11.3 Exercise Problems

Q1. List the attributes that populations possess but not individual.

Answer: The term population refers to a group of individuals of any species living in a well-defined geographical area, sharing or competing for similar kinds of resources and potentially interbreeding.

Population Attributes:

A population has certain attributes that an individual organism does not have. For Example, Individuals may have births and deaths, but a population has birth rates and death rates. The various population attributes are as follows:

• 1. Birth rate (natality)- It is the ratio of live births in an area to the population of an area. It is expressed as the number of individuals added to the population.
• 2. Death rate (mortality)- It refers to the live deaths in an area to the population of an area. It is expressed as the number of individuals removed from the population.
• 3. Sex ratio- It is the ratio of the number of male or female individuals per 1000 individuals.
• 4. Age distribution- Population at a given time is composed of different individuals belonging to different age groups. If the age distribution is plotted for the population, the resulting structure is called age pyramids. The shape of pyramids reflects the growth status of the population.
• 5. Population density- The number of individuals of a population present per unit area at a given time.

Q2. If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?

Answer: A population grows exponentially if sufficient amounts of food resources are available to the individual. Its exponential growth can be calculated by the following integral form of the exponential growth equation:
\(
N_t=N_0 e^{r t}
\)
Where,
\(N_t=\) Population density after time \(t\)
\(N_O=\) Population density at time zero
\(r=\) Intrinsic rate of natural increase
\(e=\) Base of natural logarithms (2.71828)
From the above equation, we can calculate the intrinsic rate of increase ( \(r\) ) of a population.
Now, as per the question, Present population density \(=x\)
Then, Population density after two years \(=2 x\)
\(\mathrm{t}=3\) years
Substituting these values in the formula, we get:
\(
\begin{aligned}
& \Rightarrow 2 x=x e^{3 r} \\
& \Rightarrow 2=e^{3 r}
\end{aligned}
\)
Applying log on both sides:
\(
\begin{aligned}
& \Rightarrow \log 2=3 r \log e \\
& \Rightarrow \frac{\log 2}{3 \log e}=r \\
& \Rightarrow \frac{\log 2}{3 \times 0.434}=r \\
& \Rightarrow \frac{0.301}{3 \times 0.434}=r \\
& \Rightarrow \frac{0.301}{1.302}=r \\
& \Rightarrow 0.2311=r
\end{aligned}
\)
Hence, the intrinsic rate of increase for the above illustrated population is 0.2311.

Q3. Name important defence mechanisms in plants against herbivory.

Answer: For plants, herbivores are the predators. Nearly 25 per cent of all insects are known to be phytophagous (feeding on plant sap and other parts of plants). The problem is severe for plants because, unlike animals, they cannot run away from their predators. Plants, therefore, have evolved an astonishing variety of morphological and chemical defences against herbivores. Thorns and spines (Acacia, cactus) are the most common morphological means of defence. Some plants release toxins that poison attacking herbivores, while others produce complex compounds that affect the attacker’s growth cycle or ability to digest the plant. Plants have evolved several secondary metabolites used in plant defence, known as antiherbivory chemicals. These are nitrogen compounds (alkaloids, cyanogenic glycosides, benzoxazinoids), terpenoids, and phenolics. The weed Calotropis grows in abandoned fields. This plant produces highly poisonous cardiac glycosides. This is the reason why any cattle or goats are not commonly seen grazing on this plant.

Q4. An orchid plant is growing on the branch of a mango tree. How do you describe this interaction between the orchid and the mango tree?

Answer: An orchid that grows on a mango tree represents an interaction called commensalism. In this type of interaction, one species is benefited while another one remains unaffected. Orchid acts as an epiphyte on the mango tree as it does not derive nutrition from the mango tree but uses it for support while the mango tree remains unaffected.

Q5. What is the ecological principle behind the biological control method of managing pest insects?

Answer: The biological principle involved in the biological control method of managing pest insects is checking their population through predators and parasites. The predators act as biological control by preying upon pest insects.

Q6. Define population and community.

Answer: Population: It is a group of individuals of the same species which can reproduce among themselves and occupy a particular area in a given time.

Community: It is an assemblage of several populations in a particular area and exhibits interaction and interdependence through trophic relationships.

