CBSE · Class 12 · Biology · Organisms and PopulationsExplain the various population interactions observed in nature between different species. Describe mutualism, competition, predation, parasitism, commensalism, and amensalism with suitable examples.

Introduction\nIn any ecosystem, biological organisms do not exist in isolation. Instead, populations of different species interact with one another in multiple ways to form a biological community. These interactions can be beneficial, detrimental, or neutral to the participating species.

1. Mutualism (+/+)

• Definition: An interaction where both interacting species are mutually benefitted.
• Example: Lichens represent an intimate mutualistic relationship between a fungus and algae or cyanobacteria. The fungus provides shelter, water, and minerals to the algae, while the algae prepare food through photosynthesis for the fungus. Another classic example is the pollination of fig trees by specific wasp species, where the wasp gets a safe oviposition site and nourishment, while the plant gets pollinated.

2. Competition (-/-)

• Definition: A process in which the fitness of one species is significantly lowered in the presence of another due to the struggle for common limiting resources.
• Example: Gause's Competitive Exclusion Principle states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will eventually be eliminated. For instance, the abingdon tortoise in Galapagos islands became extinct within a decade after goats were introduced on the island, due to greater browsing efficiency of goats.

3. Predation (+/-)

• Definition: An interspecific interaction where an organism, called a predator, kills and consumes another organism, called the prey.
• Example: Tigers hunting deer, or the prickly pear cactus introduced in Australia in the early 1920s being managed effectively by introducing a cactus-feeding moth from its natural habitat.
• Significance: Predators control prey populations, maintain species diversity in a community, and act as conduits for energy transfer across trophic levels.

4. Parasitism (+/-)

• Definition: A mode of interaction where one species (parasite) depends on another organism (host) for food and shelter, causing harm to the host.
• Example: Human liver fluke, ticks on dogs, and Cuscuta growing on hedge plants. Parasites often evolve special adaptations like loss of unnecessary sense organs, presence of adhesive organs or suckers, and high reproductive capacity.

5. Commensalism (+/0)

• Definition: An interaction in which one species benefits and the other is neither harmed nor benefited.
• Example: An orchid growing as an epiphyte on a mango branch, or barnacles growing on the back of a whale.

6. Amensalism (-/0)

• Definition: An interaction where one species is harmed while the other is unaffected.
• Example: The secretion of chemical substances by certain fungi or bacteria (like Penicillium secreting penicillin) that inhibit the growth of surrounding bacteria without deriving any direct benefit from them.