Zoology is studied as a ‘pure science’ (knowledge gaining) and it has application in other branches such as euphenics, eugenics, biotechnology, bioenergetics, bioinformatics, etc. As applied science, it has tremendous scope in agriculture, aquaculture, animal husbandry, human health, diseases, ...
8.3 POPULATION INTERACTIONS
Can you think of any natural habitat on earth that is
8.3.1 inhabited just by a single species? There is no such habitat
Inter-specific Interactions
8.3.2 Predation and such a situation is unimaginable. For any species, the
8.3.3 Competition minimal requirement is one more species on which it can feed.
8.3.4 Parasitism
Even a plant species, which makes its own food, cannot survive
8.3.5 Commensalism
alone; it needs soil microbes to break down the organic matter
8.3.6 Mutualism
in soil and return the inorganic nutrients for absorption. And
then, how will the plant manage pollination without an animal agent? It is obvious that
in nature, animals, plants and microbes do not and cannot live in isolation but interact
in various ways to form a biological community. Even in minimal communities, many
interactive linkages exist, although all may not be readily visible.
8.3.1 Inter-specific Interactions
Inter-specific interactions arise from the interaction of populations of two different
species. They could be beneficial, detrimental or neutral (neither harmful nor beneficial)
to one of the species or both. Assigning a ‘+’ sign for beneficial interaction, ‘–’ sign for
detrimental and ‘0’ for neutral interaction, let us look at all the possible outcomes of
inter-specific interactions.
The interactions between species are grouped into six types. They are mutualism,
commensalism, parasitism and amensalism. Both the species benefit in mutualism
and both lose in competition in their interactions with each other. The interaction
where one species is benefitted and the other is neither benefitted nor harmed is called
commensalism. In amensalism on the other hand one species is harmed whereas the
other is unaffected. In both parasitism and 6predation’ only one species benefits (parasite
and predator, respectively) and the interaction is detrimental to the other species (host
and prey, respectively). Predation, parasitism and commensalisms share a common
characteristic - the interacting species live closely together.
Table 8.1: Population Interactions-Types
Name of Interaction Species A Species B
Mutualism + +
Competition – -
Predation + -
Parasitism + -
Commensalism + 0
Amensalism – 0
,Ecology and Environment 221
8.3.2 Predation
What would happen to all the energy fixed by autotrophic organisms if the
community has no animals to eat the plants? You can consider predation as nature’s
way of transferring the energy fixed by plants to higher trophic levels. When we think of
predator and prey, most probably it is the tiger and the deer that readily come to our
mind, but a sparrow eating any seed is also a type of predator (a seed predator-also
called granivore). Although animals eating plants are categorized separately as
herbivores, they are, in a broad ecological context, not very different from predators.
Besides acting as ‘conduits’/ ‘pipelines’ for energy transfer across trophic levels,
predators play other important roles. They keep the prey populations under control. In
the absence of predators, the prey species could achieve very high population densities
and cause instability in the ecosystem. Predators have different types of functions to
play in nature. They include:
A. Predator as a biological control agent
The prickly pear cactus introduced into Australia in the early 1920s caused havoc
by spreading rapidly into millions of hectares of rangeland (vast natural grass lands).
Finally, the invasive cactus was brought under control only after a cactus-feeding
predator (a moth) was introduced into the country. Biological control methods adopted
in agricultural pest control are based on the ability of the predators to regulate prey
populations.
B. Predators maintain ‘species diversity’
Predators also help in maintaining species diversity in a community, by reducing
the intensity of competition among competing prey species. 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 in that
experimental area, because of increased inter - specific competition.
C. Predators are prudent (practical) pertaining to preys
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 due to lack of food. This is
the reason why predators in nature are ‘prudent’.
