Organisms and Populations

Given/Concept: A population is a group of individuals of the same species in a defined area. Populations have emergent properties that a single individual cannot possess.

Attributes that populations possess but not individuals:

1. Birth Rate (Natality): The number of births per capita in a population per unit time. An individual can give birth, but a 'birth rate' is a property of the whole population.

2. Death Rate (Mortality): The number of deaths per capita in a population per unit time. Similarly, an individual can die, but a 'death rate' belongs to the population.

3. Sex Ratio: The ratio of males to females in a population (e.g., 60% females : 40% males). A single individual has a sex but not a sex ratio.

4. Age Distribution (Age Pyramid): The proportion of individuals in different age groups (pre-reproductive, reproductive, post-reproductive). An individual has an age, but age distribution is a population attribute.

5. Population Density: The number of individuals (or biomass) per unit area or volume at a given time.

6. Population Growth Rate: The rate at which the population size changes over time (expressed as $r$ or $\lambda$).

Conclusion: Birth rate, death rate, sex ratio, age distribution, population density, and growth rate are all attributes of populations, not of individual organisms.

Given:
- Population doubles in $t = 3$ years, i.e., $N_t = 2N_0$
- Growth is exponential.

Formula used:
For exponential growth:
$$N_t = N_0 \cdot e^{rt}$$

Working:

Substituting $N_t = 2N_0$:
$$2N_0 = N_0 \cdot e^{r \times 3}$$

$$2 = e^{3r}$$

Taking natural logarithm on both sides:
$$\ln 2 = 3r$$

$$r = \frac{\ln 2}{3} = \frac{0.6931}{3}$$

$$\boxed{r \approx 0.231 \text{ per year}}$$

Conclusion: The intrinsic rate of natural increase $(r)$ of the population is approximately 0.231 per year.

Concept: Plants cannot escape herbivores by moving away, so they have evolved a variety of morphological (physical) and chemical defence mechanisms.

Important Defence Mechanisms in Plants Against Herbivory:

A. Morphological / Physical Defences:
1. Thorns and Spines: Sharp thorns (e.g., *Acacia*, *Cactus*) deter herbivores from feeding.
2. Tough / Leathery Leaves: Thick cuticle and tough texture make leaves difficult to chew.
3. Dense Trichomes (Hair): Hairy leaf surfaces make feeding difficult for insects and small herbivores.

B. Chemical Defences:
1. Toxic Alkaloids: Many plants produce alkaloids (e.g., nicotine in tobacco, caffeine in tea) that are poisonous or distasteful to herbivores.
2. Terpenes and Resins: Chemicals like terpenes in conifers repel or poison herbivores.
3. Tannins: Tannins reduce the digestibility of plant tissue.
4. Cardiac Glycosides: Present in plants like *Calotropis*; highly toxic to animals that consume them.
5. Distasteful / Poisonous Chemicals: *Calotropis* produces highly poisonous cardiac glycosides; the weed *Lantana* has chemicals toxic to cattle.

Conclusion: Plants use a combination of physical barriers and toxic/distasteful chemicals to defend themselves against herbivory.

Identification of Interaction:
The interaction between the orchid and the mango tree is Commensalism.

Definition: Commensalism is an interspecific interaction in which one species benefits and the other species is neither harmed nor benefited (unaffected).

Explanation:
- The orchid (epiphyte) benefits by using the branch of the mango tree as a physical support to grow at a height where it can receive adequate sunlight and moisture. It does not derive nutrition from the mango tree.
- The mango tree is neither harmed nor benefited by the presence of the orchid.

Symbol: $(+, 0)$ — where $+$ represents the orchid (benefited) and $0$ represents the mango tree (unaffected).

Conclusion: This is a classic example of commensalism where the orchid is the commensal and the mango tree is the host.

Ecological Principle:
The ecological principle behind biological control is Predation (and in some cases parasitism) — a natural interspecific interaction where one organism (the predator/parasite) kills or harms another (the prey/host).

