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Population Interactions
In nature, animals, plants and microbes do not and cannot live in isolation but
interact in various ways to form a biological community.
Inter specific interactions arise from the interaction of populations of two
different species. They could be beneficial, detrimental or neutral (neither
harm nor benefit) to one of the species or both.

Name of Interaction Species A Species B

Mutualism + +

Competition – –

Predation + Predator – Prey

Parasitism + Parasite – Host

Commensalism + 0 (unaffected)

Amensalism – 0 (unaffected)

Mutualism: This interaction confers benefits on both the interacting

species

1. Lichens represent an intimate mutualistic relationship between a

fungus and photosynthesizing algae or cyanobacteria.

2. The mycorrhizae are associations between fungi and the roots of higher
plants. The fungi help the plant in the absorption of essential nutrients
from the soil while the plant in turn provides the fungi with energy-
yielding carbohydrates.

3. Plant-animal relationships. Plants need the help of animals for

pollinating their flowers and dispersing their seeds. Plants offer
rewards or fees in the form of pollen and nectar for pollinators and juicy
and nutritious fruits for seed dispersers.

4. Fig-wasp relationship: In many species of fig trees, there is a tight one-

to-one relationship with the pollinator species of wasp . It means that a
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given fig species can be pollinated only by its ‘partner’ wasp species and
no other species.The female wasp uses the fruit not only for egg-laying
but also uses the developing seeds within the fruit for nourishing its
larvae. The wasp pollinates the fig inflorescence while searching for
suitable egg-laying sites.
In return for the favour of pollination the fig offers the wasp some of its
developing seeds, as food for the developing wasp larvae.

5. Orchids- bees relationship : The Mediterranean orchid Ophrys employs

‘sexual deceit’ to get pollination done by a species of bee. One petal of its
flower bears an uncanny resemblance to the female bee in size, colour
and markings. The male bee is attracted to what it perceives as a female,
‘pseudocopulates’ with the flower, and during that process is dusted
with pollen from the flower. When this same bee ‘pseudocopulates’ with
another flower, it transfers pollen to it and thus, pollinates the flower.
(Note: Co-evolution - If the female bee’s colour patterns change even
slightly for any reason during evolution, pollination success will be
reduced unless the orchid flower co-evolves to maintain the
resemblance of its petal to the female bee).

Competition:

Gause’s ‘Competitive Exclusion Principle’

It states that two closely related species competing for the same resources
cannot co-exist indefinitely and the competitively inferior one will be
eliminated eventually. This may be true if resources are limiting, but not
otherwise.

Evidences in support:
1. The Abingdon tortoise in Galapagos Islands became extinct within a
decade after goats were introduced on the island, apparently due to the
greater browsing efficiency of the goats.
2. Another evidence for the occurrence of competition in nature comes
from what is called ‘competitive release’. The larger and competitively
superior barnacle Balanus dominates the intertidal area, and excludes
the smaller barnacle Chathamalus from that zone. (Barnacle-a marine
crustacean with an external shell, which attaches itself permanently to a
surface and feeds by filtering particles from the water using its modified
feathery legs.)
In ‘competitive release,’ a species whose distribution is restricted to a
small geographical area because of the presence of a competitively
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superior species, is found to expand its distributional range

dramatically when the competing species is experimentally removed)
Evidences not in support:
1. Totally unrelated species could also compete for the same resource.
Visiting flamingoes and resident fishes compete for the zooplankton,
their common food, in the lake.
2. Resources need not be limiting for competition to occur; in interference
competition, the feeding efficiency of one species might be reduced due
to the interfering and inhibitory presence of the other species, even if
resources (food and space) are abundant.
3. Species facing competition might evolve mechanisms that promote co-
existence rather than exclusion. One such mechanism is ‘resource
partitioning’. If two species compete for the same resource, they could
avoid competition by choosing, for instance, different times for feeding
or different foraging patterns. MacArthur showed that five closely
related species of warblers (birds) living on the same tree were able to
avoid competition and co-exist due to behavioural differences in their
foraging activities.

Therefore competition is best defined as a process in which the fitness of

one species (measured in terms of its ‘r’ the intrinsic rate of increase) is
significantly lower in the presence of another species.

