Chapter 11 : Organisms and Populations

Class 12 Chapter 11 : Organisms and Populations Exercise Questions Answers  :

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

Answer : Attributes that populations possess but not individuals are :

Birth Rates and Death Rates: Populations have birth rates (per capita births) and death rates (per capita deaths), which refer to changes in numbers with respect to members of the population.

Sex Ratio: Populations have a sex ratio, indicating the proportion of males to females within the population.

Age Distribution: Populations are composed of individuals of different ages, and age distribution is often represented using an age pyramid, which reflects the population's growth status.

Population Density : The total number of individuals within a population, often measured as population density, can vary widely and is a critical attribute for understanding a population's status and ecological processes.

Relative Densities: In some cases, relative population densities, such as per cent cover or biomass, are more meaningful measures, especially when total numbers are difficult to determine.

These population-level attributes are important for studying ecological processes, understanding population dynamics, and evaluating the impact of various ecological factors on populations.

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

Answer : Using the formula for exponential population growth is : 

Where,  Population density after time t .

Population density at time zero .

  Intrinsic rate of natural increase .

  The base of natural logarithms (2.71828)

 Given that the population doubles in size in 3 years .

i.e.,     and  years .

         

We have, 

        

        

 

 

So, the intrinsic rate of increase () for this population is approximately 0.2312 per year.

3. Name important defence mechanisms in plants against herbivory.

Answer : Important defense mechanisms in plants against herbivory include chemical defenses, such as the production of toxic secondary metabolites (e.g., cardiac glycosides), and storage of chemicals that deter herbivores (e.g., nicotine, caffeine). Morphological defenses, like thorns and spines (e.g., Acacia and Cactus), also play a role. Some plants use mimicry or camouflage, while others are unpalatable due to toxins acquired from their environment (e.g., Monarch butterflies). These defenses help plants deter herbivores and ensure their survival.

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

Answer : The interaction between the orchid plant and the mango tree can be described as a form of commensalism. In this relationship, the orchid benefits by using the mango tree as a support structure to grow and access sunlight, while the mango tree is neither harmed nor benefited directly. The orchid benefits from being elevated and exposed to more sunlight in its epiphytic growth, while the mango tree is not significantly affected by the presence of the orchid.

5. What is the ecological principle behind the biological control method of managing with pest insects?

Answer : The ecological principle behind the biological control method of managing pest insects is leveraging natural predator-prey relationships to regulate prey populations. Predators play a crucial role in keeping prey populations in check, preventing them from reaching unnaturally high densities that could disrupt ecosystems. By introducing or enhancing the presence of natural predators of pest insects, biological control helps maintain ecological balance, reduces the need for chemical pesticides, and contributes to ecosystem stability and species diversity.

6. Define population and community.

Answer : Population : A population is a group of individuals of a given species sharing or competing for similar resources in a defined geographical area. Populations have attributes that individual organisms do not, including birth rates, death rates, sex ratio, and age distribution. The age pyramid, a graphical representation of the proportion of different age groups of males and females in a population, is often used to determine whether a population is stationary, growing, or declining based on its shape.

Community : A community in ecology refers to a group of various species of organisms that inhabit a particular ecosystem or habitat and interact with each other. It represents the collective assemblage of different species coexisting and influencing each other within a defined geographic area, forming complex ecological relationships. These interactions can include competition, predation, mutualism, and more, contributing to the overall structure and dynamics of the ecosystem.

7. Define the following terms and give one example for each:

(a) Commensalism

(b) Parasitism

(c) Camouflage

(d) Mutualism

(e) Interspecific competition

Answer : (a) Commensalism : Commensalism is an ecological interaction in which one species benefits, and the other is neither harmed nor benefited.

An example is the relationship between cattle egrets and grazing cattle. The egrets forage near the cattle, benefiting from insects stirred up by the cattle's movement. The cattle, in turn, are neither harmed nor benefited by the presence of the egrets.

(b) Parasitism : arasitism is an ecological interaction in which one organism (the parasite) benefits at the expense of another organism (the host). The parasite derives its nourishment and often shelter from the host, harming the host in the process.

Example: A common example of parasitism is the relationship between a tapeworm and a human. The tapeworm lives in the human's digestive system, absorbing nutrients from the host's food. This harms the human by causing nutritional deficiencies and other health issues while benefiting the tapeworm.

