Q: Q3. Most living organisms are stenothermal. Is this true or false? Give reasons.

True.\n\n**Reason:**\nMost living organisms are indeed stenothermal. Stenothermal organisms are those that can tolerate only a narrow range of temperatures. Their metabolic processes and enzyme systems are optimized to function efficiently within a specific, limited temperature window. Even slight deviations from this optimal range can cause significant physiological stress, inhibit enzyme activity, or even denature proteins, leading to impaired function or death.\n\nWhile there are some organisms, known as eurythermal organisms (e.g., certain bacteria, archaea, some fish like carp, and intertidal invertebrates), that can tolerate a wide range of temperatures, they represent a minority. The vast majority of plants, animals, and microorganisms are adapted to specific thermal conditions found in their natural habitats and cannot survive significant fluctuations. This is why geographical distribution of species is often strongly correlated with temperature zones.

Q: Q14. An orchid plant is growing on the branch of mango tree. How do you describe this interaction?

This interaction is described as **commensalism**.\n\n**Commensalism** is an interspecific interaction where one species benefits, and the other species is neither harmed nor benefited. In this specific case:\n\n* **Orchid (the commensal):** The orchid benefits by growing on the branch of the mango tree. It gets support, a suitable place to grow (often higher up to receive more sunlight), and access to moisture and nutrients from the air and accumulating debris. It does not derive nutrition directly from the mango tree.\n* **Mango tree (the host):** The mango tree is neither harmed nor benefited by the presence of the orchid. The orchid uses the tree merely as a substrate for support and does not extract water or nutrients from the tree, nor does it significantly shade the tree or compete for resources.\n\nTherefore, it's a classic example of commensalism, where the orchid benefits from the association, and the mango tree remains unaffected.

Question 1

Q1. How is diapause different from hibernation?

Accepted Answer

Diapause and hibernation are both states of dormancy, which organisms enter to cope with adverse environmental conditions. However, they differ in several key aspects:

**Hibernation:**
Hibernation is a state of metabolic depression in endothermic (warm-blooded) animals, primarily mammals and some birds. It is characterized by inactivity and a dramatic drop in body temperature, heart rate, breathing rate, and metabolic rate. Organisms enter hibernation typically during winter months to conserve energy when food is scarce and temperatures are very low. It is a physiological response to cold and lack of resources.

**Diapause:**
Diapause is a state of suspended development or arrested growth, typically observed in insects and some other invertebrates (like zooplankton). It is a genetically determined, hormonally controlled state of dormancy that occurs at a specific stage of development (e.g., egg, larva, pupa, or adult). Diapause is often an anticipatory response, triggered by environmental cues like changes in photoperiod (day length) or temperature, in anticipation of future adverse conditions such as extreme cold, drought, or lack of food. It allows the organism to 'pause' its life cycle until favorable conditions return.

**Key Differences Summarized:**
| Feature          | Hibernation                                     | Diapause                                          |
| :--------------- | :---------------------------------------------- | :------------------------------------------------ | 
| **Organisms**    | Primarily mammals (e.g., bears, ground squirrels, bats) and some birds. | Primarily insects and some other invertebrates (e.g., zooplankton). |
| **Trigger**      | Adverse cold temperatures and food scarcity during winter. | Often anticipatory; triggered by photoperiod, temperature, or drought. |
| **Physiological State** | Reduced metabolic rate, significant drop in body temperature, heart rate, and respiration. | Suspended development or arrested growth; metabolic activity is greatly reduced but not necessarily a drop in body temperature (as often poikilothermic). |
| **Control**      | Primarily a physiological response to current environmental conditions. | Genetically programmed and hormonally controlled; often an anticipatory strategy. |
| **Duration**     | Typically for a season (e.g., winter).          | Can last for varying periods, often tied to specific developmental stages. |

Question 2

Q2. If a marine fish is placed in a freshwater aquarium, will the fish be able to survive? Why or why not?

Accepted Answer

No, a marine fish will generally not be able to survive for long if placed in a freshwater aquarium.

**Reason:**
This is due to the fundamental differences in osmoregulation required for survival in marine versus freshwater environments.

