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Clownfish in Aquarium



Ecology is a branch of biology which studies the interactions among organisms and their bio-physical environment.

A single organism cannot live alone, due to which there is always seen an inter-relationship between organisms and their surrounding. The branch of biology which deals with different principles that control this relationship and consists of various levels is known as ecology.


It is a branch of biology which studies the interactions among organisms and their bio-physical environment, which includes both biotic and abiotic components.


Organisational Levels of Ecology

  • Organism: Living component of the environment at individual level is called organism.

  • Ecology at the organismic level is physiological ecology which reveals how different organisms are adapted to their environments. The organism is the smallest level of ecological hierarchy.

  • Population: Population is defined as the sum total of all individuals of a species in a specific geographical area.

  • Species: The species are the group of individuals of one or more populations which resemble each other and can interbreed among themselves.

  • Biotic community: The assemblage of all the populations of different species present in an area that interact among themselves are called biotic community.

  • Biotic community are of three types:

  1. Plant community

  2. Animal community

  3. Microbial community

  • Ecosystem: The sum total of the biotic (living) and abiotic (non-living) components of a particular geographical area being integrated through exchange of energy and recycling of nutrients are collectively called ecosystem.

  • Biome: The large unit of environment consisting of a major vegetation type and its associated fauna in a specific climatic zone is called a biome.

  • Biosphere: All the ecosystems of the world are collectively called biosphere.

  • Niche: The ecological niche of an organism represents the range of conditions that it can tolerate, the resources it utilises and its functional role in the ecological system. Each species occupies a distinct niche and no two species occupy the same niche.

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  • Environment is referred to as the sum total of all the physical and biotic conditions which influence the organism in terms of survival and reproduction.

  • Different seasons result due to

  1. Rotation of earth around the sun.

  2. Tilting of the earth on its axis.

  • The major biomes of the world include desert, grassland, rainforest and tundra.

  • Formation of different biomes is due to

  1. Annual variations in intensity and duration of temperature.

  2. Annual variations in precipitation.

  • The above annual variations together with annual variation in precipitation (remember precipitation include both rain and snow) thus form major biomes.

  • The biomes may be desert, rainforest and tundra.

  • Regional and local variations within each biome lead to the formation of different habitats.

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Major Abiotic Factors

1. Temperature

  • It is the most ecologically relevant environmental factor.

  • It is observed that seasonally, the average temperature on land varies.

  • The temperature decreases progressively from the equator to the poles and from plains to the mountain top.

  • The range of temperature varies from subzero levels in polar areas to >50oC at high altitude in tropical deserts in summer.

  • The temperature can affect the kinetics of enzymes and through it the basal metabolism and other physiological functions of the organisms.

  • The organism tolerating the high range of temperature is called eurythermal and the organism which can tolerate narrow range of temperature is called stenothermal.


2. Water

  • It is the next important factor as life is unsustainable without water.

  • The amount of water in an environment determines the productivity and distribution of plants.

  • For aquatic habitat, the quality of water becomes important like pH value, salinity and temperature of water.

  • The organisms tolerating wide range of salinities are called euryhaline and the organisms that tolerate only narrow range of salinities are called stenohaline.


3. Light

  • Light is important because autotrophs make food with the help of light (photosynthesis) and 02 is evolved during this process.

  • The small plants like herbs and shrubs can perform photosynthesis under very low light conditions as they are overshadowed by tall trees.

  • The plants depend on sunlight to meet their photoperiodic requirement for flowering.

  • For many animals, light is important in that they use the diurnal and seasonal variations in light intensity and duration (photoperiod) as cues for timing their foraging, reproductive and migratory activities.


4. Soil

  • The nature and properties of soil varies with different places.

  • The nature and properties of soil depend on the climate and weathering process.

  • The soil composition, grain size and aggregation determine the percolation and water holding capacity of the soil.

  • The vegetation in an area is determined by some soil parameters like pH, mineral composition and topography.

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Responses to Abiotic Factors

  • During the course of million years of existence, many species would have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximum efficiency and thus, enhances the overall fitness of the species.

  • The organisms try to maintain the constancy of its internal environment (a process called homeostasis) despite varying external environmental conditions that tend to upset its homeostasis.


How do Living Organisms Cope with Environment?