Q7. Define the following terms and give one example for each
a. Commensalism
b. Parasitism
c. Camouflage
d. Mutualism
e. Interspecific competition

Answer: (a) Commensalism- It is a kind of population interaction in which one species gets benefited while the other one remains unaffected. E.g., Orchids growing on trees.
(b) Parasitism- It is the interaction wherein one species (parasite) depends on the other species (host) for food and shelter. This is a kind of negative interaction as the parasite gets benefited but the host is harmed. E.g., Taenia solium living in the intestine of human.
(c) Camouflage- This is an adaptive strategy adapted by prey species to escape the predators. For example, many species of frogs and insects camouflage in their surroundings and escape their predators.
(d) Mutualism– The symbiotic relationship between two species in which both species are benefited is called mutualism. For example, lichens show a mutual symbiotic relationship between fungi and blue-green algae, where both are equally benefited from each other. For example, Rhizobium and legumes.
(e) Interspecific competition: In this interaction, both species (different) get negatively affected. For example, the competition between flamingoes and resident fishes in South American lakes for common food resources i.e., zooplankton.

Q8. With the help of suitable diagrams describe the logistic population growth curve.

Answer: The growth of a population takes place according to the availability of food, presence of predators and parasites, presence of other biotic and abiotic factors affecting a population, and other conditions in a particular environment. There are two main types of models i.e., exponential growth model and logistic growth model. The logistic growth curve represents the logistic growth model. It represents growth when there is competition between individuals of a population for food and space. In this case, the fittest organism survives, reproduces and population growth is seen. In this type of growth, firstly the lag phase is shown followed by phases of acceleration and deacceleration.

The different phases of the logistic growth curve are:

• A. Lag phase- In this phase, no growth is seen due to the absence of food and other resources.
• B. Positive acceleration phase- Limited growth in population is seen as the resources become available
• C. Exponential growth phase- The population grows suddenly and shows very rapid growth. The curve rises steeply.
• D. Negative acceleration phase- In this phase, environmental resistance increases and the growth rate of the population decreases.
• E. Stationary phase- In this phase, the growth becomes stationary and becomes stable.

A Verhulst-pearl logistic is also known as s shaped growth curve. A diagrammatic representation of Population growth curves is given below. The curve represents exponential growth and curve b represents log c growth.

The equation of the logistic growth curve is as follows:
Where \(\mathrm{N}=\) Population density at time \(t\)
\(R=\) Intrinsic rate of natural increase
\(\mathrm{K}=\) Carrying capacity Since resources for growth for populations are finite and become limiting, the logistic growth model is considered a more realistic one.

Q9. Select the statement which explains the best parasitism
(a) One organism is benefited.
(b) Both the organisms are benefited.
(c) One organism is benefited; the other is not affected.
(d) One organism is benefited; the other is affected.

Answer: (d) One organism is benefited, other is affected. In parasitism, one organism lives in the body of another. In this interaction, the parasite gets benefited while the host is negatively affected.

Q10. List any three important characteristics of a population and explain

Answer: The term population refers to a group of individuals of any species living in a well-defined geographical area, sharing or competing for similar kinds of resources and potentially interbreeding. Some characteristics of the population are as follows:

• 1. Birth rate (natality)- It is the ratio of live births in an area to the population of an area. It is expressed as the number of individuals added to the population
• 2. Death rate (mortality)- It refers to the live deaths in an area to the population of an area. It is expressed as the number of individuals removed from the population.
• 3. Sex ratio- It is the ratio of the number of male or female individuals per 1000 individuals.
• 4. Age distribution- A population at a given time is composed of different individuals belonging to different age groups. If the age distribution is plotted for the population, the resulting structure is called age pyramids. The shape of pyramids reflects the growth status of the population.
• 5. Population density- The number of individuals of a population present per unit area at a given time.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. What is the interaction between two species called?

Answer: Interspecific interaction

Q2. What is commensalism?

Answer: Commensalism is the interaction in which one species benefits and the other is neither harmed nor benefited.

Q3. Name the association in which one species produces poisonous substance or a change in environmental conditions that is harmful to
another species.

Answer: Amensalism

Q4. What is Mycorrhiza?

Answer: Mycorrhiza is a symbiotic association between a fungus and the roots of higher plants.

Q5. What is homeostasis?

Answer: To maintain the constancy of internal environment despite varying external environmental conditions is called homeostasis.