Prey species have evolved various defenses to lessen the impact of predation. They
include :
a) Preys fool (deceive) or avoid their predators: 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.
b) Preys defend by becoming distasteful to predators: The Monarch butterfly is
highly distasteful to its predator (bird) because of a special chemical present in its
,222 Zoology
body. Interestingly, the butterfly acquires this chemical during its caterpillar stage
by feeding on a poisonous weed.
c) Plants too have their defensive mechanisms: For plants, herbivores are the
predators. Nearly 25 per cent of all insects are known to be phytophagous (feeding
on plant sap and other parts of plants). The problem is particularly severe for plants
because, unlike animals, they cannot escape from their predators. Plants therefore
have evolved a variety of morphological and chemical defences against herbivores.
i) Thorns (Acacia, Cactus, etc.,) are the most common morphological means of
defense. Many plants produce and store chemicals that make the herbivore
sick when they are eaten, inhibit feeding or digestion, disrupt its reproduction
or even kill it.
ii) You must have seen the weed Calotropis growing in abandoned fields. The
plant produces highly poisonous cardiac glycosides and that is why you never
see any cattle or goats browsing on this plant.
iii) 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.
8.3.3 Competition
When Darwin spoke of the struggle for existence and survival of the fittest in nature,
he was convinced that interspecific competition is a ‘potent force’ in the process of
organic evolution, involving Natural Selection. It is generally believed that competition
occurs when closely related species compete for the same resources that are limited,
but this is not entirely true.
I. Competition among Unrelated Species
Firstly, unrelated species could also compete for the same resource (Interspecific
competition). For instance, in some shallow South American lakes visiting flamingos
and resident fishes compete for their common food, the suspended zooplankton in
the shallow waters. Secondly, 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. 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.
II. Competitive exclusion
It is relatively easy to demonstrate in laboratory experiments, as Gause and other
experimental ecologists did. When the resources are limited, the competitively superior
species will eventually eliminate the other species e.g. the Abingdon tortoise in Galapagos
, Ecology and Environment 223
Islands became extinct within a decade after goats were introduced on the island, actually
due to the greater browsing efficiency of the goats.
III. Competitive release
Another evidence for the occurrence of competition in nature comes from what is
called ‘competitive release’. Competitive release occurs when one of the two competing
species is removed from an area, thereby releasing the remaining species from one of
the factors that limited its population size. A species, whose distribution is restricted to
a small geographical area because of the presence of a competitively superior species, is
found to expand its distributional range dramatically when the competing species is
experimentally removed. This is due to the phenomenon called ‘competitive release’.
Connell’s ‘field experiments’ showed that, on the rocky sea coasts of Scotland, the larger
and competitively superior barnacle Balanus dominates the intertidal area, and excludes
the smaller barnacle Chathamalus from that zone. When the dominant one is
experimentally removed, the populations of the smaller ones increased. In general,
herbivores and plants appear to be more adversely affected by competition than the
carnivores (Ref: NCERT text book).
IV. Coexistence, rather than exclusion
Gause’s principle of ‘Competitive Exclusion’ states that two closely related species
competing for the same resources cannot co-exist indefinitely and the competitively
inferior one will be eliminated in due course of time. This may be true if resources are
limiting, but not otherwise.
More recent studies point out that species facing competition might evolve
mechanisms that promote co-existence rather than competitive 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 (food collecting) patterns. MacArthur showed that five closely related
species of warblers (a kind of birds) living on the same tree were able to avoid competition
and co-exist due to behavioural differences in their foraging activities.
8.3.4 Parasitism
Considering that the parasitic mode of life ensures free ‘lodging’ and ‘meals’, it is
not surprising that parasitism has evolved in so many taxonomic groups from plants to
higher vertebrates. Many parasites have evolved to be host-specific (they can parasitize
only a specific species of host) in such a way that both host and the parasite tend to co-
evolve; that is, if the host evolves special mechanisms for rejecting or resisting the parasite,
the parasite has to evolve mechanisms to ‘counteract’ and ‘neutralize’ them, in order to
continue successful parasitic relationship with the same host species. In order to lead
successful parasitic life, parasites evolved special adaptations, such as:
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