fungus withinwhich cells of an algae are embedded. The fungus holds moisture andmakes minerals available for both the partners. In return, thechlorophyll bearing algae manufacture carbohydrates for itself and forthe colourless fungus. The fungus of a lichen can never grow innature without the association of algae, and on the otller hand, algaeare dependent upon fungus under neutral conditions (Fig. 7.3).

2. The bacteria of genus Rhizobium furnish a common example ofmutualism, which form nodules on the roots of leguminous plants. Thebacteria obtain carbohydrates and other substances from the Juices ofplants and in turn fix nitrogen in the form of nitrates, which is utilizedby plants (Fig. 7.4).

I Q. What is the Third Law of Ecology?

upper cortex~ ~ '"..', .' ' "0 flO ~~. <:' ,".' (,: \'-.....•.•" 0" ,"''' . •

14 '\ ll"C~·' ''"(!) t"Vtl~

algal __ ~ 0" ~JL# ~cells q

fungal ~~ . - Jhyphae" CX' (3) ~ ]!I' 0

\ .' • d d 0 'OSl. 0o 1,0. ,C1 (7'0 f"() .,.",.'--< """Q " ". •

Fig. 7.4: Root system of a leguminous plant showingnodules within which live symbiotic bacteria