Exercises

1. List the attributes that populations possess but not individuals.

• Birth rate and Death rate: Individuals have births and deaths; populations have rates (per capita).
• Sex Ratio: Individuals are either male or female; populations have a ratio of both.
• Age Distribution: A population consists of individuals of different ages, which can be represented as an age pyramid.
• Population Density: The number of individuals of a species per unit area or volume.

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

• Exponential growth equation: Nt = N0 ert
• Given Nt = 2N0 at t = 3.
• 2N0 = N0 er(3) ⇒ 2 = e3r
• ln(2) = 3r ⇒ 0.693 = 3r
• r = 0.231 per year.

3. Name important defence mechanisms in plants against herbivory.

• Morphological Defences: Thorns (Acacia, Cactus) and spines.
• Chemical Defences:
  • Poisonous cardiac glycosides (Calotropis).
  • Toxic substances like nicotine, caffeine, quinine, and strychnine.
  • Inhibitors that make herbivores sick or disrupt their digestion.

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

• This is Commensalism.
• The orchid (epiphyte) benefits by getting support and exposure to sunlight, while the mango tree is neither benefited nor harmed.

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

• The principle of Predation.
• Natural predators are used to keep the population of prey (pests) under control. For example, using Ladybirds to control aphids and Dragonflies to control mosquitoes.

6. Define population and community.

• Population: A group of individuals of the same species living in a well-defined geographical area, sharing resources and potentially interbreeding.
• Community: An assemblage of populations of different species living and interacting in a common habitat.

7. Define the following terms and give one example for each:

• (a) Commensalism: One species benefits, the other is unaffected. Example: Cattle egret and grazing cattle.
• (b) Parasitism: One species (parasite) benefits at the expense of the other (host). Example: Ticks on dogs.
• (c) Camouflage: Cryptic coloration to blend with the environment and avoid detection. Example: Leaf insects or frogs.
• (d) Mutualism: Both interacting species benefit. Example: Lichens (Algae and Fungus).
• (e) Interspecific competition: Competition for resources between individuals of different species. Example: Flamingoes and fish competing for zooplankton.

8. With the help of suitable diagram describe the logistic population growth curve. (Answer involves a Sigmoid/S-shaped curve showing: 1. Lag phase → 2. Acceleration → 3. Deceleration → 4. Asymptote at Carrying Capacity K)

• Equation: dN/dt = rN [(K-N)/K]

9. Select the statement which explains best parasitism.

• (d) One organism is benefited, other is affected.
• Explanation: In parasitism, the parasite derives nutrition and shelter from the host, typically harming the host in the process.

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

1. Population Density (N): The status of a population in its habitat, often evaluated in terms of numbers, biomass, or relative abundance.
2. Birth Rate (Natality): The number of per capita births in a given period, contributing to an increase in population size.
3. Age Distribution: Represented by age pyramids, showing whether a population is growing (broad base), stable, or declining.