1.  **Marine Fish Adaptations:** Marine fish live in a hypertonic environment, meaning the surrounding seawater has a higher salt concentration than their body fluids. To counteract the constant tendency for water to leave their bodies (via osmosis) and salts to enter, marine fish have evolved specific osmoregulatory mechanisms:
    *   They constantly drink seawater.
    *   They excrete excess salts through specialized chloride cells in their gills.
    *   They produce a small volume of highly concentrated urine to conserve water.

2.  **Freshwater Environment Impact:** When a marine fish is placed in freshwater, it is suddenly in a hypotonic environment (water with a much lower salt concentration than its body fluids). In this situation:
    *   Water will rapidly move by osmosis from the freshwater into the fish's body across its gills and skin, causing the fish's cells to swell.
    *   Essential salts from the fish's body will diffuse out into the surrounding freshwater.
    *   The marine fish's osmoregulatory system is designed to conserve water and excrete salt, not to excrete excess water and conserve salt. It lacks the physiological mechanisms (like large glomeruli for filtering large volumes of water and active salt absorption by gills) that freshwater fish possess to cope with this influx of water and loss of salts.

Consequently, the marine fish will be unable to maintain its internal osmotic balance, leading to cellular swelling, disruption of physiological processes, and ultimately death.

Question 3

Q3. Most living organisms are stenothermal. Is this true or false? Give reasons.

Accepted Answer

True.

**Reason:**
Most living organisms are indeed stenothermal. Stenothermal organisms are those that can tolerate only a narrow range of temperatures. Their metabolic processes and enzyme systems are optimized to function efficiently within a specific, limited temperature window. Even slight deviations from this optimal range can cause significant physiological stress, inhibit enzyme activity, or even denature proteins, leading to impaired function or death.

While there are some organisms, known as eurythermal organisms (e.g., certain bacteria, archaea, some fish like carp, and intertidal invertebrates), that can tolerate a wide range of temperatures, they represent a minority. The vast majority of plants, animals, and microorganisms are adapted to specific thermal conditions found in their natural habitats and cannot survive significant fluctuations. This is why geographical distribution of species is often strongly correlated with temperature zones.

Question 4

Q4. What are the different ways organisms cope with abiotic stresses? Explain with suitable examples.

Accepted Answer

Organisms have evolved various strategies to cope with abiotic stresses (such as extreme temperatures, water scarcity, salinity, etc.) in their environment. These responses can be broadly categorized into four main types:

### 1. Regulate
Some organisms are able to maintain a constant internal environment (homeostasis) despite fluctuating external conditions. This process is called regulation. Regulators are typically homeostatic, meaning they can maintain a stable body temperature (thermoregulation) and osmotic concentration (osmoregulation).

*   **Mechanism:** They expend energy to maintain a constant internal environment. For instance, mammals maintain a constant body temperature of 37°C. In summer, they sweat to cool down, and in winter, they shiver to generate heat. Plants, to some extent, also regulate their internal water potential through stomatal movements.
*   **Examples:**
    *   **Mammals and Birds:** Maintain a constant body temperature regardless of ambient temperature.
    *   **Some Vertebrates and Invertebrates:** Maintain constant osmotic concentration in their body fluids.
*   **Cost:** Regulation is energetically expensive, which is why it is more common in larger, more complex organisms.

### 2. Conform
Most organisms cannot maintain a constant internal environment and allow their internal conditions to change with the external environment. These organisms are called conformers.

*   **Mechanism:** Their body temperature or osmotic concentration fluctuates with the ambient conditions. For example, the body temperature of most animals (like fish, amphibians, reptiles) and nearly all plants changes with the ambient temperature. Similarly, the osmotic concentration of many aquatic animals varies with that of the surrounding water.
*   **Examples:**
    *   **Most Animals (Poikilotherms/Ectotherms):** Fish, amphibians, reptiles, and most invertebrates. Their body temperature is largely determined by the external temperature.
    *   **Almost All Plants:** They do not have mechanisms to regulate their internal temperature or water potential to the same extent as animals.
*   **Cost:** Conforming is energetically less expensive but limits the organisms to a narrow range of environmental conditions where they can survive and thrive.

