Organisms and PopulationsClass 12 Biology NCERT Solutions
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Q1EXERCISES
List the attributes that populations possess but not individuals.
Solution
An individual organism is born and dies, but a population has attributes that individuals do not. These attributes are:
- Birth Rate and Death Rate: These are per capita rates of births and deaths in the population over a given time. An individual has a birth and a death, but not a birth rate or death rate.
- Sex Ratio: This is the ratio or percentage of males and females in the total population. An individual is either a male or a female, but does not have a sex ratio.
- Age Distribution: A population is composed of individuals of different ages. The distribution of individuals in different age groups (pre-reproductive, reproductive, and post-reproductive) can be plotted as an age pyramid. The shape of this pyramid indicates the growth status of the population.
- Population Density: This refers to the number of individuals of a population per unit area or volume. It is a measure of the size of the population in its habitat.
Q2EXERCISES
If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase ( r ) of the population?
Solution
Given:
- The population is growing exponentially.
- Time taken to double, years.
- Let the initial population density be .
- The final population density after 3 years is .
To Find:
The intrinsic rate of increase, .
Formula:
The integral form of the exponential growth equation is:
Calculation:
Substituting the given values into the formula:
Divide both sides by :
To solve for , take the natural logarithm (ln) of both sides:
Using the property :
Now, solve for :
We know that the value of is approximately 0.693.
Final Answer:
The intrinsic rate of increase (r) of the population is approximately 0.231 per individual per year.
Q3EXERCISES
Name important defence mechanisms in plants against herbivory.
Solution
Plants have evolved various defense mechanisms against herbivores because, unlike animals, they cannot run away from their predators. The important defense mechanisms are:
-
Morphological Defenses: These are physical structures that deter herbivores. The most common example is the presence of thorns (e.g., in Acacia and Cactus) and spines, which make it difficult for animals to eat the plant.
-
Chemical Defenses: Many plants produce and store a wide variety of chemical substances that are harmful or unpalatable to herbivores. These chemicals can:
- Make the herbivore sick when eaten (e.g., highly poisonous cardiac glycosides produced by Calotropis).
- Inhibit feeding or digestion.
- Disrupt the herbivore's reproduction or even kill it.
- Examples of such chemicals that humans extract for commercial use include nicotine, caffeine, quinine, strychnine, and opium. These are produced by plants as defenses against grazers and browsers.
Q4EXERCISES
An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?
Solution
The interaction between an orchid plant growing on the branch of a mango tree is an example of Commensalism.
In this type of interaction, one species benefits while the other is neither harmed nor benefited.
- The Orchid (Beneficiary): As an epiphyte, the orchid benefits by getting a surface to grow on, which provides it better access to sunlight and air, away from the competition on the forest floor.
- The Mango Tree (Unaffected): The mango tree is not harmed because the orchid does not derive any nutrition from it (unlike a parasite). It also does not appear to derive any significant benefit from the presence of the orchid.
Therefore, this is a (+/0) interaction, which defines commensalism.
Q5EXERCISES
What is the ecological principle behind the biological control method of managing with pest insects?
Solution
The ecological principle behind the biological control method of managing pest insects is the predator-prey relationship.
In any ecosystem, predators play an important role in regulating the population of their prey. When a prey species (like a pest insect) has no natural predators, its population can grow exponentially and cause significant damage.
Biological control methods are based on the ability of a predator to regulate the prey population. This involves introducing a natural predator of the pest species into the affected area. The predator feeds on the pest, thereby keeping its population under control and minimizing the damage it causes. This method is considered a natural way to manage pests, reducing the need for chemical pesticides. An example is the control of the invasive prickly pear cactus in Australia by introducing a cactus-feeding moth from its natural habitat.
Q6EXERCISES
Define population and community.
Solution
Population:
A population is defined as a group of individuals of the same species that live in a well-defined geographical area, share or compete for similar resources, and can potentially interbreed. For example, all the teakwood trees in a forest tract or all the lotus plants in a pond constitute a population.
Community:
A community, also known as a biological community, is an assemblage of populations of different species (e.g., plants, animals, and microbes) living in a defined area and interacting with one another. For example, a forest community includes all the trees, animals, insects, and microorganisms living and interacting in that forest.
