Organisms and Populations
Welcome to HSLC Guru! In this chapter we explore the fascinating relationships between organisms and their environment. You will learn about major biomes, abiotic factors, organismal responses to stress, population attributes, growth models, and the various ways species interact with each other. This chapter forms the foundation of ecology and is highly important for the ASSEB Class 12 Biology examination.
Summary
Ecology is the study of organisms and their environment. The biosphere is divided into large regional units called biomes, each characterised by distinctive climate, vegetation and animal life. The major biomes of the world include the tundra (cold, treeless polar regions), taiga (coniferous forests of the sub-arctic), temperate deciduous forest (broad-leaved trees that shed leaves seasonally), tropical rainforest (warm, humid, biologically richest biome), desert (extremely dry with sparse vegetation) and grassland (dominated by grasses with few trees). Each biome supports characteristic flora and fauna adapted to its specific climatic conditions.
The most important abiotic factors that shape life on Earth are temperature, water, light and soil. Temperature affects the kinetics of enzymes and basal metabolism. Water availability determines whether an environment is aquatic, terrestrial or arid. Light provides energy for photosynthesis and regulates photoperiodic responses. Soil composition, pH and topography influence vegetation type and animal distribution. Organisms have evolved different responses to abiotic stress: regulators maintain a constant internal environment by physiological means (homeostasis) — only birds, mammals and a few lower invertebrates are true regulators. Conformers (poikilotherms) allow their body conditions to change with the environment. Many animals migrate to escape stressful conditions, while others suspend activity through hibernation (winter sleep), aestivation (summer sleep) or diapause (a stage of suspended development in zooplankton and insects).
A population is a group of individuals of the same species inhabiting a defined geographical area, sharing or competing for similar resources and capable of interbreeding. Population attributes include birth rate (natality), death rate (mortality), sex ratio and age distribution. The age distribution plotted in a histogram is called an age pyramid, which can be expanding, stable or declining. Population growth follows two main models: exponential growth (dN/dt = rN), which occurs under unlimited resources and gives a J-shaped curve, and logistic growth (dN/dt = rN((K-N)/K)), which accounts for the carrying capacity (K) of the environment and gives an S-shaped (sigmoid) curve. Life-history variations are evolutionary strategies that maximise reproductive fitness — some organisms breed once and die (semelparity) while others breed many times (iteroparity).
Populations of different species interact in many ways. Mutualism (+/+) benefits both partners, as in lichen (fungus and alga) and mycorrhiza (fungus and plant roots). Competition (-/-) occurs when two species need the same limited resource; Gause’s competitive exclusion principle states that two species competing for the same resource cannot coexist indefinitely. In predation (+/-), the predator kills and consumes the prey, an interaction that controls prey populations and transfers energy. Parasitism (+/-) involves a parasite living on (ectoparasite) or inside (endoparasite) a host, deriving nutrition at the host’s expense. Commensalism (+/0) benefits one species without affecting the other, while amensalism (-/0) harms one species without affecting the other.
1 Mark Questions
Q1. Define the term biome.
Answer: A biome is a large regional unit of the biosphere characterised by a distinctive climate, soil, vegetation and animal life.
Q2. What is homeostasis?
Answer: Homeostasis is the maintenance of a constant internal environment (such as body temperature and osmotic concentration) by an organism despite external changes.
Q3. Name the biome that has the highest biological diversity.
Answer: The tropical rainforest biome has the highest biological diversity.
Q4. What is aestivation?
Answer: Aestivation is a state of summer dormancy in which animals avoid heat and desiccation by becoming inactive (e.g., snails and certain frogs).
Q5. Define carrying capacity.
Answer: Carrying capacity (K) is the maximum population size of a species that an environment can sustain indefinitely with the available resources.
Q6. Give two examples of mutualism.
Answer: Lichen (association between a fungus and an alga) and mycorrhiza (association between a fungus and the roots of a higher plant).
Q7. Who proposed the competitive exclusion principle?
Answer: The competitive exclusion principle was proposed by the Russian ecologist G. F. Gause.
Q8. What is the equation for exponential population growth?
Answer: The equation is dN/dt = rN, where N is the population size, r is the intrinsic rate of natural increase and t is time.
Q9. Differentiate between ectoparasite and endoparasite in one line.
Answer: Ectoparasites live on the outer body surface of the host (e.g., lice), whereas endoparasites live inside the host’s body (e.g., tapeworm).
Q10. What is diapause?