### 3. Migrate
Some organisms cope with localized or temporary stressful conditions by moving away temporarily from the stressful habitat to a more hospitable area and returning when stressful period is over.

*   **Mechanism:** This is a behavioral response, involving movement over short or long distances.
*   **Examples:**
    *   **Migratory Birds:** Many bird species undertake long-distance migrations to warmer regions during winter (e.g., Siberian cranes migrate to Keoladeo National Park in Bharatpur, Rajasthan, from Siberia).
    *   **Fish:** Some fish species migrate for spawning or to find better feeding grounds.
*   **Cost:** Migration requires significant energy expenditure and exposes individuals to new predators and environmental challenges.

### 4. Suspend
In some cases, if migration is not possible, organisms may avoid the stress by suspending their metabolic activities or development. This state of dormancy allows them to survive unfavorable conditions.

*   **Mechanism:** Entering a state of reduced metabolic activity or arrested development.
*   **Examples:**
    *   **Hibernation (Winter Sleep):** Some animals (e.g., bears, ground squirrels, bats) go into a state of inactivity and metabolic depression during winter to escape cold and food scarcity.
    *   **Aestivation (Summer Sleep):** Some animals (e.g., snails, some fish) go into a state of dormancy during hot and dry periods to avoid heat and desiccation.
    *   **Diapause:** Many zooplankton species in lakes and ponds, and many insects, enter a state of suspended development during unfavorable conditions.
    *   **Spore/Seed Formation:** Bacteria, fungi, and lower plants form thick-walled spores, and higher plants produce seeds, which are dormant structures that can withstand harsh conditions and germinate when conditions become favorable.
*   **Cost:** Loss of active life, potential vulnerability during dormancy.

### 5. Adaptations (Long-term Evolutionary Changes)
Beyond these immediate responses, organisms also exhibit long-term evolutionary adaptations (morphological, physiological, and behavioral) that allow them to survive and thrive in specific abiotic conditions.

*   **Examples:**
    *   **Desert Plants (Xerophytes):** Have thick cuticles, sunken stomata, CAM pathway, and reduced leaves (spines) to minimize water loss.
    *   **Desert Animals (e.g., Kangaroo Rat):** Can meet all its water requirements from internal fat oxidation, never drinks water, and produces highly concentrated urine.
    *   **Arctic Fish:** Possess antifreeze proteins to prevent their body fluids from freezing.
    *   **High Altitude Adaptations:** People living at high altitudes develop increased red blood cell production, higher breathing rate, and increased binding affinity of hemoglobin to cope with low atmospheric oxygen pressure.

Question 5

Q5. Elaborate on how the following factors influence an organism:
(a) Light
(b) Temperature
(c) Water
(d) Soil

Accepted Answer

Abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Here's how the specified factors influence organisms:

### (a) Light
Light is a crucial abiotic factor, primarily because it is the ultimate source of energy for most ecosystems.

1.  **Photosynthesis:** For autotrophs (plants, algae, cyanobacteria), light is essential for photosynthesis, the process by which they convert light energy into chemical energy. The intensity, quality (wavelength), and duration (photoperiod) of light directly affect primary productivity.
2.  **Photoperiodism:** The duration of light (photoperiod) influences many biological rhythms and processes in both plants and animals. In plants, it triggers flowering (short-day, long-day, day-neutral plants). In animals, it affects reproductive cycles, migration, hibernation, and daily activity patterns (diurnal, nocturnal, crepuscular).
3.  **Light Intensity:** Different plants are adapted to different light intensities (heliophytes require high light, sciophytes prefer low light). In aquatic environments, light penetration determines the depth to which photosynthetic organisms can grow, leading to distinct vertical stratification of life.
4.  **Vision and Behavior:** For many animals, light is critical for vision, navigation, foraging, and predator avoidance. The absence of light in deep-sea environments has led to unique adaptations like bioluminescence.

### (b) Temperature
Temperature is the most ecologically relevant environmental factor, as it profoundly affects the kinetics of enzymes and the metabolic activity of organisms.