Q7EXERCISES
Define the following terms and give one example for each:
(a)
Commensalism
(b)
Parasitism
(c)
Camouflage
(d)
Mutualism
(e) Interspecific competition
Solution
(a) Commensalism: This is an interspecific interaction where one species benefits and the other is neither harmed nor benefited (+/0).
* Example: An orchid growing as an epiphyte on a mango tree. The orchid gets support and sunlight, while the mango tree is unaffected.
(b) Parasitism: This is an interaction where one species (the parasite) benefits by deriving nutrition from another species (the host), which is harmed (+/−).
* Example: Cuscuta (a parasitic plant) growing on a hedge plant. Cuscuta has lost its chlorophyll and derives its nutrition from the host plant.
(c) Camouflage: This is an adaptation where an organism blends in with its surroundings or background to avoid detection by predators. It is also known as cryptic coloration.
* Example: Some species of frogs and insects are cryptically colored to match their environment, making it difficult for predators to spot them.
(d) Mutualism: This is an interaction that confers benefits on both the interacting species (+/+).
* Example: Lichens, which are a mutualistic relationship between a fungus and photosynthesizing algae. The fungus provides shelter and absorbs nutrients, while the alga provides food through photosynthesis.
(e) Interspecific Competition: This is an interaction between individuals of two different species where the fitness of both species is significantly lowered in the presence of each other (−/−). This usually occurs when they compete for the same limited resources.
* Example: In some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton.
Q8EXERCISES
With the help of suitable diagram describe the logistic population growth curve.
Solution
The logistic population growth curve describes the growth of a population in a habitat with limited resources. This model is considered more realistic than exponential growth. The plot of population density (N) in relation to time (t) results in a sigmoid or S-shaped curve (as shown in Figure 11.3b in the textbook).
Diagram:
The logistic growth curve has a characteristic S-shape.
- The Y-axis represents Population Density (N).
- The X-axis represents Time (t).
- The curve starts near zero, rises slowly, then rapidly, and finally flattens out at the carrying capacity (K).
Description of the Curve:
The curve can be divided into distinct phases:
- Lag Phase: Initially, the population is small and adapting to the new environment. The growth rate is slow.
- Acceleration Phase (Log Phase): Resources are abundant, and the population begins to grow rapidly, approaching its maximum rate of increase.
- Deceleration Phase: As the population size increases, resources such as food and space become limited. This leads to increased competition among individuals. The growth rate starts to slow down.
- Asymptote Phase: The population density reaches the carrying capacity (K) of the habitat. The carrying capacity is the maximum population size that the environment can sustain indefinitely. At this point, the birth rate equals the death rate, and the population growth rate becomes zero. The curve flattens out.
The Verhulst-Pearl Logistic Growth is described by the following equation:
Where:
- = Population density at time
- = Intrinsic rate of natural increase
- = Carrying capacity
Q9EXERCISES
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.
Solution
The correct statement that best explains parasitism is:
(d) One organism is benefited, other is affected.
Explanation:
Parasitism is a type of interspecific interaction where one organism, the parasite, lives in or on another organism, the host, and derives benefits (like nutrition and shelter) from it. In this process, the host is harmed, meaning its survival, growth, or reproduction is negatively affected. Therefore, one organism benefits (+) and the other is affected detrimentally (−).
- (a) is incomplete as it does not mention the effect on the other organism.
- (b) describes mutualism.
- (c) describes commensalism.
Q10EXERCISES
List any three important characteristics of a population and explain.
Solution
Three important characteristics (or attributes) of a population are:
-
Population Density: This is the size of a population in relation to a unit of space, expressed as the number of individuals per unit area or volume. For example, 200 carrot grass plants per square meter. It is a fundamental property that helps in understanding the status of the population in its habitat and evaluating the effects of ecological processes like competition or predation.
-
Birth Rate (Natality): This is the per capita births in a population during a given period. It is expressed as the number of new individuals produced per individual of the population per unit time (e.g., 0.4 offspring per lotus per year). Birth rate is a primary factor that contributes to an increase in population density.
-
Age Distribution: This is the proportion of individuals of different ages or age groups within a population. It is often graphically represented as an age pyramid. The shape of the age pyramid (e.g., expanding, stable, or declining) reflects the growth status of the population and can be used to predict future population trends.