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. (a) 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?
(b) 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.
Student to attempt either sub-part (c) or (d):
(c) 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?
\begin{center} OR \end{center} (d) Name two methods to measure the population density of tigers.

Hint:

Population density is dynamic and varies with birth, death, immigration, and emigration. Different species require different techniques to estimate their population effectively.

Answer 1

(a): Monarch butterflies acquire the chemical by feeding on poisonous milkweed plants during their larval stage. These chemicals accumulate in their body making them unpalatable to predators. (b):
Population at \( t + 1 = 800 \)
Using formula:
\[ N_{t+1} = N_t + \text{Natality} + \text{Immigration} - \text{Mortality} - \text{Emigration} \] \[ 800 = N_t + 200 + 200 - 150 - 100 \] \[ 800 = N_t + 150 \Rightarrow N_t = 650 \] Since natality and immigration are higher than mortality and emigration, it indicates a growing population. Therefore, the type of age pyramid formed would be **expanding**. (c):
For carrot grass (uniform, small individuals), population density is best measured in terms of **number of individuals per unit area**.
For a huge banyan tree (single large organism with many offshoots), population is better measured in terms of **biomass** or **canopy cover**. \begin{center} OR \end{center} (d):
Two methods to measure tiger population density:
1. **Pugmark method** – tracking and counting footprints.
2. **Camera trapping** – using motion-sensor cameras to photograph and identify individual tigers.