1.  **Metabolic Rate:** Every organism has an optimal temperature range for its enzymes to function efficiently. Deviations from this range can slow down metabolic reactions or even denature enzymes, leading to physiological stress or death.
2.  **Geographical Distribution:** Temperature largely determines the geographical distribution of species. For example, polar bears are found in cold regions, while cacti thrive in hot deserts. Organisms are classified as stenothermal (tolerate narrow temperature range) or eurythermal (tolerate wide temperature range).
3.  **Thermoregulation:** Organisms have evolved various strategies to cope with temperature fluctuations. Endotherms (warm-blooded animals like mammals and birds) regulate their internal body temperature, while ectotherms (cold-blooded animals like fish, amphibians, reptiles) largely conform to ambient temperatures.
4.  **Reproduction and Development:** Temperature can influence reproductive success, developmental rates, and even sex determination in some reptiles (e.g., turtles, crocodiles).
5.  **Dormancy:** Extreme temperatures can induce dormancy states like hibernation (winter sleep) and aestivation (summer sleep) in animals.

### (c) Water
Water is the most crucial factor influencing the life of organisms, as it constitutes 70-90% of the cellular mass and is essential for all biochemical reactions.

1.  **Life's Medium:** Water is the universal solvent and the medium for all metabolic processes within cells. Its availability directly impacts an organism's survival and physiological functions.
2.  **Productivity and Distribution:** Water availability is the primary factor determining the productivity of terrestrial ecosystems. The distribution of plants (hydrophytes, mesophytes, xerophytes) and animals is strongly linked to water availability.
3.  **Osmoregulation:** Organisms must maintain a constant internal water and salt balance (osmoregulation). Marine organisms face desiccation, freshwater organisms face water influx, and terrestrial organisms face desiccation, leading to diverse adaptations.
4.  **Adaptations:** Organisms in water-scarce environments (deserts) have evolved remarkable adaptations to conserve water, such as thick cuticles, sunken stomata, CAM photosynthesis in plants, and concentrated urine, nocturnal activity, and metabolic water production in animals (e.g., kangaroo rat).
5.  **Habitat:** Water bodies provide unique habitats for a vast array of aquatic organisms, influencing their morphology, physiology, and behavior.

### (d) Soil
Soil is the uppermost layer of the Earth's crust, providing anchorage and nutrients for terrestrial plants and serving as a habitat for numerous organisms.

1.  **Substrate and Anchorage:** Soil provides the physical support and anchorage for terrestrial plants, allowing them to grow upright and access sunlight.
2.  **Water and Nutrient Availability:** The composition of soil (particle size, aggregation, pH, mineral content) determines its water-holding capacity and the availability of essential nutrients (nitrogen, phosphorus, potassium, etc.) to plants. These edaphic (soil-related) factors directly influence plant growth and distribution.
3.  **Habitat for Organisms:** Soil is a complex ecosystem itself, teeming with a diverse community of microorganisms (bacteria, fungi), invertebrates (worms, insects, nematodes), and small vertebrates. These organisms play vital roles in decomposition, nutrient cycling, and soil aeration.
4.  **Soil Profile:** Different layers of soil (horizons) have distinct physical and chemical properties, supporting different types of organisms and influencing root penetration.
5.  **pH and Salinity:** Soil pH affects nutrient solubility and microbial activity, while salinity can be a major stress factor, especially in arid and semi-arid regions, influencing the types of plants that can grow.

Question 6

Q6. List the attributes that populations but not individuals possess.

Accepted Answer

Populations, as a group of individuals of the same species living in a well-defined geographical area, possess certain attributes that are not characteristic of an individual organism. These collective attributes are:

1.  **Birth Rate (Natality):** It refers to the per capita births in a population over a given period. An individual gives birth, but a birth rate is a population characteristic.
2.  **Death Rate (Mortality):** It refers to the per capita deaths in a population over a given period. An individual dies, but a death rate is a population characteristic.
3.  **Sex Ratio:** It is the ratio of males to females in a population. An individual is either male or female, but a sex ratio describes the population structure.
4.  **Age Distribution:** It describes the proportion of individuals of different age groups (pre-reproductive, reproductive, post-reproductive) in a population. An individual has an age, but age distribution is a population characteristic, often represented by age pyramids.
5.  **Population Density:** It is the number of individuals per unit area or volume at a given time. An individual occupies space, but density is a measure of the population's size relative to its space.
6.  **Growth Forms:** Populations exhibit characteristic growth patterns (e.g., exponential or logistic growth) over time, which are not applicable to individuals.

