Question

Read the following passage and answer the questions that follow:

In nature, we rarely find isolated, single individuals of any species; majority of them live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population. The population has certain attributes whereas, an individual organism does not. A population at a given time is composed of individuals of different ages. The size of the population tells us a lot about its status in the habitat. Whatever ecological processes we wish to investigate in a population, be it the outcome of competition with another species, the impact of the predator or the effect of pesticide application, we always evaluate in terms of any change in the population size. The size, in nature, could be low or go into millions. Population size, technically called population density (N) need not necessarily be measured in numbers only. The size of a population for any species is not a static parameter. It keeps on changing with time depending on various factors including food availability, predation pressure and adverse weather.

1. The Monarch butterfly is highly distasteful to its predator because of a special chemical present in its body. How does the butterfly acquire this chemical?
2. If population density at a time t + 1 is 800, Emigration = 100, Immigration = 200, Natality = 200 and Mortality = 150, calculate the population density at time t and comment upon the type of age pyramid that will be formed in this case.
3. What is the difference in a method of measuring population density in an area if there are 200 carrot grass plants to only single huge banyan tree?
   OR
4. Name two methods to measure the population density of tigers.

Answer 1

a. Monarch Butterfly and Chemical Defense: Toxic cardiac glycosides found in milkweed plants are consumed by monarch butterfly larvae. Toxins build up in their bodies, which makes them repulsive to predators (chemical defense).

b Population Density Calculation:

Given: Population density (N) at t + 1 = 800

Emigration (E) = 100

Immigration (I) = 200

Natality (birth rate) (B) = 200

Mortality (death rate) (D) = 150

Formula: N_(t) + (B + I) − (D + E) = `N_((t + 1))`

N_(t) + (200 + 200) − (150 + 100) = 800

N_(t) + 400 − 250 = 800

N_(t) + 150 = 800

N_(t) = 800 − 150

N_(t) = 650

∴ The population density at t = 650.

Types of age pyramid: The population is growing because the birth rate is higher than the mortality rate, creating an upright or expanding age pyramid. The fact that this is wide at the base suggests that there are more young people.

c. The method of measuring population density differs based on the organism’s size and distribution.

• Numerical density, or the number of individuals per unit area (e.g., 200 plants per square meter), is used for carrot grass, which is made up of numerous little plants.
• On the other hand, a single massive banyan tree is better quantified using biomass density or canopy cover; scientists estimate the area covered by the tree’s branches and roots rather than counting individuals.

OR

d. Methods used to measure the population density of tigers are:

• Camera Trapping: They are infrared motion-sensor cameras in the habitats of tigers. The camera automatically takes pictures when a tiger moves in front of it. Scientists can recognize and count tigers thanks to their distinctive stripe patterns.
• Pugmark Analysis (Footprint Identification Method): On mud or soft dirt, tigers create distinctive footprints known as pugmarks. To determine the number of individuals, wildlife experts gather plaster casts of pugmarks and examine their size, shape, and pattern.