Explanation:
- In nature, predators and parasites keep the population of their prey/host under check, preventing any one species from becoming excessively abundant.
- In biological control, a natural enemy (predator, parasite, or pathogen) of the pest is deliberately introduced or encouraged to reduce the pest population to a manageable level.
- This exploits the predator–prey relationship to control pest populations without the use of chemical pesticides.

Example:
- The Cochineal insect (*Dactylopius*) was used to control the invasive prickly pear cactus (*Opuntia*).
- The Gambusia fish is used to control mosquito larvae in ponds.
- *Bacillus thuringiensis* (Bt) is used as a biopesticide against caterpillars.

Conclusion: The principle is that predators/parasites naturally regulate prey/host populations, and this natural regulation is harnessed in biological control methods.

Population:

A population is defined as a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding.

- Populations have attributes such as birth rate, death rate, sex ratio, age distribution, and population density.
- *Example:* All the tigers in the Sundarbans, or all the mango trees in an orchard.

Community:

A community (also called a biotic community) is defined as an assemblage of populations of different species living in the same geographical area at the same time, interacting with one another.

- A community includes all the plants, animals, fungi, and microorganisms in a given habitat.
- *Example:* All the organisms (plants, animals, microbes) living in a pond or a forest constitute a community.

Key Difference:
| Feature | Population | Community |
|---|---|---|
| Species | Single species | Multiple species |
| Interaction | Intraspecific | Interspecific |
| Example | All frogs in a pond | All organisms in a pond |

(a) Commensalism:

Definition: An interspecific interaction in which one species benefits and the other species is neither harmed nor benefited (unaffected). It is represented as $(+, 0)$.

Example: An orchid (epiphyte) growing on the branch of a mango tree. The orchid benefits by getting support and sunlight; the mango tree is unaffected.

Another example: The barnacles growing on the back of a whale — barnacles benefit (transport, access to food-rich waters) while the whale is unaffected.

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(b) Parasitism:

Definition: An interspecific interaction in which one species (the parasite) benefits at the expense of the other species (the host), which is harmed. It is represented as $(+, -)$.

Example: *Cuscuta* (dodder plant) is a parasitic plant that grows on a host plant, deriving water and nutrients from it, thereby harming the host.

Another example: The human liver fluke (*Fasciola hepatica*) parasitises the liver of its host.

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(c) Camouflage:

Definition: A defence adaptation in which an organism blends with its surrounding environment in terms of colour, shape, or pattern, making it difficult for predators (or prey) to detect it. It is a form of cryptic coloration.

Example: The leaf insect (*Phyllium*) resembles a green leaf so closely that predators cannot distinguish it from real leaves. Similarly, the stick insect (*Carausius*) resembles a twig.

Another example: The Kallima butterfly resembles a dry leaf when it sits with its wings closed.

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(d) Mutualism:

Definition: An interspecific interaction in which both the interacting species benefit. It is represented as $(+, +)$.

Example: Lichens — a mutualistic association between a fungus and a photosynthetic alga/cyanobacterium. The fungus provides shelter and absorbs minerals; the alga provides food through photosynthesis.

Another example: The relationship between the fig tree (*Ficus*) and the fig wasp (*Agaonidae*) — the wasp pollinates the fig flowers and in turn gets a safe site to lay eggs and food for its larvae.

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(e) Interspecific Competition:

Definition: An interspecific interaction in which two different species compete for the same limited resources (food, space, light, etc.), and both species are adversely affected. It is represented as $(-, -)$.

Example: Flamingoes and resident fishes in South American lakes compete for the same food resource — zooplankton. Both species are negatively affected by the presence of the other.

Another example: Competition between two grass species for soil nutrients and space in a meadow.

Concept: When resources are limited, a population cannot grow indefinitely. The growth follows a logistic (sigmoid/S-shaped) pattern.