Question 7

Q7. If a population growing exponentially doubles in size in 3 years, what is the intrinsic rate of natural increase (r) of the population?

Accepted Answer

To find the intrinsic rate of natural increase (r) for a population growing exponentially that doubles in size in a given time, we can use the formula for doubling time in exponential growth.

**Given:**
Doubling time (t_d) = 3 years

**Formula for doubling time in exponential growth:**
t_d = ln(2) / r

Where:
t_d = doubling time
ln(2) ≈ 0.693 (natural logarithm of 2)
r = intrinsic rate of natural increase

**Rearranging the formula to solve for r:**
r = ln(2) / t_d

**Substituting the given values:**
r = 0.693 / 3 years
r = 0.231 per year

Therefore, the intrinsic rate of natural increase (r) of the population is 0.231 per year.

Question 8

Q8. Name important defence mechanisms in plants against herbivory.

Accepted Answer

Plants have evolved a wide array of defence mechanisms to protect themselves from herbivores. These mechanisms can be broadly categorized into morphological and chemical defences.

### I. Morphological Defences
These involve physical structures that deter herbivores.

1.  **Thorns and Spines:** Many plants, such as *Acacia*, *Cactus*, and *Rosa* (rose), develop sharp thorns or spines on their stems and leaves. These structures make it difficult and painful for herbivores to feed on them.
2.  **Prickles:** Similar to thorns, prickles are sharp outgrowths of the epidermis (e.g., on roses) that deter grazing animals.
3.  **Trichomes:** Some plants have hair-like structures (trichomes) on their leaves and stems. These can be stiff and irritating, or glandular, releasing sticky or toxic substances (e.g., stinging nettle).
4.  **Toughness/Sclerophyllous Leaves:** Leaves with thick cuticles, high lignin content, or a leathery texture are harder to chew and digest, making them less palatable (e.g., many evergreen plants).
5.  **Waxes and Resins:** A waxy or resinous coating on leaves can make them slippery, difficult to grasp, or unpalatable.

### II. Chemical Defences
Plants produce a vast array of secondary metabolites that are not directly involved in primary metabolic processes but serve as potent deterrents or toxins to herbivores. These chemicals can be:

1.  **Toxins:** Directly poisonous to herbivores.
    *   **Nicotine:** Found in tobacco plants, it is a potent neurotoxin.
    *   **Caffeine:** Present in coffee, tea, and cocoa, it acts as a stimulant and can be toxic in high doses.
    *   **Opium alkaloids:** Derived from poppy plants, these include morphine and codeine, which are narcotics.
    *   **Cardiac Glycosides:** Produced by plants like *Calotropis* (milkweed), these chemicals interfere with heart function and are highly toxic to many animals.
    *   **Quinine:** From the Cinchona tree, it has anti-malarial properties but is bitter and can be toxic to some herbivores.
    *   **Cyanogenic Glycosides:** Release hydrogen cyanide upon tissue damage, which is highly toxic (e.g., cassava, wild cherry).

2.  **Digestibility Reducers:** These chemicals interfere with the digestion of plant material by herbivores.
    *   **Tannins:** Found in many plants (e.g., oak, tea), tannins bind to proteins, reducing their digestibility and making the plant material astringent and unpalatable.
    *   **Resins and Gums:** Can make plant tissues sticky and difficult to digest.
    *   **Lignins:** Make plant tissues tough and indigestible.

3.  **Growth Regulators/Hormone Mimics:** Some plants produce compounds that mimic insect hormones, disrupting their growth and development (e.g., phytoecdysteroids).

### III. Indirect Defences
Plants can also employ indirect defences by attracting natural enemies of herbivores.

1.  **Volatile Organic Compounds (VOCs):** When attacked by herbivores, some plants release specific VOCs that attract parasitic wasps or predatory mites, which then attack the herbivores.
2.  **Extrafloral Nectaries:** Some plants produce nectar outside of flowers to attract ants, which then protect the plant from herbivores.