Predation
Role of Predators:

1. Predators act as ‘conduits’ for energy transfer (the energy fixed by

plants) to higher trophic levels.. Plants- Herbivores, Tiger – Deer, Seed-
Sparrow etc.
2. They keep prey populations under control, they act as bio control agents
also (Prey species could achieve very high population densities and
cause ecosystem instability).
The prickly pear cactus introduced into Australia in the early 1920’s
caused havoc by spreading rapidly into millions of hectares of
rangeland. Finally, the invasive cactus was brought under control only
after a cactus-feeding predator (a moth) from its natural habitat was
introduced into the country.
1. Predators also help in maintaining species diversity in a community, by
reducing the intensity of competition among competing prey species.
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(In the rocky intertidal communities of the American Pacific Coast the
starfish Pisaster is an important predator. In a field experiment, when all
the starfish were removed from an enclosed intertidal area, more than
10 species of invertebrates became extinct within a year, because of
interspecific Competition)

If a predator is too efficient and overexploits its prey, then the prey might
become extinct and following it, the predator will also become extinct for lack
of food. This is the reason why predators in
nature are ‘prudent’.

Defense mechanisms developed by prey:

➢ Some species of insects and frogs are cryptically-coloured

(camouflaged) to avoid being detected easily by the predator.
➢ Some are poisonous and therefore avoided by the predators.
➢ The Monarch butterfly is highly distasteful to its predator (bird)
because of a special chemical present in its body. Interestingly, the
butterfly acquires this chemical during its caterpillar stage by
feeding on a poisonous weed.

Defenses evolved by Plants (Morphological and chemical) against

herbivores:

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

defence.
• Calotropis growing in abandoned fields produces highly poisonous
cardiac glycosides.
• A wide variety of chemical substances that we extract from plants on a
commercial scale
(Nicotine, caffeine, quinine, strychnine, opium, etc.,) are produced by
them actually as defences against grazers and browsers.

Parasitism :

Majority of the parasites harm the host; they may reduce the survival, growth
and
reproduction of the host and reduce its population density. They might render
the host more vulnerable to predation by making it physically weak.
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The life cycles of parasites are often complex, involving one or two
intermediate hosts or vectors
to facilitate parasitisation of its primary host. The human liver fluke (a
trematode parasite) depends on two intermediate hosts (a snail and a fish) to
complete its life cycle. The malarial parasite needs a vector (mosquito) to
spread to other hosts.

Ectoparasites : Endoparasites :

Parasites that feed on the external Endoparasites are those that live
surface of the host organism are inside the host
called. body at different sites (liver, kidney,
The lice on humans and ticks on dogs, lungs, red blood cells, etc.) Malarial
Copepods on many marine fish, parasite-plasmodium
Cuscuta, a parasitic plant

Brood parasitism:

It is a fascinating example of parasitism in birds in which the parasitic bird lays

its eggs in the nest of its host and lets the host incubate them. During the
course of evolution, the eggs of the parasitic bird have evolved to resemble the
host’s egg in size and colour to reduce the chances of the host bird detecting
the foreign eggs and ejecting them from the nest.

Brood parasitism in action is seen in the movements of the cuckoo (koel) and
the crow during the breeding season (spring to summer)

Commensalism: This is the interaction in which one species benefits

and the other is neither harmed nor benefited.

1. An orchid growing as an epiphyte on a mango branch, benefit while the

mango tree does not get any benefit.

2. Barnacles growing on the back of a whale benefit while the whale does
not derive any apparent benefit.
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3. The cattle egret and grazing cattle in close association The egrets always
forage close to where the cattle are grazing because the cattle, as they
move, stir up and flush out insects from the vegetation that otherwise
might be difficult for the egrets to find and catch.

4. sea anemone and the clown fish that lives among them The fish gets
protection from predators which stay away from the stinging tentacles
of sea anemone.The anemone does not appear to derive any benefit by
hosting the clown fish.

Amensalism: In amensalism, one species is harmed whereas the other is

unaffected.

1. Moulds: Penicillium notatum, a mould, produces an antibiotic,

penicillin, which checks the growth of a variety of bacteria, e.g.
staphylococcus and apparently does not gain by their death.

2. Bacteria: The bacterium, streptomyces griseus, produce the antibiotic

streptomycin, which inhibit the growth of many bacteria.

3. Plants : Certain plants like the walnut tree produces a substance

juglone that inhibits the growth of seedlings including its own. Such
chemicals are called allomones and the phenomenon is called
allelopathy.
Sunflower, sorghum and barley check the growth of weeds near them
and Ocimum sanctum (Tulsi) checks the growth of plants near it.

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