Karnataka 2nd PUC Biology Notes Chapter 13 Organisms and Populations


The term ecology was coined by H. Reiter (1868) and is defined by E.Haekel (1870) as a branch of biology which deals with the inter relationship and interactions between living organisms and their natural environment. (Smith 1947).

The Types of Ecology:

→ Autecology: It is the study, of an individual organism or particular species in relation with its natural environment.

→ Synecology: It is the study of (different species of plant and mammals) belonging to various populations in relation with their natural environment.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

Levels of organization:
1. Population: Population is a local group of individuals of a species. Study of population is called demography, e.g., insect population, reptile population.

2. Community: Community is a localised collection of population of different species of plants and animals, e.g., plant community, animal community.

3. Biome: It is an unit of biosphere with a specific environmental condition to support and sustain a specific interaction of biotic and abiotic factors, e.g., pond biome, marine biome, estuarine biome, forest biome, grass land biome, desert biome.

4. Biosphere: It is the totality of earth with its life supporting environment. It consists of hydrosphere (water), lithosphere (soil), and atmosphere (air). Functionally biosphere is the collection of several ecosystems (Biomas).

5. Ecotone and Edge effect:

  • A transitional zone between the 2 eco-systems constitutes an ecotone.
  • It will be a mixture of environmental conditions of both the eco-systems. e.g., Grassland is an ecotone in the terrestrial habitat. Estuary is an ecotone in the aquatic habitat.
  • When we look at the flora (total plants) and fauna (total animals) in an ecotone, we can observe their increased number.
  • This increase of biota in an ecotonic habitat is called edge effect, e.g., Owl keeps its maximum number in grassland.

6. Ecological Dominance:
In a community of individuals, if the environmental conditions are supportive to one community, they may outnumber the others and dominate over the other communities. Such a phenomenon is called ecological dominance. It may be because of their number, size or biomass, e.g. In a forest the oak tree population has an ecological dominance.

7. Ecological succession: If the ecological dominance is magnified to succeed over the other community, it is called ecological succession that leads to the absence of the other community and growth of the ecologically dominant community only.

8. Climatic climax: Since climatic conditions are supportive for the ecological dominance and ecological succession, it is also called as climatic climax. Here climatic conditions are very vital in upholding ecological dominance.

9. Ecological niche: If the habitat of an individual is compared to the address, an ecological niche suggests the profession of an individual. It is a factor to describe the overall activities of an individual in its community and habitat. It is also a dynamic entity, that changes from place to place.

Any Eco system or Biomes is the sum total of all (living) and abiotic (non – living) factors.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

Biotic Components:

Consists of producers [P], consumers [C] and decomposers [D], Producers include all green plants. Autotrophs capable of photosynthesis, synthesise food for themselves and for all organisms of the ecosystem. Producers of pond include macrovegetation and microvegetation.

Macrovegetation: It refers to large aquatic plants. They can be rooted , submerged or free floating. e.g.:Typha, Chara, Hydrilla.

Microvegetation: It refers to microscopic, free floating plants called phytoplanktons. They represent the bulk producers of the pond ecosystem, e.g.: Diatoms, volvox, nostoc, scytonema, chlamydomonas, anabena.


It includes all animals which are heterotrophs i.e. incapable of photosynthesis. They depend on producers for food, energy and food directly or indirectly. They are of three grades namely,
(a) Primary consumers (C1) – Zooplankton (Mosquito larva, tadpole) Daphnia, Desmids, Nematodes; Crustaceans.
(b) Secondary consumers (C2) – Water, beetles, hydra, small fishes , frog.
(c) Tertiary consumers (C3) -Bigfish, water snake.
(d) Decomposers – Bacteria and fungi (Saprophytes).

(a) Primary consumers: Also called herbivorous, feed on green plants directly for food and energy, and thereby regulate the population size qf producers. Their numbers and energy level would be greater than that of C2 . C1 of pond includes tadpoles, pond snakes and zooplanktons like cyclops, deaphuria, crustaceans, daphnia, nematodes cypris etc.,
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(b) Secondary consumers : Also called primary carnivores, feed on C1 for food and energy thereby regulates the population size of Cr Their number and energy level would be much greater than that of C3. C2 of pond includes rotifers, hydra, water beetles, small fish and frogs.

(c) Tertiary consumers: Secondary carnivore feed on C2 for food and energy thereby regulate the population size of C2, their number and energy level would be least of all. C3 of pond include water snakes and big fish like carps and cat fishes.

