CBSE Class 12 Biology – Population Interactions & Predation (Point-wise Notes)

Life History Variation

  • Populations evolve to maximize reproductive fitness (Darwinian fitness).
  • Darwinian fitness is associated with a high intrinsic rate of increase (r).
  • Organisms evolve reproductive strategies according to habitat conditions.
  • Some organisms breed once in a lifetime (Semelparity):
    • Pacific salmon fish
    • Bamboo
  • Some organisms breed many times in a lifetime (Iteroparity):
    • Most birds
    • Most mammals
  • Some produce many small offspring:
    • Oysters
    • Pelagic fishes
  • Some produce few large offspring:
    • Birds
    • Mammals
  • Life-history traits evolve due to biotic and abiotic constraints of the habitat.

Population Interactions

  • Interaction between populations of different species is called interspecific interaction.
  • Interactions may be:
    • Beneficial (+)
    • Harmful (−)
    • Neutral (0)

Types of Population Interactions

InteractionEffect
Mutualism(+,+)
Competition(−,−)
Predation(+,−)
Parasitism(+,−)
Commensalism(+,0)
Amensalism(−,0)

Predation

  • Predation is an interaction where:
    • Predator benefits (+)
    • Prey is harmed (−)
  • Predators transfer energy from lower to higher trophic levels.
  • Herbivores are also considered predators in a broad ecological sense.

Ecological Importance of Predators

  • Control prey population size.
  • Prevent ecosystem instability.
  • Maintain species diversity.
  • Reduce competition among prey species.
  • Used in biological control of pests.

NCERT Examples

Prickly Pear Cactus

  • Introduced into Australia.
  • Became invasive and spread rapidly.
  • Controlled by introducing a cactus-feeding moth.

Starfish (Pisaster)

  • Important predator in rocky intertidal communities.
  • Removal led to extinction of more than 10 invertebrate species due to competition.

Prey Adaptations Against Predation

Camouflage

  • Insects and frogs use cryptic colouration.
  • Helps avoid detection by predators.

Chemical Defence

  • Some animals are poisonous.
  • Predators avoid them.

Monarch Butterfly

  • Distasteful to birds.
  • Obtains toxic chemicals during caterpillar stage from poisonous weeds.

Plant Defences Against Herbivores

Morphological Defence

  • Thorns in:
    • Acacia
    • Cactus

Chemical Defence

  • Toxic chemicals:
    • Inhibit feeding
    • Inhibit digestion
    • Disrupt reproduction
    • May kill herbivores

Calotropis

  • Produces poisonous cardiac glycosides.
  • Avoided by cattle and goats.

Important Plant Defence Chemicals

  • Nicotine
  • Caffeine
  • Quinine
  • Strychnine
  • Opium

CBSE Exam One-Liners

  • Darwinian fitness = Reproductive success of an organism.
  • Predation = (+,−) interaction.
  • Mutualism = (+,+) interaction.
  • Competition = (−,−) interaction.
  • Commensalism = (+,0) interaction.
  • Amensalism = (−,0) interaction.
  • Pisaster is a keystone predator.
  • Monarch butterfly is protected by toxic chemicals.
  • Calotropis contains cardiac glycosides.
  • Prickly pear cactus in Australia was controlled by a cactus-feeding moth.

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