(c) Camouflage : Camouflage refers to the adaptation of an organism's appearance or behavior to blend in with its surroundings, making it less visible or indistinguishable from its environment. This adaptation helps the organism avoid detection by predators or prey and increases its chances of survival.

Example: The peppered moth (Biston betularia) is a classic example of camouflage. In industrial areas with pollution, tree bark turned dark due to soot deposition. Peppered moths with dark-colored wings had a survival advantage because they were less visible to predators on the darkened trees. Conversely, in non-polluted areas with lighter tree bark, light-colored moths were more successful in evading predators. This example demonstrates how camouflage can influence an organism's survival in changing environments.

(d) Mutualism : Mutualism is an ecological interaction in which both interacting species benefit from the relationship. Both partners gain advantages from the association, which can include resources, protection, or other services.

Example: A classic example of mutualism is the relationship between flowering plants and their pollinators, such as bees. Bees obtain nectar and pollen from flowers, which serve as food sources. In the process, bees transfer pollen from one flower to another, facilitating the plant's reproduction through pollination. The plant benefits from this by achieving fertilization and producing seeds, while the bee gets food.

(e) Interspecific competition : Interspecific competition is an ecological interaction in which individuals of different species compete for shared, limited resources, such as food, water, or habitat. This competition can affect the fitness (measured in terms of intrinsic rate of increase) of the species involved and may lead to changes in their distribution and abundance.

Example: An example of interspecific competition is the competition for food resources between visiting flamingoes and resident fishes in some South American lakes. Both the flamingoes and the fishes rely on the same resource, zooplankton, as their food source. This competition for zooplankton can influence the feeding efficiency and survival of both species and may impact their population dynamics within the ecosystem.

8. With the help of suitable diagram describe the logistic population growth curve.

Answer : The logistic population growth curve, also known as the Verhulst-Pearl Logistic Growth curve, illustrates the population growth of a species in a habitat with limited resources. It is characterized by four main phases are :

Lag Phase: Initially, when a population is introduced into a habitat with limited resources, it experiences a lag phase. During this phase, the population grows slowly because individuals are adjusting to the new environment and resource availability is not fully utilized. The growth is relatively flat at this stage.

Acceleration Phase: As individuals adapt and compete for resources, the population begins to grow more rapidly. Resources are being utilized more efficiently, resulting in an exponential increase in population size. This phase represents a steep upward slope on the growth curve.

Deceleration Phase: As the population size approaches the carrying capacity (K) of the habitat, resource availability becomes limited. Competition for resources intensifies, and the population growth rate starts to slow down. The curve begins to level off during this phase.

Stationary phase : Finally, when the population density reaches the carrying capacity (K) of the habitat, the population stabilizes. The carrying capacity is the maximum number of individuals that the habitat can support sustainably. At this point, the population growth rate levels off, and the curve reaches an asymptote, indicating that the population has reached its maximum sustainable size in that environment.

The simple diagram of the logistic population growth curve:

    

a – When responses are not limiting the growth , plot is exponential ,

b – When responses are limiting the growth , plot is logistic ,

K – carrying capacity .  

9. Select the statement which explains best parasitism.

(a) One organism is benefited.

(b) Both the organisms are benefited.

(c) One organism is benefited, other is not affected.

(d) One organism is benefited, other is affected.

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

This statement best explains parasitism, where one organism (the parasite) benefits, often at the expense of the other organism (the host), which is affected negatively by the interaction.

10. List any three important characteristics of a population and explain.

Answer : Three important characteristics of a population and their explanations are :

Birth Rates and Death Rates : A population has birth rates and death rates, which refer to the number of births and deaths per capita within the population. For example, if 8 new lotus plants are born in a pond with 20 existing plants, the birth rate is 0.4 offspring per lotus per year. These rates help us understand the population's growth or decline over time.

Sex Ratio : In a population, there is a sex ratio, which indicates the proportion of males to females. For instance, if 60% of the population are females and 40% are males, the sex ratio is 60:40. This information is crucial for understanding reproductive dynamics and genetic diversity within the population.

Age Distribution (Age Pyramid) : Populations consist of individuals of different ages, and their age distribution can be represented as an age pyramid. This pyramid provides insights into the population's structure and can help determine if it is growing, stable, or declining. The shape of the pyramid reflects the population's growth status, and it is a valuable tool for ecological analysis.

Population size, which is also an important characteristic, varies widely in nature and is a fundamental parameter for studying ecological processes within a population. It can be measured in absolute numbers or, in some cases, through relative measures like per cent cover or biomass when counting individuals is impractical.