These diverse defence mechanisms highlight the co-evolutionary arms race between plants and herbivores.

Question 9

Q9. An orchid plant is growing on the branch of mango tree. How do you describe this interaction?

Accepted Answer

The interaction between an orchid plant growing on the branch of a mango tree is described as **commensalism**.

**Commensalism** is an interspecific interaction where one species benefits, and the other species is neither harmed nor benefited. It can be represented as (+, 0).

In this specific case:
*   **Orchid (the commensal):** Benefits by getting support, access to sunlight (being higher up), and better air circulation without expending energy to grow tall itself. It does not derive nutrition from the mango tree; it synthesizes its own food through photosynthesis and absorbs moisture and nutrients from the atmosphere and accumulated debris on the branch.
*   **Mango tree (the host):** Is neither harmed nor benefited by the presence of the orchid. The orchid does not parasitize the mango tree for nutrients or water, nor does it significantly impede the tree's growth or access to resources.

Therefore, it is a classic example of commensalism.

Question 10

Q10. What is the significance of the equation:
N_t = N_0 e^(rt)
where N_t = population density after time t
N_0 = population density at time zero
r = intrinsic rate of natural incre

Accepted Answer

The equation N_t = N_0 e^(rt) is a fundamental mathematical model used in ecology to describe **exponential population growth**. Its significance lies in explaining how populations grow under ideal conditions and providing insights into the potential for rapid population increase.

### I. Description of Exponential Growth
This equation models population growth when resources (food, space, etc.) are unlimited, and there are no predators, diseases, or other environmental resistances. Under such conditions, the population grows at an accelerating rate, meaning the rate of increase itself increases with population size. This type of growth is often observed in newly introduced species in a suitable habitat or during periods of abundant resources.

### II. Significance of 'r' (Intrinsic Rate of Natural Increase)
1.  **Measure of Biotic Potential:** The 'r' (intrinsic rate of natural increase) is a crucial parameter. It represents the maximum potential rate at which a population can increase under ideal conditions (i.e., unlimited resources, no competition, no predation). It is a species-specific characteristic, though it can vary with environmental factors.
2.  **Indicator of Fitness:** A higher 'r' value generally indicates a species with a greater capacity for reproduction and survival, often associated with 'r-selected' species (e.g., bacteria, insects, weeds) that produce many offspring with little parental care.
3.  **Predictive Power:** 'r' allows ecologists to predict the future size of a population if conditions remain ideal. For example, a positive 'r' means the population is growing, while a negative 'r' indicates a declining population.

### III. Graphical Representation (J-shaped Curve)
When N_t is plotted against time (t), the exponential growth equation produces a **J-shaped growth curve**. This curve starts slowly and then rises steeply as the population size increases, reflecting the ever-increasing rate of growth. This shape signifies that the population is growing without any checks or limits.

### IV. Ecological Implications
1.  **Understanding Population Explosions:** The equation helps explain phenomena like bacterial growth in a culture medium or the rapid increase of invasive species in a new environment where resources are initially abundant.
2.  **Basis for Logistic Growth:** While exponential growth is rarely sustained indefinitely in nature due to limited resources, it serves as a theoretical baseline. The exponential model forms the initial phase of the more realistic logistic growth model (S-shaped curve), which incorporates environmental carrying capacity.
3.  **Conservation and Management:** Understanding 'r' and exponential growth is vital for conservation efforts (e.g., predicting the recovery of endangered species) and for managing pest populations (e.g., understanding their potential for rapid increase).

In summary, the equation N_t = N_0 e^(rt) is significant because it provides a mathematical framework for understanding the maximum potential for population growth, highlights the importance of the intrinsic rate of natural increase, and serves as a foundational concept in population ecology, even if real-world populations rarely sustain such growth indefinitely.

Question 11

Q11. List the attributes that a population has but an individual organism does not.