Answer: Diapause is a stage of suspended development seen in many zooplankton and insects under unfavourable conditions.
2-3 Marks Questions
Q1. Differentiate between regulators and conformers.
Answer: Regulators are organisms that maintain a constant internal environment (homeostasis) regardless of external changes by physiological and behavioural means; only birds, mammals and a few lower invertebrates are true regulators. Conformers, also called poikilotherms, are organisms whose internal body temperature and osmotic concentration change with the surrounding environment. The vast majority of animals (about 99%) and nearly all plants are conformers because regulation is energetically expensive.
Q2. Explain the term population and list its main attributes.
Answer: A population is a group of individuals of the same species inhabiting a defined geographical area, sharing or competing for similar resources and capable of interbreeding among themselves. The main attributes of a population are: (i) birth rate (natality), (ii) death rate (mortality), (iii) sex ratio (the ratio of males to females), and (iv) age distribution (the proportion of individuals in different age classes, often shown as an age pyramid).
Q3. What is migration? Give one example.
Answer: Migration is the seasonal movement of animals from one habitat to another to escape stressful conditions such as severe cold, lack of food or unsuitable breeding conditions, and to return when conditions become favourable again. For example, the Siberian crane migrates from Siberia to Keoladeo National Park (Bharatpur), Rajasthan, India during winter.
Q4. Briefly describe the logistic model of population growth.
Answer: When resources in an environment are limited, populations grow according to the logistic model. The equation is dN/dt = rN((K-N)/K), where N is population size, r is the intrinsic rate of increase and K is the carrying capacity. Initially the population grows slowly (lag phase), then rapidly (log phase), and finally levels off as it approaches K (asymptote). The resulting growth curve is sigmoid or S-shaped and is considered more realistic than the exponential model because no environment has infinite resources.
Q5. Distinguish between commensalism and amensalism.
Answer: Commensalism (+/0) is an interaction in which one species is benefited while the other is neither benefited nor harmed (e.g., orchids growing on a mango tree). Amensalism (-/0) is an interaction in which one species is harmed while the other is unaffected (e.g., the bread mould Penicillium produces penicillin, which kills bacteria, without affecting the mould itself).
Q6. Mention any three adaptations of desert plants.
Answer: (i) Desert plants such as Opuntia have leaves modified into spines and the photosynthesis is taken over by the green flattened stem to reduce water loss. (ii) They possess a thick waxy cuticle on the body surface to minimise transpiration. (iii) Many follow the CAM (Crassulacean Acid Metabolism) photosynthetic pathway, in which stomata remain closed during the day and open at night to conserve water.
5-7 Marks Questions
Q1. Describe the major biomes of the world with their characteristic features.
Answer: The major biomes of the world are described below:
(i) Tundra: Found in the Arctic and on high mountain tops, the tundra is treeless, with permanently frozen subsoil (permafrost). Vegetation consists of mosses, lichens and dwarf shrubs. Animals include reindeer, musk ox, polar bear and arctic fox.
(ii) Taiga (Coniferous forest): Located south of the tundra, this biome has long cold winters and short summers. It is dominated by evergreen conifers such as pine, fir and spruce. Typical fauna includes wolves, bears, moose and lynx.
(iii) Temperate deciduous forest: Found in temperate regions with moderate rainfall and four distinct seasons. Trees such as oak, maple and beech shed their leaves in autumn. Animals include deer, foxes, squirrels and many songbirds.
(iv) Tropical rainforest: Located near the equator with high rainfall and warm temperatures throughout the year. It has the greatest species diversity of any biome and shows distinct vertical stratification (canopy, understory, forest floor). Examples include the Amazon and Congo rainforests.
(v) Desert: Characterised by very low rainfall (less than 25 cm per year) and extreme temperature fluctuations. Vegetation is sparse and includes cacti and thorny shrubs. Animals such as camels, lizards and rodents have remarkable adaptations to conserve water.
(vi) Grassland: Receives moderate rainfall (25-75 cm) and is dominated by grasses with few scattered trees. Examples include the savanna of Africa, the prairies of North America and the steppes of Eurasia. Large herbivores such as zebras, bison and wildebeest are typical of this biome.
Q2. Explain the various responses (adaptations) of organisms to abiotic stress.
Answer: Organisms cope with abiotic stress in four main ways:
(i) Regulate: Some organisms maintain a constant internal environment (homeostasis) through physiological and biochemical means. They keep their body temperature, osmotic concentration and other internal conditions stable regardless of external changes. Only birds, mammals and a small number of lower invertebrates are capable of true regulation. For example, humans maintain a body temperature of about 37 degrees Celsius despite ambient temperature changes.