(d) Decomposers : Saprotrophs feed on the dead bodies of plants and animals and reduce them into simpler chemical elements to be reused by producers for photosynthesis and as nutrients. Thus they serve in cycling of matter. Decomposers of pond include bacteria and fungi abundant in the muddy bottom of pond.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

Abiotic Components:

Consists of sunlight, soil, water and its pH, temperature, dissolved gases like oxygen and carbon dioxide, mineral salts like P04, S04, N03, Na, K and Ca and organic substance like Humic acid, carbohydrates, fats.

Major abiotic factors:
1. Temperature:
→ It is the most important ecological factor affecting almost all the metabolic activities of the organisms. The metabolic activities begin at a certain minimum temperature and increase with increase of temperature, until they reach a maximum temperature called optimum. Further rise of temperature, is accompanied by a fall in the metabolic activity until it ceases at a maximum temperature. Most of the organisms can survive in a narrow range of temperature (0° – 35°C).

→ Effects of temperature on plants: The metabolic reactions, transpiration, mineral absorption and water uptake and reproduction in plants, are affected by changes in temperture. All metabolic reactions are controlled by enzymes and variation of temperature affects enzyme action. The rate of transpiration increases with the increase of temperature.

→ When the temperature is low, the minerals remain tightly attached to soil. Many species of plant show flowering with low temperature treatment and this phenomenon is called vernalization.

→ Equatorial region, tropics, temperate and polar regions have their own particular type of vegetation.

Effect of temperature on animals:
→ The variations of temperature seems to have remarkable effect on the metabolism of certain animals. Temperature affects the reproductive capacity of organisms and enzyme linked chemical reactions in the cells. Temperature also effects growth, development and morphology. It shows that different animals have different range of temperature tolerance. Based on this, animals are divided into the following categories.

→ Eurythermal organisms: These are the animals which can tolerate and thrive in a wide range of temperatures
e.g.: Cyclops, toad, man, lizards etc.

Stenothermal organisms:
→ These are the animals which tolerate only a narrow range of temperatures and the vast majority animals are stenothermal, e.g. : fishes, corals and snail.

2. Water:
→ The amount of water available, determines the type and distribution of plants in different areas. Water availability is so limited in deserts, that only plants with special adaptations (xerophytes) make it possible to grow there. The survival, productivity and distribution of plants mostly depend on water.

→ Even though there is a lot of water in the oceans, lakes, seas and rivers, the plants growing there face problem of quality of water. The chemical composition and pH of water becomes important in the distribution of plants. The salt concentration is less than 5 in inland waters, 30-35 in the seas and more than 100 percent in some hypersaline lagoons.

→ Based on the tolerance of salt, there are two types of organisms, euryhaline organisms which tolerate a wide range of’salinities and stenohaline organisms restricted to a narrow range of salinities. Many fresh water animals cannot live for long in sea water due to exosmosis of water from the cells and sea water animals cannot live in fresh water due to endosmosis of water.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

3. Light:
→ Light is directly responsible for the growth, development and differentiation of plants. Sunlight is a source of energy for plants to produce food through photosynthesis. Intensity, quality and duration of sunlight controls the activities of both plants and animals. Certain plants require less intense light for optimum photosynthesis and grow in shady places. They are called shade tolerant species or sciophytes. On the other hand certain plants require high intensity for optimum photosynthesis. Such plants are called shade intolerant species or heliophytes.

Note: Extremely intense light results in photooxidation of cellular components including photosynthetic apparatus and finally their death. The phenomenon is called solarization.

→ The duration of sunlight in a day is called photoperiod and response of plants to this is called photoperiodism.

→ Light also affects movements of plants. Leaves of some leguminous plants fold up or droop at night. These movements are called nictinastic movement.

→ Light also influences daily movement of animals. Majority of animals are diurnal (active during the day) e.g.: Most of birds, man, butterflies etc.

→ Some animals, such as cockroaches, bats, owls, moths etc hide during the day, and become active at night. They are called nocturnal animals. Photoperiod also affects the breeding cycles of animals.

4. Soil:
→ The vegetation on the earth is the direct result of the nature of soil. Soil provides water, mineral ssalts and anchorage to plants. The characteristics of soil such as its constituents, origin, temperature range, water holding capacity, aeration, minerals etc., determine the flora and fauna of a particular place.

