NCERT Solutions for Class 12 Biology Chapter 11: Organisms and Populations

Populations, unlike individual organisms, show collective characteristics that help us understand their behavior and growth. These attributes include:

• Birth Rate:
  The number of births per 1,000 individuals in a population per unit time. It shows how fast a population is growing.

• Death Rate:
  The number of deaths per 1,000 individuals in a population per unit time. It helps track the decline in population size.

• Sex Ratio:
  The proportion of males to females in a population, which affects reproduction and growth.

• Age Structure:
  The number of individuals in different age groups (pre-reproductive, reproductive, and post-reproductive). This is often shown as an age pyramid and helps predict future population trends.

• Population Density:
  The number of individuals of a species in a given area or volume. It tells us how crowded the population is.

These features apply only to populations and not to individuals. They help ecologists study population growth, reproduction, and survival patterns.

To calculate the intrinsic rate of increase (r) for a population that doubles in size in 3 years under exponential growth, we use the formula:

Nt = N0 × e^(rt)

Where:

• Nt = population size at time t

• N0 = initial population size

• r = intrinsic rate of increase

• t = time in years

Since the population doubles in 3 years:
Nt = 2 × N0 and t = 3

Substitute into the formula:
2 × N0 = N0 × e^(3r)

Divide both sides by N0:
2 = e^(3r)

Take the natural log of both sides:
ln(2) = 3r

Solve for r:
r = ln(2) / 3 ≈ 0.693 / 3 ≈ 0.231

Therefore, the intrinsic rate of increase (r) is approximately 0.231 per year, or 23.1%.

Plants have developed several defence mechanisms to protect themselves from herbivores (plant-eating animals).

1. Morphological (physical) defences:

   • Some plants have sharp thorns or spines that prevent animals from eating them.

   • Examples: Acacia and Cactus have thorns to keep herbivores away.

2. Chemical defences:

   • Some plants produce toxic or bitter chemicals that discourage animals from feeding on them.

   • Example: Calotropis produces cardiac glycosides, which are poisonous to grazing animals.

   • Other examples of defensive chemicals include nicotine, caffeine, quinine, strychnine, and opium, which can harm or kill herbivores or make the plant taste bad.

These adaptations help plants survive by reducing damage caused by herbivores and increasing their chances of reproduction.

The relationship between an orchid growing on the branch of a mango tree is called commensalism. In this type of interaction, one organism benefits while the other is neither harmed nor helped.

The orchid benefits by getting support and better access to sunlight and air by growing on the mango tree. It does not take nutrients from the mango tree.

The mango tree remains unaffected by the presence of the orchid. It is neither harmed nor benefited.

This is a typical example of commensalism in nature.

The ecological principle behind biological control of pest insects is based on predation and natural population regulation.

In this method, a natural predator or parasite of the pest is introduced. The predator feeds on the pest and helps keep its population under control, preventing it from causing serious damage.

A well-known example is the control of prickly pear cactus in Australia, which became a major pest. It was successfully controlled by introducing a moth (Cactoblastis) that feeds on the cactus.

This method maintains balance in the ecosystem and reduces the use of chemical pesticides, making it a more eco-friendly and sustainable way to manage pests.

A population is a group of individuals of the same species living in a specific area. They share or compete for similar resources and can interbreed.
Example: All the cormorants in a wetland or all the rats in an abandoned building.

A community is a group of different populations (different species) living in the same area and interacting with each other. It includes all the plants, animals, and microorganisms in that place.

Understanding both population and community is important in the study of ecology.

Ecological Interactions

(a) Commensalism:

Definition: An interaction where one species benefits, and the other is neither harmed nor benefited.

Example: An orchid growing on a mango tree branch. The orchid benefits from the support and access to sunlight, while the mango tree is unaffected.

(b) Parasitism:

Definition: An interaction where one species (the parasite) benefits, and the other (the host) is harmed.

Example: Ticks on dogs. The ticks feed on the dog's blood, harming the dog while benefiting themselves.

(c) Camouflage:

Definition: A defense mechanism where an organism blends in with its surroundings to avoid detection by predators or prey.

Example: Cryptically-colored insects that resemble leaves or bark, making it difficult for predators to spot them.

(d) Mutualism:

Definition: An interaction where both species benefit.

Example: Mycorrhizae, where fungi help plant roots absorb nutrients, and the plant provides carbohydrates to the fungi.

(e) Interspecific Competition:

Definition: An interaction where two different species compete for the same limited resources, negatively affecting both.

Example: Visiting flamingoes and resident fishes in South American lakes competing for zooplankton.

The logistic population growth curve shows how a population grows when resources are limited.

• At first, the population grows slowly (lag phase) as individuals adjust to the environment.

• Then comes the exponential (acceleration) phase, where the population grows rapidly.

• As resources become limited, growth slows down (deceleration phase).

• Finally, the population size becomes stable, reaching the carrying capacity (K) of the environment.

• The curve is S-shaped (called a sigmoid curve).