A. Regulate

  1. Some organisms maintain homeostasis by physiological and behavioural means.

  2. All birds and mammals and few lower vertebrate and invertebrate species maintain homeostasis by thermoregulation and osmoregulation.

  3. The success of mammals is largely due to their ability to maintain a constant body temperature.

  4. In summers, when outside temperature is more than our body temperature, we sweat profusely, and the resulting evaporative cooling brings down the body temperature.

  5. In winters, when temperature is lower, we shiver, a kind of exercise that produces heat' and raises the body temperature.

  6. Plants do not have such mechanism to maintain internal temperatures.


B. Conform

  1. Majority (99%) of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature is determined by ambient temperature.

  2. The osmotic concentration of the body fluids changes with that of the ambient water osmotic concentration, such animals and plants are simply called conformers.

  3. Loss or gain of heat is a function of surface area. The small animals have larger surface area relative to their volume. They lose body heat very fast in low temperature. So, they expend energy to generate body heat through metabolism for adjusting. Therefore, very small animals are rarely found in polar regions.


C. Migration

  1. The temporary movement of organisms from the stressful habitat to a more hospitable area and return when favourable conditions reappear, is called migration.

  2. The long-distance migration is very common in birds. In winter, famous Keoladeo National Park (Bharatpur) in Rajasthan hosts thousands of migratory birds coming from Siberia and other extremely cold northern regions.


D. Suspend

  1. Some bacteria, fungi and lower plants, under unfavourable conditions slow down metabolic rate and form a thick-walled spore to overcome stressful conditions.

  2. These spores germinate under onset of suitable environment.

  3. The animals that fail to migrate might avoid the stress by escaping in time, e.g., bear goes into hibernation (winter sleep) during winter.

  4. Snail and fish go into aestivation (summer sleep) to avoid summer. 

  5. Zooplanktons under unfavourable conditions enter diapause, a stage of suspended development.

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  • Any morphological, physiological and behavioural attribute of the organism that enables it to survive and reproduce in its habitat is called adaptation.

  • Over a long period of time, many adaptations have evolved and are incorporated in the gene, thus becoming heritable.


1. Adaptation in Kangaroo rat (Dipodomys merriami)

  • The Kangaroo rat in North American deserts is capable to meet its internal water requirement by oxidation of fat where water is a by-product.

  • It has the ability to concentrate its urine for minimum loss of water through excretory products.

  • Prevents water loss by living in burrows during day.

  • Solidification of faeces.

  • Nasal counter current mechanism to retrieve moisture from air being exhaled.


2. Adaptation in desert plants

  • Desert plants have thick waxy coating on leaves called cuticle, for minimum loss of water through Sweating.

  • They have special photosynthetic pathway (CAM) that enables minimum loss of water during daytime because stomata remain closed.

  • Some desert plants develop spines instead of leaves and photosynthetic function is carried out by the flattened stem.

  • Stomata are arranged in deep pits (sunken stomata) to minimize loss through transpiration.


3. Adaptation in mammals in cold climate

  • Mammals have shorter ears and limbs to minimize heat loss. This is called Allen's rule.

  • Seals (aquatic mammals) have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces excessive loss of body heat.


4. Adaptation in desert lizards

  • They absorb heat from the sun when the body temperature drops below the comfort zone.

  • They move into shade when ambient temperature rises above the comfort levels.


5. Adaptation at high altitude in humans

  • The people living in high altitudes compensate low oxygen by increasing production of red blood cells (RBCs).

  • The binding capacity of haemoglobin decreases and breathing rate increases.

  • People living at high altitudes of Himalayas have higher RBC count or total Hb than people living in plains.

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Population Attributes

  • population is defined as the total number of individuals of a species in a specific geographical area, which can interbreed under natural conditions to produce fertile offsprings and function as a unit of biotic community.

  • Population ecology links ecology to population genetics and evolution.

  • Characteristics of a population:

  1. Population size or density: of a species is the number of individuals of a species per unit area or volume

  2. Birth or natality rate: It is expressed as the number of births per 1,000 individuals of a population per year.

  3. Death or mortality rate: It is expressed as the number of deaths per 1,000 individuals of a population per year.

  4. Sex ratio: It is expressed as the number of females per 1,000 males of a population in given time.

  • A population at any given time is composed of individuals of different ages. When the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called age pyramid.