→ Parameters such as pH, mineral composition and topography determine the type of vegetation and animals of any area. Most organisms survive in an optional pH range. Plants and aquatic animals require acidic conditions and others need neutral or alkaline conditions.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

Response to Abiotic Factors:
→ Many organisms have evolved (during the course of million years of their existence) a relatively constant internal environment (homeostasis), that allows all the biochemical reactions to proceed with maximum efficiency. They are called the regulators.
Their constancy could be in terms of optimal temperature, osmotic concentration of body fluids, etc.

→ Other organisms may be

  • partial regulators or
  • conformers.

→ Partial regulaters try and regulate their internal environment by physiological or behavioural means, but beyond a certain limit, they just conform to the external environmental conditions.

→ Conformers are those organisms, which change their body temperature or osmolarity of the body fluid according to the medium they inhabit; they do not have any internal mechanism to regulate and no energy is spent.

(a) Regulation: The organisms maintain homeostasis by physiological and/or behavioural means, and ensure a constant body temperature (thermoregulation), osmotic concentration,
(osmoregulation), etc.
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→ The levels of thermal tolerance of the species determine their geographical distribution.

→ Evolutionary biologists believe that the success of mammals is mainly due to their ability to maintain a constant body temperature, that they are able to survive in Antarctica as well as in Sahara desert.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

(b) Conformation:
→ Most of the animals and nearly all plants cannot maintain a constant internal environment.

→ Their body temperature changes with the ambient temperature.

→ The osmotic concentration of body fluids of aquatic animals changes with those of the ambient water or osmotic concentration and such animals are called osmoconformers.

→ Thermoregulation is an energy-expensive process where heat loss or gain is a function of surface area.

→ Smaller animals have a larger surface area compared to body volume and lose heat f very fast when the outside is cold. They have to spend more energy to produce the body heat through metabolism and hence small animals (shrews, humming bird, etc.) are not found in polar regions.

(c) Migration:
→ By this mechanism, the organisms can move away temporarily from the stressful conditions in the habitat to another habitat with hospitable conditions.

→ Birds undertake long distance migrations during winter, e.g., Keol Dev Ghana (Bharatpur) National Park hosts thousands of migratory birds from Siberia and other extremely cold northern regions.

(d) Suspension:
→Those organisms which cannot/do not migrate, suspend their metabolic functions during the stressful period and resume their functions at the return of the favourable condition, Hibernation (in frogs, certain reptiles and polar bears) and aestivation (some snails and fish) are examples of suspension.

→ Diapause is a stage of suspended development shown by certain zooplanktons of lakes and ponds under unfavourable conditions.

→ Adaptations: Any attribute of the organism (morphological, physiological or behavioural)
that enables the organism to survive and reproduce in its habitat is called adaptation.

→ Many adaptations have evolved over a long evolutionary time and are genetically controlled. The ultimate aim of all the adaptations is to make the individual fit to obtain food and space for its survival.

Some of the examples of adaptations are given below:
(a) In the absence of external water, the Kangaroo rat in North American deserts is capable of meeting all its water requirements through its internal fat oxidation, in which water is a by product.
Also it can concentrate its urine to a minimal volume.

(b) Mammals living in colder climates have shorter ears and limbs to minimize heat loss. This is called Allen’s Rule.

(c) Aquatic mammals like seals, living in polar seas have a thick layer of fat (Blubber) below their skin, that acts as an insulator and reduces loss of body heat.

(d) Fishes thriving in Antartic waters where the temperature is below zero degrees, have an array of biochemical adaptations to such extreme environments.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

(e) Desert plants possess the following adaptations:

  • Thick cuticle on their leaf surface.
  • Posses sunken stomata.
  • They have a special photosynthetic pathway i.e., CAM pathway that enables their stomata to remain closed during day time.
  • In opuntia, the leaves are reduced to spines and the photosynthetic functions are taken over by the flattered stems.

(f) Archaebacteria have adaptations to survive at temperature more than 100°C.

(g) When human beings go from plains to high altitudes (> 3500 m) [eg : Rohtang pass] they experience altitude sickness, due to the low atmospheric pressure and non availability of oxygen. They develop symptoms like fatigue nausea and heart palpitations.
But gradually the body gets adjusted by increasing red blood cell production, decreasing binding capacity of haemoglobin and increasing the breathing rate.


All the individuals of a species occurring in a locality constitute a population.

Population attributes :
A population has certain attributes that an individual organism does riot. A population has group attributes like birth and death rates, sex ratio, age distribution, population size etc.