Q6. Define aestivation.

Answer: Aestivation is a state of dormancy characterized by inactivity and a lowered metabolic rate in response to high temperatures and arid conditions.

Q7. What is diapause and its significance?

Answer: Under unfavourable conditions many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development.

Q8. What would be the growth rate pattern, when the resources are unlimited?

Answer: Exponential.

Q9. Give a suitable example for commensalism.

Answer: Cattle egret and grazing cattle.

Q10. Define ectoparasite and endoparasite and give suitable examples.

Answer:

• Parasites that feed on the external surface of the host organism are called ectoparasites. The most familiar examples of this group are the lice on humans and ticks on dogs.
• Endoparasites are those that live inside the host body at different sites (liver, kidney, lungs, red blood cells, etc.).’”The human liver fluke (a trematode parasite) is an endoparasite.

Q11. What is brood parasitism? Explain with the help of an example.

Answer: Brood parasitism in birds is a fascinating example of parasitism in which the parasitic bird lays its eggs in the nest of its host and let the host incubate them. During the course of evolution, the eggs of the parasitic bird have evolved to resemble the host’s egg in size and colour to reduce the chances of the host bird detecting the foreign eggs and ejecting them from the nest.

SHORT ANSWER TYPE QUESTIONS

Q1. The density of a population in a habitat per unit area is measured in
different units. Write the unit of measurement against the following:
a. Bacteria _______________
b. Banyan _______________
c. Deer _______________
d. Fish _______________

Answer: a. Nos. / Vol; b. Coverage / area; c. Biomass / area; d. Nos. / area; e. Wt. / area

Q2. a. Label the three tiers 1, 2, 3 given in the above age pyramid.
b. What type of population growth is represented by the above age pyramid?

Answer: (a) 1. Pre-reproductive population
2. Reproductive population
3. Post-reproductive population
(b) Expanding or growing population

Q3. In an association of two animal species, one is a termite which feeds on wood and the other is a protozoan Trichonympha present in the gut of the termite. What type of association they establish?

Answer: They shows mutualism.

Q4. Lianas are vascular plants rooted in the ground and maintain erectness of their stem by making use of other trees for support. They do not maintain direct relation with those trees. Discuss the type of association the lianas have with the trees.

Answer: This association is called commensalism.

Q5. Give the scientific names of any two microorganisms inhabiting the human intestine.

Answer: 1. Escherichia coli
2. Enterococcus faecalis 

Q6. What is a tree line?

Answer: When we go up the altitude, beyond a particular height no trees are found and the vegetation comprises only of shrubs and herbs. The altitude beyond which no tree is seen is known as tree line.

Q7. Define ‘zero population growth rate’. Draw a age pyramid for the same.

Answer: Yes. An inverted bell shaped age pyramid is obtained. The young of pre- reproductive age group individuals are less in number and both pre- reproductive and reproductive stages are in the same level.

Q8. List any four characters that are employed in human population census.

Answer: 1. Birthrates
2. Death rates
3. Sex ratio
4. Age distribution

Q9. Fill in the blanks:

Answer: 

Q10. Observe the set of 4 figures A, B, C and D and, answer the following questions
i. Which one of the figures shows mutualism?
ii. What kind of association is shown in D?
iii. Name the organisms and the association in C.
iv. What role is the insect performing in B?

Answer: (i) Figure ‘A’ shows’ mutualism (plant-animal relationship).
(ii) Figure ‘D’ shows predation (leopard killing deer and eating it)
(iii) Figure ‘C’ shows commensalism (cattle egret and grazing cattle)
(iv) In figure ‘B’ insect is phytophagous that feed on sap of the flower.

LONG ANSWER QUESTIONS

Q1. Comment on the following figures: 1, 2 and 3: A, B, C. D, G, P, Q, R, S are species

Answer: Fig. 1: It is a single population and all individuals are of the same species, i.e. A – Individual interact among themselves and their environment.
Fig. 2: It is a community and it contains three populations of species A, B and C. They interact with each other and their environment.
Fig. 3: It is a biome. It contains three communities of which one is in climax and other two are in different stages of development. All three communities are in the same environment and they interact with each other and their environment.

Q2. An individual and a population has certain characteristics. Name these attributes with definitions.