This pattern is explained by the Verhulst-Pearl logistic growth model, given by the equation:

dN/dt = rN × (K – N)/K

Where:

• N = population size

• r = intrinsic rate of increase

• K = carrying capacity

• dN/dt = rate of change of population over time

This model is more realistic than exponential growth because it considers environmental limits and resource availability.

One organism is benefited, other is affected.

Three important characteristics of a population are:

1. Birth Rate: This refers to the per capita number of births in a population during a specific period. It is a crucial factor influencing population growth. For example, if a population of 100 individuals has 10 births in a year, the birth rate is 0.1 offspring per individual per year.
2. Death Rate: This is the per capita number of deaths in a population during a specific period. It significantly impacts population decline. For instance, if a population of 100 individuals experiences 5 deaths in a year, the death rate is 0.05 individuals per individual per year.
3. Sex Ratio: This represents the proportion of males to females in a population. It influences reproductive potential and population dynamics. For example, a population with a sex ratio of 60% females and 40% males may have a higher reproductive capacity than one with an equal sex ratio.

Night-blooming flowers are generally white because white reflects light more effectively than other colors, making them easier to spot by nocturnal pollinators such as moths and bats. At night, visibility is low, and pollinators rely more on visual cues to locate flowers. The white color contrasts against the dark background, increasing the flower's visibility. Additionally, white flowers often have a strong fragrance, which also aids pollinators in finding them in the dark, ensuring successful pollination.

Bees know which flowers have nectar through a combination of visual and olfactory cues. They can see ultraviolet patterns on flowers that act as nectar guides, leading them to the nectar source. Additionally, bees have a highly developed sense of smell and can detect the specific fragrances emitted by flowers that contain nectar. They also learn from experience; if a bee finds nectar in a particular type of flower, it will remember and preferentially visit similar flowers in the future.

Cacti have many thorns as a defense mechanism against herbivores. In arid environments where cacti thrive, water is scarce, making cacti a valuable source of moisture for animals. The thorns deter animals from eating the cactus, thus protecting the plant's water reserves and ensuring its survival.

Additionally, thorns provide shade, reducing water loss from the plant's surface, and help to trap moisture from the air.

Chickens recognize their mothers through a combination of visual and auditory cues. Chicks imprint on their mother hen shortly after hatching, learning her unique calls and appearance. They can distinguish her from other hens by her specific vocalizations and visual features like comb shape and feather patterns. This recognition is essential for the chick's survival, as the mother provides protection, warmth, and guidance in foraging for food.

When dealing with a dense laboratory culture of bacteria in a petri dish, the best measure to report its density is often the Colony Forming Units (CFU) per milliliter (CFU/mL). This involves diluting the bacterial culture, plating a known volume onto agar plates, incubating them, and then counting the number of colonies that grow. Each colony is assumed to have originated from a single bacterial cell, allowing an estimation of the viable bacterial count in the original sample. Alternatively, optical density (OD) using a spectrophotometer can provide a quick estimate of bacterial density by measuring the turbidity of the culture.

Yes, the growth of a population over time often shows a specific and predictable pattern.

There are two main types of population growth patterns:

1. Exponential Growth:

   • This occurs when resources are unlimited.

   • The population grows rapidly and forms a J-shaped curve.

   • The growth rate increases continuously as the population size increases.

2. Logistic Growth:

   • This happens when resources are limited.

   • The population grows fast at first, then slows down, and finally becomes stable at the carrying capacity (K).

   • This forms an S-shaped or sigmoid curve.

These patterns are predictable and help ecologists understand how populations behave in different environmental conditions.

Gathering population figures for India over the last 100 years from government census data reveals a growth pattern that initially shows a slow increase followed by a rapid acceleration. When plotted, the graph exhibits a curve resembling a logistic growth model. The initial phase represents a lag period with moderate growth due to factors like disease and limited resources. As healthcare improved and resources became more available, the population experienced exponential growth. More recently, due to family planning initiatives and increased awareness, the growth rate has begun to stabilize, indicating a transition towards the carrying capacity.

While it might seem intuitive that an ideal parasite should thrive without harming its host, natural selection has not necessarily led to the evolution of totally harmless parasites due to the dynamics of resource allocation and reproductive success. Parasites that extract resources from their hosts, even if it causes some harm, may have a higher reproductive rate or competitive advantage compared to those that cause no harm. Additionally, the co-evolutionary arms race between hosts and parasites often favors parasites that can overcome host defenses, which may inevitably cause some degree of harm. Therefore, the selective pressures on parasites often prioritize their own survival and reproduction over the well-being of the host.

The female mosquito is not considered a parasite, even though she requires blood for reproduction, because the interaction is temporary and does not involve a prolonged dependence on the host. Parasitism typically involves a long-term relationship where the parasite lives on or inside the host, deriving nourishment over an extended period, often causing harm. In contrast, a mosquito takes a blood meal and then leaves, without establishing a persistent dependency or causing significant harm beyond the immediate bite. Therefore, the mosquito's behavior is more accurately described as predation.