Accepted Answer

A population is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding. Unlike an individual organism, a population possesses several unique attributes:

1.  **Birth Rate (Natality):** It refers to the per capita births in a population over a given period. For example, if in a pond, 20 lotus plants were present last year and through reproduction, 8 new plants are added, the current population is 28. The birth rate is 8/20 = 0.4 offspring per lotus per year.
2.  **Death Rate (Mortality):** It refers to the per capita deaths in a population over a given period. For example, if 4 individuals in a population of 40 fruit flies died in a week, the death rate is 4/40 = 0.1 individuals per fruit fly per week.
3.  **Sex Ratio:** It is the ratio of males to females in a population. For example, a population might have 60% females and 40% males.
4.  **Age Distribution:** It describes the proportion of individuals of different age groups (pre-reproductive, reproductive, and post-reproductive) in a population. This can be graphically represented by age pyramids.
5.  **Population Density:** It is the number of individuals of a species per unit area or volume at a given time. It helps in understanding the size of the population.
6.  **Population Growth Forms:** Populations exhibit characteristic patterns of growth (e.g., exponential or logistic growth) depending on resource availability and environmental conditions.
7.  **Dispersal:** It refers to the movement of individuals into (immigration) or out of (emigration) a population, affecting its size and distribution.

Question 12

Q12. If a population growing exponentially doubles in size in 3 years, what is r for the population?

Accepted Answer

The intrinsic rate of natural increase (r) for the population is approximately 0.231 per individual per year.

Question 13

Q13. Name important defence mechanisms in plants against herbivory.

Accepted Answer

Plants have evolved a variety of sophisticated defence mechanisms to protect themselves from herbivores. These defences can be broadly categorised into morphological and chemical defences.

### I. Morphological Defences
These are physical structures that deter herbivores from feeding on plants.

1.  **Thorns and Spines:** Many plants, such as *Acacia*, cactus, and *Rosa* (rose), develop sharp thorns or spines on their stems and leaves. These structures make it difficult and painful for herbivores to consume the plant material.
2.  **Tough Leaves/Fibrous Stems:** Some plants have tough, leathery leaves or fibrous stems that are difficult to chew and digest. This reduces their palatability and nutritional value for herbivores.
3.  **Hairs and Trichomes:** The presence of epidermal hairs (trichomes) on leaves and stems can deter small herbivores by creating a physical barrier or by irritating their mouthparts.
4.  **Waxes and Resins:** A thick layer of wax or resin on the plant surface can make it difficult for insects to attach or feed, and can also reduce water loss.
5.  **Silica:** Some grasses accumulate silica in their tissues, making them abrasive and difficult to digest for grazing animals.

### II. Chemical Defences
Plants produce a vast array of secondary metabolites that are not directly involved in primary metabolic processes but serve as potent deterrents against herbivores. These chemicals can be toxic, unpalatable, or interfere with herbivore digestion or development.

1.  **Toxins/Poisons:** Many plants produce highly toxic compounds that can cause illness or death to herbivores. Examples include:
    *   **Cardiac glycosides:** Found in *Calotropis* (milkweed), these compounds are highly poisonous and can disrupt heart function in animals. This is why cattle and goats usually do not browse on *Calotropis*.
    *   **Cyanogenic glycosides:** Release hydrogen cyanide upon tissue damage, which is highly toxic.
    *   **Alkaloids:** A diverse group of nitrogen-containing compounds, many of which are bitter and toxic. Examples include:
        *   **Nicotine:** Found in tobacco.
        *   **Caffeine:** Found in coffee and tea.
        *   **Quinine:** From Cinchona bark.
        *   **Strychnine:** From *Strychnos nux-vomica*.
        *   **Opium:** From poppy plants.
    These alkaloids can interfere with nervous systems, muscle function, or digestion in herbivores.
2.  **Digestive Inhibitors:** Some plants produce compounds that interfere with the digestion of plant material by herbivores. For example, tannins bind to proteins and reduce their digestibility, while protease inhibitors block the action of digestive enzymes.
3.  **Repellents/Antifeedants:** These chemicals make the plant unpalatable or unattractive to herbivores, causing them to avoid feeding. Examples include:
    *   **Neem compounds:** Azadirachtin from neem acts as a powerful antifeedant and insect growth regulator.
    *   **Resins and Gums:** Sticky substances that can trap or deter insects.
4.  **Hormone Mimics:** Some plants produce compounds that mimic insect hormones, disrupting their growth and development cycles.

