Organisms and Populations — NCERT Solutions

A population is a group of individuals of the same species living in a defined geographical area and sharing or competing for similar resources; they potentially interbreed.

A community is an assemblage of populations of different species living together in an area and interacting with one another.

(a) Commensalism: An interaction in which one species benefits and the other is neither harmed nor benefited.
Example: Orchid growing as an epiphyte on a mango branch.

(b) Parasitism: An interaction in which one organism (parasite) benefits and the other organism (host) is harmed.
Example: Cuscuta on a host plant, or lice on humans.

(c) Camouflage: A protective adaptation in which an organism blends with its surroundings so that it is not easily detected by predators.
Example: Some insects and frogs are cryptically coloured.

(d) Mutualism: An interaction in which both species benefit.
Example: Lichen is a mutualistic association between a fungus and an alga.

(e) Interspecific competition: A relationship between two different species that compete for the same resources, so the fitness of one species is reduced in the presence of the other.
Example: Flamingoes and resident fishes in shallow South American lakes competing for zooplankton.

The logistic population growth curve occurs when resources are limited.

Description

• The population first shows a lag phase.
• Then it enters a phase of acceleration.
• After that growth slows down in a deceleration phase.
• Finally it reaches an asymptote at the carrying capacity (K).

The curve is sigmoid or S-shaped and is described by:
$$\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)$$
where $N$ is population density, $r$ is intrinsic rate of increase, and $K$ is carrying capacity.

A simple diagram:

N
|                         ______ K
|                    ____/
|                ___/
|            ___/
|        ___/
|    ___/
|___/
+-------------------------------> t
   lag   accel.  decel.  asymptote

In parasitism, one organism benefits while the other is harmed. So the correct statement is one organism is benefited, other is affected.

Any three important characteristics of a population are:

1. Birth rate (natality)

• The number of births added to the population in a given time period.
• Example: If 8 new lotus plants are added to 20 plants, the birth rate is $8/20 = 0.4$ per lotus per year.

2. Death rate (mortality)

• The number of deaths in the population during a given time period.
• Example: If 4 fruitflies die out of 40 in a week, the death rate is $4/40 = 0.1$ per fruitfly per week.

3. Sex ratio

• The proportion of males and females in a population.
• Example: 60% females and 40% males.

4. Age distribution

• The proportion of individuals in different age groups.
• Example: An age pyramid showing pre-reproductive, reproductive and post-reproductive individuals.

These characteristics help describe the status and growth of a population.