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Population Interactions

• For any species, the minimal requirement is one more species on which it can feed.
• Even a plant species, which makes its own food, cannot survive alone; it needs soil microbes to break down the organic matter in soil and return the inorganic nutrients for absorption.
• In nature, animals, plants and microbes interact in various ways to form a biological community.
• Interspecific 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.
• Assigning a ‘+’ sign for beneficial interaction, ‘-’ sign for detrimental and 0 for neutral interaction, look at all the possible outcomes of interspecific interactions.

Species A	Species B	Name of Interaction
+	+	Mutualism
-	-	Competition
+	-	Predation
+	-	Parasitism
+	0	Commensalism
-	0	Amensalism

• Both the species benefit in mutualism and both lose in competition in their interactions with each other.
• In both parasitism and predation only one species benefits (parasite and predator, respectively) and the interaction is detrimental to the other species (host and prey, respectively).
• 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.
• Predation, parasitism and commensalism share a common characteristic– the interacting species live closely together.

Interactions Among Population Of Different Species

• There are interactions among population of different species as below -
  1. Predation -
     • In this, predator feed on prey. e.g. it is the tiger(predator) and the deer(prey) but a sparrow eating any seed is also a predator in nature.
     • Predation is the nature’s way of transferring to higher trophic levels the energy fixed by plants.
     • Predator keep prey populations under control.
     • Biological control methods adopted in agricultural pest control are based on the ability of the predator to regulate prey population.
     • 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, because of inter-specific 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’.
     • Prey species have evolved various defenses to lessen the impact of predation.
       • 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.
     • 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).
     • Plants have evolved an astonishing variety of morphological and chemical defences against herbivores.
       • Thorns (Acacia, Cactus) are the most common morphological means of defence.
       • 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.
       • The weed Calotropis grow 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.
       • 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.
  2. Competition -
     • Competition is defined as a process in which the fitness of one species is significantly lower in the presence of another species.
     • In this, no species get benifitted, both are at equal risk.
     • e.g. 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.
     • Competitive release -
       • 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.
       • Connell’s elegant 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.
     • 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.
     • Resource partitioning
       • If two species compete for the same resource, they could avoid competition by choosing different times for feeding or different foraging patterns.
       • MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioural differences in their foraging activities.
  3. Parasitism -
     • In this, one(prey) feed on other species(host) without actual killing.
     • Parasitism has evolved in so many taxonomic groups from plants to higher vertebrates.
     • Many parasites have evolved to be host-specific (they can parasitise only a single species of host) in such a way that both host and the parasite tend to co-evolve.
       • If the host evolves special mechanisms for rejecting or resisting the parasite, the parasite has to evolve mechanisms to counteract and neutralise them, in order to be successful with the same host species.
     • In accordance with their life styles, parasites evolved special adaptations such as the loss of unnecessary sense organs, presence of adhesive organs or suckers to cling on to the host, loss of digestive system and high reproductive capacity.
     • 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.
     • Majority of the parasites harm the host; they may reduce the survival, growth and reproduction of the host and reduce its population density.
     • Ectoparasites -
       • Parasites that feed on the external surface of the host organism are called ectoparasites. e.g. lice on humans and ticks on dogs
       • Many marine fish are infested with ectoparasitic copepods
       • Cuscuta, a parasitic plant that is commonly found growing on hedge plants, has lost its chlorophyll and leaves in the course of evolution. It derives its nutrition from the host plant which it parasitises.
     • Endoparasites -
       • Endoparasites are those that live inside the host body at different sites (liver, kidney, lungs, red blood cells, etc.).
       • The life cycles of endoparasites are more complex because of their extreme specialisation.
       • Their morphological and anatomical features are greatly simplified while emphasising their reproductive potential.
       • Brood parasitism in birds is a fascinating example of parasitism 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.e.g. the cuckoo (koel) and the crow during the breeding season (spring to summer) show brood parasitism in action.
  4. Commensalism -
     • This is the interaction in which one species benefits and the other is neither harmed nor benefited.
     • An orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of a whale benefit while neither the mango tree nor the whale derives any apparent benefit.
     • The cattle egret and grazing cattle in close association is a classic example of commensalism. 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.
     • The interaction between sea anemone that has stinging tentacles and the clown fish that lives among them. The fish gets protection from predators which stay away from the stinging tentacles. The anemone does not appear to derive any benefit by hosting the clown fish.
  5. Mutualism -
     • This interaction confers benefits on both the interacting species.
     • Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.
     • Similarly, 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.
     • Plant-animal relationships -
       • Plants need the help of animals for pollinating their flowers and dispersing their seeds.
       • Plants offer rewards in the form of pollen and nectar for pollinators and juicy and nutritious fruits for seed dispersers.
       • Plant-animal interactions often involve co-evolution of the mutualists, that is, the evolutions of the flower and its pollinator species are tightly linked with one another.
       • In many species of fig trees, there is a tight one-to-one relationship with the pollinator species of wasp. It means that a given fig species can be pollinated only by its ‘partner’ wasp species and no other species. The female wasp uses the fruit not only as an oviposition (egg-laying) site but 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.
       • Orchids show a bewildering diversity of floral patterns many of which have evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination by it.
       • 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 of the 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.
       • 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.

One Mark Questions

Q1. What is ecology?

Ans. Ecology is a branch of science in which studies the interactions among organisms and between the organism and its physical (abiotic) environment.

Q2. What is population ecology?

Ans. It is the study of understanding the patterns and processes of change over time for populations of a single species.

Q3. Define amensalism.

Ans. In this interaction, one species inhibit growth of other species without getting any harm. e.g. The bread mould penicillium secretes penicillin that ultimately kills bacteria.