Birth and death rates:
In a population, birth and death rates refer to the per capita birth and death respectively. Therefore, these rates are expressed as change in numbers (increase or decrease) with respect to members of the population. Consider the following example to understand the birth and death rates.

(i) In a pond there were 20 lotus plants last year, and 8 new plants were added to the existing population. Thus, the current number of plants in the population is 28.
2nd PUC Biology Notes Chapter 13 Organisms and Populations 3

(ii) In a laboratory population of 40 fruit flies, 4 flies died during a specified time interval (say a week).
2nd PUC Biology Notes Chapter 13 Organisms and Populations 42nd PUC Biology Notes Chapter 13 Organisms and Populations 4

Sex Ratio:
Sex ratio is another attribute characteristic of a population. An individual belongs to either male or female sex, but a population has a sex ratio. For example, 60 per cent of the population are females and 40 percent males.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

Age distribution:
A population at any given time is composed of individuals of different ages. Ecologically a population has three age groups-pre-reproductive, reproductive and post reproductive. Their comparative abundance determines the reproductive status of population. A population having larger number of young individuals will show rapid increase (positive growth). A population with almost equal number of various age groups show stable growth (zero growth). A population with large number of post-reproductive or older individuals and lesser number of pre- reproductive individuals will show declining population (negative growth).

If the age distribution (per cent individuals of a given age or age group) is plotted for the population, with pre-reproductive groups at the base, reproductive one in the middle and post- reproductive grouops at the top, the resulting structure is called an age pyramid. For human population, the age pyramids generally show age distribution of males and females in a combined diagram. The growing, stable and declining populations respectively show broad base, bell shaped and urn shaped pyramids.
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Population size:
→ The size of a population may be the outcome of competition with another species, the impact of predator or the effect of a pesticide application. It tells a lot about its status in the habitat. Various ecological processes in a population, are evaluated in terms of change in the population size. The size of a population in nature could be as low as < 10 (e.g., Siberian cranes at Bharatpur wetlands in any year), or go into millions (e.g., Chlamydomonas in a pond). Population size, more technically called population density (designated as N) need not necessarily be measured in numbers only.

→ Although the total number is most appropriate measure of population density, still it is difficult to determine population density in some cases. For instance, if there are 200 Parthenium (carrot grass) plants in a forest area, but only a single huge banyan tree with a large canopy, indicating that the population density of banyan is low relative of that of Parthenium, amounting under estimation of the enormous role of the banyan tree in that community.

→ In such cases the per cent cover or biomass is a more meaningful measure of the population size. If the population is huge and counting is impossible or very time consuming, the total number is not easily measured . For a dense laboratory culture of bacteria, the number of colonies in the petri dish may be the best measure to report its density. Sometimes, for certain ecological investigations, there is no need to know the absolute population densities then, relative densities serve the purpose equally well.

→ For example, the number of fish caught per trap is good enough measure of its population density in the lake. Sometimes population size is indirectly estimated without actually counting them. The census in our national parks and tiger reserves is often evaluated on the basis of pug marks (animal’s foot print) and faecal pellets.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

Population Growth:

The size of population of any species in not a static parameter. It keeps on changing with time, depending on various factors such as availability of food, predation pressure and prevailing weather. Such changes give some idea of what is happening to the population, whether it is increasing or decreasing. Though a number of factors affect population size, the density of a population in a given habitat during a given period, fluctuates due to four basic processes-natality, mortality, immigration and emigration.

The population density is the number of individuals of a species per unit area/space at a given time. Mathematically population density is expressed as:

D = \(\frac{\mathrm{N}}{\mathrm{S}}\)

where, D stands for the population density, N denotes the number of individuals of a species at- a specific time and S represents the number of units of the space.

1. Natality: It refers to the number of births during a given period in the population that are added to the initial density.

2. Mortality: It is the number of deaths in the population during a given period.

3. Immigration: It is the numbed of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.

4. Emigration and gone elsewhere during the time period under consideration. Natality and immigration contribute to an increase in population density, while mortality and emigration to a decrease.