  • 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 population and is of three types:

  1. Expanding (Triangular shaped pyramid)

  2. Stable (Bell shaped pyramid)

  3. Declining (Urn shaped pyramid).

  • The pyramids also indicate the ratio of pre-reproductive, reproductive and post-reproductive individuals in a population.

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Population Growth

  • The size of a population depends on food availability, predation pressure and weather. Therefore, size of the population is not a static parameter.

  • The population density depends on few basic processes:

  1. Natality: It is the number of births during a given period of time. It increases the population density.

  2. Mortality: It is the number of deaths in a given time period. It decreases the population density.

  3. Immigration: It is the number of individuals of same species added to a habitat in a given time period. It increases the population density.

  4. Emigration: It is the number of individuals of same species that move to a different habitat in a given time period. It decreases the population density.

  • The population density is given by the following equation:

Nt=No + [(B + I) – (D + E)]


Nt = Population density at time t,

B = Birth rate,

I = Immigration,

D = death rate,

E = Emigration, and

No = Population in the beginning.

  • This equation shows that the population density will increase, if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants, i.e., (D + E), otherwise it will decrease.

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Population Growth Models

There are two models of population growth:

  1. The exponential growth

  2. Logistic growth


1. Exponential Growth

  • The exponential or geometric growth is common where the resources (food + space) are unlimited.

  • The equation of exponential growth can be derived as follows:

Nt = No ert

N = Population size

Nt = Population density after time t,

No = Population density at time zero

r = Intrinsic rate of natural increase

e = The base of natural logarithms (2.71828)

b = Birth rate

d = Death rate

  • 'r' is an important parameter assessing impacts of biotic and abiotic factor on population growth.

  • In exponential growth, when ‘N’ in relation to time is plotted on graph, the curve becomes ‘J’ shaped.


2. Logistic growth

  • The resources become limited at certain point of time, so no population can grow exponentially.

  • This growth model is more realistic.

  • Every ecosystem or environment or habitat has limited resources to support a particular maximum number of individuals called its carrying capacity (K).

  • When ‘N’ is plotted in relation to time ‘t’, the logistic growth shows sigmoid curve and is also called Verhulst—Pearl logistic growth. It is given by the following equation:

dN/dt = rN [K-N/K]

N = Population density at time t

r = Intrinsic rate of natural increase

K = Carrying capacity.

Life History Variation

  • Darwinian fitness refers to the populations where they evolve to maximise their reproductive fitness, i.e., high 'r' value.

  • Under selection pressures, organisms evolve towards the most efficient reproductive strategy.

  • The rate of breeding varies from species to species, as some organisms breed once in their lifetime (Pacific salmon fish, bamboo), while others breed many times during their lifetime (most birds and mammals).

  • Some organisms produce a large number of small-sized offsprings (oysters, pelagic fishes), while Others produce a small number of large-sized offsprings (birds, mammals).

  • Ecologists suggest that life history traits of organisms have evolved in relation to the constraints, imposed by the abiotic and biotic components of the habitat, in which they live.

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Population Interaction

  1. Interspecific interactions are interactions of populations of two different species.

  2. The interactions may be

  • beneficial/positive effect indicated by +.

  • harmful/detrimental/negative effect indicated by –.

  • neutral interaction/no effect on the species indicated by 0.

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1. Predation

  • It is an interspecific interaction, where an animal, called predator, kills and consumes the other weaker animal called prey.

  • Predation is nature's way of transferring energy to higher trophic levels, e.g., a tiger (predator) eating a deer (prey), a sparrow (predator) eating fruit or seed (prey), etc.

  • The role of predators:

  1. Predators keep prey population under control. This is called biological control.

  2. Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among prey species.

  3. Besides acting as 'conduits' for energy transfer across trophic levels, predators play other important roles. In absence of predator species, prey species could achieve very high population densities and lead to ecosystem instability.

  • When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have its natural predators.

  • If a predator is too efficient and over-exploits its prey, then the prey might become extinct and following it, the predator will also become extinct due to the lack of food.

  • The prey defense mechanisms:

  1. To avoid being detected easily by the predators, some species of insects and frogs are cryptically coloured (camouflaged).

  2. The Monarch butterfly is highly distasteful to its predator (birds) because of a special chemical present in its body which is acquired by the butterfly by feeding on a poisonous weed in its caterpillar stage.