(ii) Conform: A vast majority of animals (~99%) and nearly all plants cannot maintain a constant internal environment. Their body temperature changes with ambient temperature; in aquatic forms the osmotic concentration of body fluids changes with that of the surrounding water. Such organisms are called conformers or poikilotherms.
(iii) Migrate: Many animals temporarily move away from a stressful habitat to a more hospitable area and return when the stressful period is over. The Siberian crane migrating to Keoladeo National Park is a classic example.
(iv) Suspend: Some organisms suspend or reduce their metabolic activity to survive unfavourable periods. Bears hibernate during winter (winter sleep). Snails and some fish aestivate during dry summer (summer sleep). Many zooplankton and insects undergo diapause, a stage of suspended development. In plants, seeds and other vegetative reproductive structures often serve as means to tide over periods of stress.
Q3. Compare exponential and logistic models of population growth, deriving their equations.
Answer: When resources are unlimited, every species shows its full intrinsic potential to grow in number — this is exponential growth. If a population of size N has a per capita birth rate b and a per capita death rate d during a unit time period, then the rate of change in population is given by:
dN/dt = (b – d) N = rN
Where r = (b – d) is the intrinsic rate of natural increase. Plotting N against t produces a J-shaped curve. The equation in integrated form is Nt = N0 ert.
However, since resources are finite in nature, exponential growth cannot continue indefinitely. The environment can support only a maximum number of individuals, called the carrying capacity (K). The logistic growth model takes K into account:
dN/dt = rN ((K – N) / K)
The growth curve is sigmoid (S-shaped) with four phases: lag phase (slow growth), log phase (rapid growth), deceleration phase (slowing growth) and asymptote (population stabilises at K). The logistic model is regarded as a more realistic description of population growth in nature because no habitat has unlimited resources.
Q4. Describe the various types of population interactions with examples.
Answer: Populations of different species interact in several ways:
(i) Mutualism (+/+): Both species benefit. Example: lichens (fungus + alga) and mycorrhiza (fungus + plant roots). The fig tree and its pollinator wasp also share an obligate mutualism.
(ii) Competition (-/-): Both species are harmed because they compete for the same limiting resource. Gause’s competitive exclusion principle states that two closely related species competing for the same resource cannot coexist indefinitely; the inferior competitor will be eventually eliminated.
(iii) Predation (+/-): The predator benefits while the prey is harmed (killed and eaten). Predation transfers energy across trophic levels, controls prey populations and is essential for maintaining biodiversity. For example, lion preying on deer.
(iv) Parasitism (+/-): The parasite benefits at the cost of the host. Parasites that live on the body surface of the host are called ectoparasites (e.g., lice on humans, ticks on dogs); those that live inside the host’s body are called endoparasites (e.g., Plasmodium and tapeworms in humans). Brood parasitism in birds (e.g., the cuckoo laying eggs in the crow’s nest) is also well known.
(v) Commensalism (+/0): One species benefits and the other is neither harmed nor benefited. Example: an orchid growing on a mango tree, or barnacles attached to whales.
(vi) Amensalism (-/0): One species is harmed while the other is unaffected. Example: Penicillium produces penicillin which kills bacteria but does not benefit Penicillium itself.
Q5. Describe the major abiotic factors that influence the distribution and survival of organisms.
Answer: The four key abiotic factors are:
(i) Temperature: Temperature affects the kinetics of enzymes and basal metabolism. It varies from sub-zero in polar regions to over 50 degrees Celsius in deserts. Eurythermal organisms tolerate a wide range of temperatures, while stenothermal ones tolerate only a narrow range. Body forms and physiological adaptations such as fur, sweat glands and migration patterns are all temperature-related.
(ii) Water: Water is essential for all life. On land, water availability determines whether an area is forest, grassland or desert. In aquatic habitats, the salt concentration (salinity) of water is critical. Euryhaline organisms tolerate a wide salinity range; stenohaline organisms tolerate only a narrow range.
(iii) Light: Light is the ultimate source of energy for photosynthesis. The intensity, duration and quality of light influence plant growth, flowering (photoperiodism), animal foraging, breeding and migration. In deep oceans, the absence of light supports unique chemosynthetic communities.
(iv) Soil: Soil characteristics such as texture, depth, mineral composition, pH and water-holding capacity determine the type of vegetation. The vegetation in turn influences the animal community. Soil microorganisms play vital roles in nutrient cycling.