Therefore, population growth or change in the size of population in a given time is determined by the above factors.
Change in population size = (Births + Immigration) – (Deaths + Emigration)
If N is the population density at time t, then its density at time t + 1 is
Nt + 1 = Nt + [(B + I) – (D + E)]
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It is clear from the above equation, that population density increases if the number of births plus the number of immigrants (B +1) is more than the number of deaths plus the number of emigrants (D + E), otherwise it will decrease. Under normal conditions, births and deaths are the most important factors influencing population density. If a new habitat is just being colonized, than immigration may contribute more significantly to the population growth than birth rates.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

Differences between Natality and Mortality

Natality  Mortality
1. It refers to the number of births per unit population per unit time e.g., per thousand. per year in humans. 1. It refers to the number of deaths per unit population per unit time, e.g., per thousand individuals per year in humans.
2. It adds new members to the population.  2. It removes individuals from the population.
3. It increases the size of population.  3. It decreases the size of population.
4. Natality adds to population density.  4. Mortality reduces density of population.
5. It maintains continuity of population.  4. Mortality reduces density of population.
6. It is high when population size is small and low when population size is large.  6. Mortality is low when population size is small and high when population size is large.

Differences between Immigration and Emigration

Immigration  Emigration
1. It refers to the inward movement of some individuals into a local population. 1. It is a permanent outward movement of some individual from a local population.
2. It is caused by the availability of better living conditions. 2. It is caused by the occurrence of deficiencies and calamities.
3. It results in an increase in the size of gene pool and local population. 3. It results in a decrease in the size of gene pool and local population.

Growth Models:

The growth of a population with time shows specific pattern that can be predicted. The uncontrolled human population growth and the problems created by it in our country is a matter of concern. It is, therefore, essential to be curious if different animal populations in nature behave the same way. Perhaps, we can learn from nature, how to control population growth.

1. Exponential Growth:
→ The availability of resources (food and space) is essential for the unimpeded growth of a population. When the food and space in the habitat are unlimited, each species has the ability to realize fully its inherited potential to grow, as observed by Darwin while developing his theory of natural selection. Then the population grows in an exponential or geometric ratio.

→ If in a population of size n, the birth rate (per capita births) is represented as b and death rates (per capita death) as d, the increase or decrease in n during a unit time period t will be \(\frac{d n}{d t}\) Here, r is called ‘intrinsic rate of natural increase’. It is a very important parameter selected for assessing impacts of any biotic or abiotic factor on population growth.

→ Following are some examples to give some idea about the magnitude of r values.

→ For the Norway rat the r is 0.015, and for the flour beetle it is 0.12. The r value for human population in India in the year 1981,was 0.0205. The current value of r can be calculated by knowing the birth rates and death rates.

→ The equation given above describes the exponential or geometric growth pattern and results in a J-shaped curve when we plot n in relation to time. By applying basic calculus, you can derive the integral form of the exponential growth equation as nt = n0en

→ where, nt = Population density after time t; n0 = Population density at time zero; r = intrinsic rate of natural increase; e = the base of natural logarithms (2.71828).
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2nd PUC Biology Notes Chapter 13 Organisms and Populations

If any species is growing exponentially under unlimited resources, it can reach enormous population densities in a short time. Darwin showed how even a slow growing animal like elephant could reach unlimited numbers in the absence of checks.

2. Logistic Growth:
→ In nature, no population of any species gets unlimited resources to permit exponential growth. As a result, competition occurs between individuals for limited resources and only the fittest individual survives and reproduces. Many countries have also realized this fact and introduced various restraints with the view to limit human population growth. In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. The maximum number of individuals of a population that can be sustained indefinitely in a given habitat represents its carrying capacity (K).

→ If a population is growing in a habitat with limited resources, shows initially a lag phase, followed by phases of acceleration and deceleration and finally reaches a constant level, when the population density reaches the carrying capacity. In such a case, if the population size (N) is plotted over time (t) a S-shaped or sigmoid curve is obtained. This type of population growth is called Verhulst Pearl Logistic Growth. It is described by the following equation,
dn/dt = rN\(\left(\frac{\mathrm{K}-\mathrm{N}}{\mathrm{K}}\right)\)
N = Population density at a time t
r = Intrinsic rate of natural increase and
K = carrying capacity.
For most animal populations, the resources for growth are finite and become limiting sooner or later. The logistic growth model is considered to be a more realistic one.

Population Interaction:
In nature, no species can live in isolation. For any species there must be atleast one more species on which it can feed. Plants and animals show interdependence on each other. A plant species which makes its own food cannot survive alone, it needs soil microbes to break-down the organic matter so as the inorganic nutrients become available for reuse by the plant. Animals help in pollination and dispersal of fruits and seeds of plants.

Populations of different species often interact in various ways in their habitats.