Logistic Growth — Key Points:

Equation:
$$\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$$

Where:
- $N$ = Population size at time $t$
- $r$ = Intrinsic rate of natural increase
- $K$ = Carrying capacity of the environment
- $\frac{K-N}{K}$ = Environmental resistance (the unused portion of carrying capacity)

Description of the Logistic Growth Curve (S-shaped / Sigmoid Curve):

The curve has three distinct phases:

1. Lag Phase: Initially, the population is small and growth is slow as individuals establish themselves.

2. Log (Exponential) Phase: The population grows rapidly as resources are still relatively abundant. The curve rises steeply.

3. Deceleration Phase: As population size approaches $K$, resources become limiting. The growth rate slows down.

4. Plateau (Asymptote) Phase: The population stabilises at the carrying capacity ($K$). The growth rate becomes zero ($dN/dt = 0$) because $N = K$.

Diagram:

```
Population
Size (N)
|
K|_ _ _ _ _ _ _ _ _ _ _ _ _ _ (Carrying Capacity)
| /‾‾‾‾‾‾
| /
| /
| /
| /
| /
| / (Lag phase)
|/________________________
Time (t)
```

*(The curve is S-shaped/sigmoid, starting slowly, rising steeply, then levelling off at K)*

Key Features:
- The population never exceeds $K$ in ideal logistic growth.
- The growth rate is maximum when $N = K/2$.
- This is a more realistic model of population growth in nature compared to exponential growth.
- Also called the Verhulst-Pearl Logistic Growth model.

Conclusion: The logistic growth curve is a sigmoid (S-shaped) curve that shows how a population grows rapidly when small, then slows as it approaches the carrying capacity $K$, ultimately stabilising at $K$.

Correct Answer: (d) One organism is benefited, other is affected.

Justification:
In parasitism, the parasite benefits by deriving nutrition, shelter, or other resources from the host, while the host is harmed (negatively affected) — it loses nutrients, may suffer tissue damage, reduced fitness, or even death. This is represented as a $(+, -)$ interaction.

- Option (a) is incomplete — it does not mention the effect on the other organism.
- Option (b) describes mutualism $(+, +)$.
- Option (c) describes commensalism $(+, 0)$.
- Option (d) correctly and completely describes parasitism $(+, -)$.

Three Important Characteristics of a Population:

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1. Birth Rate (Natality):

- Birth rate is defined as the number of births per capita (per individual) in a population per unit time.
- It is expressed as: $$\text{Birth Rate} = \frac{\text{Number of births}}{\text{Population size}}$$
- It contributes to the increase in population size.
- *Example:* If a pond has 20 water hyacinths and 8 new plants are added in a week, the birth rate = $8/20 = 0.4$ offspring per individual per week.

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2. Death Rate (Mortality):

- Death rate is defined as the number of deaths per capita in a population per unit time.
- It is expressed as: $$\text{Death Rate} = \frac{\text{Number of deaths}}{\text{Population size}}$$
- It contributes to the decrease in population size.
- *Example:* If 4 out of 40 frogs in a pond die in a week, the death rate = $4/40 = 0.1$ individuals per week.
- The difference between birth rate and death rate determines whether a population grows, declines, or remains stable.

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3. Age Distribution (Age Pyramid):

- A population is composed of individuals of different ages. The proportion of individuals in different age groups (pre-reproductive, reproductive, and post-reproductive) is called age distribution.
- It is graphically represented as an age pyramid.
- Three types of age pyramids:
- Expanding (triangular): Broad base — high proportion of young individuals — rapidly growing population.
- Stable (bell-shaped): Equal proportion of young and middle-aged individuals — stable/stationary population.
- Declining (urn-shaped): Narrow base — low proportion of young individuals — declining population.
- *Example:* India's age pyramid has a broad base, indicating a young, rapidly growing population.

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Conclusion: Birth rate, death rate, and age distribution are fundamental characteristics of a population that determine its size, growth rate, and future trajectory.