These defence mechanisms represent a crucial aspect of plant evolution, allowing them to survive and thrive in environments where herbivory is a constant threat.

Question 14

Q14. An orchid plant is growing on the branch of mango tree. How do you describe this interaction?

Accepted Answer

This interaction is described as **commensalism**.

**Commensalism** is an interspecific interaction where one species benefits, and the other species is neither harmed nor benefited. In this specific case:

*   **Orchid (the commensal):** The orchid benefits by growing on the branch of the mango tree. It gets support, a suitable place to grow (often higher up to receive more sunlight), and access to moisture and nutrients from the air and accumulating debris. It does not derive nutrition directly from the mango tree.
*   **Mango tree (the host):** The mango tree is neither harmed nor benefited by the presence of the orchid. The orchid uses the tree merely as a substrate for support and does not extract water or nutrients from the tree, nor does it significantly shade the tree or compete for resources.

Therefore, it's a classic example of commensalism, where the orchid benefits from the association, and the mango tree remains unaffected.

Question 15

Q15. What is the significance of the equation:
`dN/dt = rN (K-N)/K`

Accepted Answer

The equation `dN/dt = rN (K-N)/K` is the mathematical representation of the **Logistic Growth Model**, also known as Verhulst-Pearl Logistic Growth. This model describes a more realistic pattern of population growth when resources are limited, unlike the exponential growth model which assumes unlimited resources.

### I. Understanding the Components of the Equation:

*   **`dN/dt`**: This term represents the **rate of change of population size (N) with respect to time (t)**. It indicates how fast the population is growing or declining at any given moment.
*   **`N`**: This is the **current population size**.
*   **`r`**: This is the **intrinsic rate of natural increase**. It represents the maximum potential per capita growth rate of the population under ideal conditions (unlimited resources, no predators, etc.).
*   **`K`**: This is the **carrying capacity**. It represents the maximum population size that a particular environment can sustain indefinitely, given the available resources (food, water, space) and environmental conditions. When `N` approaches `K`, the term `(K-N)/K` approaches zero, and thus `dN/dt` also approaches zero.
*   **`(K-N)/K`**: This term is the **environmental resistance or limiting factor**. It reflects how much the environment is resisting further population growth. As `N` gets closer to `K`, this term becomes smaller, reducing the population growth rate. When `N = K`, this term becomes 0, and the population growth stops.

### II. Significance of the Logistic Growth Model:

1.  **Realistic Representation:** It provides a more realistic model for population growth in nature, where resources are finite and competition, predation, and disease limit population expansion. Most natural populations exhibit logistic growth.
2.  **S-shaped Curve (Sigmoid Curve):** When `N` is plotted against `t`, the logistic growth equation yields a characteristic S-shaped curve. This curve typically has three phases:
    *   **Lag Phase:** Initial slow growth when the population is small and adapting to the new environment.
    *   **Logarithmic (Exponential) Phase:** A period of rapid growth when resources are abundant and the population grows almost exponentially. `(K-N)/K` is close to 1, so `dN/dt` is close to `rN`.
    *   **Stationary Phase:** Growth slows down as `N` approaches `K`. Eventually, `N` equals `K`, and the population size stabilizes, meaning `dN/dt` becomes zero. Birth rates equal death rates, and the population fluctuates around the carrying capacity.
3.  **Concept of Carrying Capacity (K):** The model explicitly incorporates the concept of carrying capacity, which is a fundamental ecological principle. It highlights that no population can grow indefinitely; there are always limits imposed by the environment.
4.  **Resource Management:** Understanding logistic growth is crucial for managing natural resources and populations. For example, in fisheries or wildlife management, knowing the carrying capacity and the maximum sustainable yield (often occurring at `K/2`, where the growth rate is highest) helps in making informed decisions about harvesting or conservation.
5.  **Competition and Density Dependence:** The `(K-N)/K` term implicitly accounts for density-dependent factors like competition for resources, predation, and disease, which become more intense as the population density increases and approaches `K`.

In summary, the logistic growth equation is significant because it offers a practical and ecologically sound framework for understanding how populations grow and are regulated in environments with limited resources, leading to a stable population size at the carrying capacity.

Chapter 13: Organisms and Populations