Population interactions

Species A  Species B Name of interaction
+ + Mutualism
+ 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.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

1. Predation:
It is an interaction between members of two species in which membersof one species capture, kill and eat up, members of other species. The killer is called predator and the one getting killed is called prey. The predator cannot survive without the prey. They keep prey population under control. Without the predators, the prey species could multiply and produce high density and cause ecosystem instability. When any exotic species is introduced into a geographical area, they establish and spread fast because the invaded land does not have its natural predators. When prickly pear cactus (Opuntia) was introduced in Australia in early 1920’s, it started multiplying and spreadihg rapidly into millions of hectares of rangeland. But, it was controlled, only after a cactus-feeding predator (cochineal insect) introduced from its natural habitat.

→ Predators may increase the biodiversity of communities by preventing a single species from becoming dominant. Such predators are called Keystone species and may have a profound influence on the balance of organisms in a particular ecosystem.

→ When a predator is too efficient and over-exploits its prey, then the prey becomes extinct and following that, the predator also becomes extinct for lack of food. That is why the predators in nature are ‘prudent’.

→ 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). Since plants are stationary, they have evolved different morphological and chemical defences against herbivores, e.g., thorns in Acacia, Cactus and production of posionous chemicals in Calotropis. When herbivores eat such plant they feel sick. That is why, cattle or goats never browse on these plants.

2. Competition:
→ Competition is defined as ‘an interaction that occurs between two or more organisms when the resources necessary for them are limited and adversely affect them. Gause’s Competition Exclusion Principle, states that two closely related species competing for the same resources cannot co-exist independently and competitively and the inferior one will get eliminated eventually, e.g. When two species of Paramecium i.e, Paramecium caudatum and Paramecium aurelia are kept together in one habitat for long time, it was observed that one species got eliminated.

→ 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. The larger and competitively superior barnacle Balanus dominates the intertidal area, and excludes the smaller barnacle Chathamalus from that zone. In general, herbivores and plants appear to be more adversely affected by competition than carnivores.

3. Parasitism:
→ It is an interaction or relationship between two living organisms of different species in which one species (usually smaller) called parasite obtains its food directly from another living organism (usually larger) called host. The parasite spends a part or whole of its life on or in the body of the host. Many parasites are host specific (they can parasitise only a single species of host) where the 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 be successful with the same host species.

→ 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 parasitization of its primary host.

→ The parasites may be of the following types as listed below.

→ Ectoparasites are found on the outer, surface of host’s body e.g., head lice and humans, ticks on the skin of dogs, bed bugs etc.

→ Endoparasites are those that live inside the host body at different sites (liver, kidney, intestine, lungs, red blood cells etc.) The life cycle of endoparasite is more complex because of their extreme specialization, e.g., Malarial parasite in RBG of man, Ascaris and Taenia in the gut of man etc.

→ 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.
Eg : Cuckoo lays eggs in the nest of crow.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

4. Mutualism or symbiosis:
→ In which both the species are mutually benefited but association is obligatoiy and they cannot live separately under normal conditions.

→ e.g., mycorrhizae with association of fungus (e.g., Boletus) and roots of higher plants. The fungi help the plant in the absorption of water and essential nutrients from the soil while the plant in turn provides the fungi with carbohydrates.

→ Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.

→ Root nodules of legume plants, Casuarina, Alnus and leaf nodules of Ardisia have symbiotic association of nitrogen fixing bacteria – Rhizobium. The plants provide food and shelter to the bacteria and the bacteria fix free atmospheric nitrogen to the plant.

→ The most interesting example of mutualism are found in plant-animal relationships. Plants take the help of animals for pollinating their flowers and dispersing their seeds. Animals get rewards or fees in the form of pollen and nectar (for pollinators) and juicy and nutritious fruits (for seed dispersals).

→ In many species of fig trees, there is a tight one-to-one relationship with the pollinator species of wasp. 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. The Mediterranean orchid orchid ophrys employs sexual deceit to get pollination done by a species of bee. One petal of its flower bears an uncarry resemblance to the female of the bee in size, colour and markings.

→ The male bee is attracted to what is perceives as a female, ‘pseudo copulates’ with the flower and during that process is dusted with pollan from the flower. When this same bee pseudocopulates with another flower, it transfer pollen to it and thus pollinates the flower.

2nd PUC Biology Notes Chapter 13 Organisms and Populations

5. Commensalism:
→ This is the interaction in which one species benefits and the other is neither harmed nor benefited.

→ Example: 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.

→ Another example of commensalfsm is 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.

2nd PUC Biology Notes

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