  3. Some plants have thorns or spines for defence mechanism, e.g., Acacia, cactus.

  4. Some plants produce highly poisonous chemicals like cardiac glycosides, nicotine, caffeine, quinine, strychnine, opium, etc., are produced by plants actually as defences against grazers and browsers.

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2. Competition

  • Competition is a type of interaction where both the species suffer. It may exist between some species (interspecific competition) or between individuals of same species (intraspecific competition).

  • The competition occurs due to limited resources between closely related species.

  • Some totally unrelated species could also compete for the same resource, e.g., in some shallow south American lakes, visiting flamingoes and resident fishes compete for their common food, zooplanktons.

  • In interspecific competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, although the resources are abundant.

  • For example, after the introduction of goats in Galapagos Islands, the Abingdon tortoise became extinct within a decade due to greater browsing efficiency of the goats.

  • Competitive release refers to the phenomenon of 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.

  • Gause's competitive exclusion principle states that two closely related species competing for the same resource cannot coexist indefinitely and the competitively inferior one will be eliminated eventually by the superior one.

  • Resource partitioning: It refers to the phenomenon in which species facing competition might evolve mechanisms that promote coexistence rather than exclusion. MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and coexist due to behavioural differences in their foraging activities.

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3. Parasitism

  • It is the mode of interaction between two species in which one species (parasite) depends on the other species (host) for food and shelter, and in this process damages the host. In this process one organism is benefited (parasite) while the other is being harmed (host).

  • Adaptation of parasite:

  1. The parasite has evolved to be host-specific in such a manner that both host and parasite tend to co-evolve.

  2. Loss of unnecessary sense organs.

  3. Presence of adhesive organs or suckers.

  4. Loss of digestive system.

  5. High reproductive capacity.

  • The life cycles of some parasites are complex where one or more intermediate host or vectors to facilitate parasitisation are present.

  1. The human liver fluke depends on two intermediate hosts, a snail and a fish, to complete its life cycle.

  2. Malarial parasite (Plasmodium) needs a vector (mosquito) to complete its life cycle.

  • Majority of parasites harm the host by reducing the survival, growth and reproduction of the host. They reduce its population density by making it physically weak.

  • Parasites may be of two types: ectoparasites and endoparasites.

  • The Phenomenon in which one organism (parasite) lays its eggs in the nest of another organism is called brood parasitism.

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4. Commensalism

  • Commensalism is referred to as the interaction between two species where one species is benefited and the other is neither harmed nor benefited.

  • Few examples of commensalism:

  1. An orchid growing as an epiphyte on a mango tree. The orchid gets shelter and nutrition from mango tree while the mango tree is neither benefited nor harmed.

  2. Barnacles growing on the back of whale. Barnacles are benefited to move to location for food as well as shelter while the whales are neither benefited nor harmed.

  3. The egrets are in close association of grazing cattle. The cattle egrets are benefited by the cattle to detect insects because cattle stir up the bushes and insects are flushed out from the vegetation, to be detected by cattle egrets.

  4. The commensalism is also found between sea anemones and the clown fish. The fish is protected from predators and sea anemones are neither benefited nor harmed.

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5. Amensalism

  • Amensalism is referred to as the interaction between two different species, in which one species is harmed and the other is neither benefited nor harmed.

  • For example, the mould Penicillium secretes penicillin which kills bacteria but the mould is unaffected.

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6. Mutualism

  • Mutualism is referred to as the interspecific interaction in which both the interacting species are benefited.

  • Some examples of mutualism

  1. Lichens represent close association between fungus and photosynthetic algae or cyanobacteria, where the fungus helps in the absorption of nutrients and provides protection while algae or cyanobacterium prepares the food.

  2. Mycorrhizae are close mutual association between fungi and the roots of higher plants, where fungi help the plant for absorption of nutrients while the plant provides food for the fungus.

  3. Mutualism are found in plant-animal relationships. Plants take the help of animals for pollination and dispersal of their seeds and animals are rewarded in the form of nectar or edible pollen or oviposition (site for laying egg).

  4. The male bee pseudocopulates with it and during this process of pseudocopulation, the pollen grains are dusted on the body of male bees. With such pollen dusts, male bee pseudocopulates to another flower of the same species and pollination takes place.

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