Answer: A population has certain attributes that an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capita births and deaths, respectively. The rates, hence, expressed as change in numbers (increase or decrease) with respect to members of the population.

• Another attribute characteristic of a population is sex ratio. An individual is either a male or a female but a population has a sex ratio (e.g., 60 per cent of the population are females and 40 per cent males).
• A population at any given time is composed of individuals of different ages. If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid. For human population, the age pyramids generally show age distribution of males and females in a combined diagram. The shape of the pyramids reflects the growth status of the population (a) whether it is growing, (b) stable or (c) declining.

Q3. The following diagrams are the age pyramids of different populations. Comment on the status of these populations. 

Answer: Fig. A: It is a pyramid shaped age pyramid. In this figure, the base, i.e., pre-reproductive stage is very large when compared with the reproductive
and past reproductive stages of the population. This type of age structure indicates that the population would increase rapidly.
Fig. B: It is an inverted bell shaped pyramid. In this figure, the pre- reproductive and reproductive stages are same. This type of age structure indicates that the population is stable.
Fig. C: It is ‘Urn’ shaped pyramid. In this figure, the pre-reproductive and reproductive stages are less than the post reproductive stage of this population. In this population, more older people are present. This type of age structure indicates that the population definitely is declining,

Q4. Comment on the growth curve given below

Answer:  A population growing in a habitat with limited resources show initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity. A’ plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth and is described by the following equation:

\( \frac{d N}{d t}=r N\left(\frac{K-N}{K}\right)\)

\( \frac{d N}{d t}=r N\left(1-\frac{N}{K}\right)\)

\( \left(1-\frac{N}{K}\right) \quad \text {Environmental resistance }\)

Q5. A population of Paramecium caudatum was grown in a culture medium. After 5 days the culture medium became over crowed with Paramecium and had depleted nutrients. What will happen to the population and what type of growth curve will the population attain? Draw the growth curve.

Answer: Pattern of growth: The population of Paramecium shows a sigmoid pattern of growth with lag, log and stationary phase.

Lag phase: First the population will be in the lag phase of growth as the Paramecium is acclimatising itself to the resources. So, the numbers don’t increase rapidly.

Log phase: Since food and space for the population are abundant, the population of Paramecium reproduces exponentially resulting in a log phase right after the lag phase.

Stationary phase: After $5$ days, the culture becomes crowded representing the stationary phase. The nutrients get depleted showing that the carrying capacity of the environment is reached ($K$ value).

Growth curve:

The curve is sigmoid or S-shaped in nature. It shows a population growing in a habitat having limited resources and has the following growth trends:

• Lag phase: Represented by the first part of the curve, this is the time when the population is getting adjusted to the environment.
• Log phase: The second part of the curve shows exponential increase in the population as the resources are available.
• Stationary phase: As population density increases, competition among species increases, resources become a limiting factor resulting in the slowdown of growth. The population reaches the carrying capacity (K) of the environment.

Diagram

Q6. Discuss the various types of positive interactions between species.

Answer: Both the species benefit in mutualism. The interaction where one species is benefitted and the other is neither benefitted nor harmed is called commensalism.

\(
\begin{array}{|c|c|c|}
\hline \text { Species A } & \text { Species B } & \text { Nature of interaction } \\
\hline+ & + & \text { Mutualism } \\
\hline+ & 0 & \text { Commensalism } \\
\hline
\end{array}
\)

Q7. In an aquarium two herbivorous species of fish are living together and feeding on phytoplanktons. As per the Gause’s Principle, one of the species is to be eliminated in due course of time, but both are surviving well in the aquarium. Give possible reasons.

Answer: Each species has a specific position or functional role within the community, called niche. According to the Gausse’s principle, no two species can live in the same niche. In this case, two herbivorous species are living in the same niche and feeding on phytoplanktons. It may be because of the availability of sufficient phytoplanktons and or less number of individuals of the fish species. Of the two species might have occurred and though neither of the species have been eliminated, niche overlapping may effect the growth and development of individuals of the species.

Q8. While living in and on the host species, the animal parasite has evolved certain adaptations. Describe these adaptations with examples.

Answer: While living in and on the host species, the animal parasite has evolved certain adaptations. Describe these adaptations with examples.

Q9. Give one example for each of the following:
i. A parasitic angiosperm _______________

Answer:  A parasitic angiosperm – Cuscuta reflexa