Multiple Choice Questions (MCQ)
Q1. Which of the following is NOT a major biome?
(a) Tundra (b) Taiga (c) Estuary (d) Tropical rainforest
Answer: (c) Estuary
Q2. The intrinsic rate of natural increase is denoted by:
(a) K (b) r (c) N (d) t
Answer: (b) r
Q3. The competitive exclusion principle was given by:
(a) Darwin (b) Tansley (c) Gause (d) Odum
Answer: (c) Gause
Q4. Lichen is an example of:
(a) Predation (b) Mutualism (c) Parasitism (d) Amensalism
Answer: (b) Mutualism
Q5. Which of the following animals is a true regulator?
(a) Frog (b) Fish (c) Bird (d) Earthworm
Answer: (c) Bird
Q6. The S-shaped population growth curve is associated with:
(a) Exponential growth (b) Logistic growth (c) Geometric growth (d) Arithmetic growth
Answer: (b) Logistic growth
Q7. Tapeworm in the human intestine is an example of:
(a) Ectoparasite (b) Endoparasite (c) Commensal (d) Mutualist
Answer: (b) Endoparasite
Q8. Aestivation is shown by:
(a) Bears in winter (b) Snails in summer (c) Insects during favourable periods (d) Fishes in spawning season
Answer: (b) Snails in summer
Q9. The maximum population size that an environment can support is called:
(a) Biotic potential (b) Carrying capacity (c) Population density (d) Birth rate
Answer: (b) Carrying capacity
Q10. Mycorrhiza is an association between:
(a) Two algae (b) Two fungi (c) Fungi and roots of higher plants (d) Algae and bacteria
Answer: (c) Fungi and roots of higher plants
Fill in the Blanks
Q1. The biome with the highest biodiversity is the __________.
Answer: tropical rainforest
Q2. The exponential growth equation is dN/dt = __________.
Answer: rN
Q3. Animals whose body temperature changes with ambient temperature are called __________.
Answer: poikilotherms (conformers)
Q4. The interaction in which one species is benefited and the other is unaffected is called __________.
Answer: commensalism
Q5. The Siberian crane migrates to __________ National Park in Rajasthan.
Answer: Keoladeo
True or False
Q1. Tundra biome supports dense forest vegetation.
Answer: False
Q2. All plants are conformers.
Answer: True
Q3. Logistic growth produces a J-shaped curve.
Answer: False
Q4. Lice are ectoparasites of humans.
Answer: True
Q5. Diapause is observed in mammals during winter.
Answer: False
Glossary
| Term | Definition |
|---|---|
| Biome | A large regional unit of the biosphere with characteristic climate, vegetation and animal life. |
| Homeostasis | Maintenance of a constant internal environment by an organism. |
| Conformer | An organism whose internal conditions vary with the surroundings (poikilotherm). |
| Regulator | An organism that maintains a constant internal environment by physiological means. |
| Migration | Seasonal movement of animals to escape stress and return when conditions improve. |
| Hibernation | Winter sleep undergone by some animals to escape cold. |
| Aestivation | Summer sleep undergone by animals to avoid heat and desiccation. |
| Diapause | A stage of suspended development in zooplankton and insects. |
| Population | A group of individuals of the same species inhabiting a given area. |
| Birth rate (Natality) | Number of births per individual per unit time. |
| Death rate (Mortality) | Number of deaths per individual per unit time. |
| Sex ratio | Proportion of males to females in a population. |
| Age pyramid | Graphical representation of age distribution in a population. |
| Carrying capacity (K) | Maximum population size an environment can sustain. |
| Exponential growth | Growth under unlimited resources following dN/dt = rN. |
| Logistic growth | Growth under limited resources following dN/dt = rN((K-N)/K). |
| Mutualism | Interaction in which both species benefit (+/+). |
| Competition | Interaction in which both species are harmed (-/-). |
| Predation | Interaction in which a predator kills and eats prey (+/-). |
| Parasitism | Interaction in which a parasite benefits at the host’s cost (+/-). |
| Ectoparasite | Parasite that lives on the body surface of the host. |
| Endoparasite | Parasite that lives inside the host’s body. |
| Commensalism | Interaction in which one species benefits and the other is unaffected (+/0). |
| Amensalism | Interaction in which one species is harmed and the other is unaffected (-/0). |
| Gause’s principle | Two species competing for the same resource cannot coexist indefinitely. |