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

Introduction to Population Interactions\nIn nature, animals, plants, and microbes do not live in isolation; rather, they interact in diverse ways to form a biological community. Interspecific interactions arise from the interaction of populations of two different species. These interactions could be beneficial, detrimental, or neutral to one of the species or both.

1. Mutualism (+/+)\nThis interaction confers benefits to both the interacting species.

• Example: Lichens represent an intimate mutualistic relationship between a fungus and photosynthesizing algae or cyanobacteria. Similarly, mycorrhizae are associations between fungi and the roots of higher plants. Another classic example is the plant-pollinator relationship, such as the Mediterranean orchid Ophrys employing sexual deceit to get pollinated by a species of bee.

2. Competition (-/-)\nInterspecific competition is a potent force in organic evolution, occurring when closely related species compete for the same limiting resources.

• Example: Visiting flamingos and resident fishes in shallow South American lakes compete for their common food, zooplankton. 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.

3. Predation (+/-)\nNature functions as an ecological theatre where predation is a vital conduit for energy transfer across trophic levels. Predators control prey populations, maintaining species diversity in a community.

• Example: Tigers hunting deer, or the prickly pear cactus introduced in Australia in the early 1920s being effectively brought under control by a cactus-feeding moth (predator).

4. Parasitism (+/-)\nIn this mode, only one species (the parasite) benefits while the other (the host) is harmed. Parasites have evolved specialized adaptations such as loss of unnecessary sense organs, presence of adhesive organs or suckers, loss of digestive system, and high reproductive capacity.

• Example: Human liver fluke, ticks on dogs, and Cuscuta growing on hedge plants.

5. Commensalism (+/0)\nIn this interaction, one species benefits, while the other is neither harmed nor benefited.

• Example: An orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of a whale.

6. Amensalism (-/0)\nIn amensalism, one species is harmed while the other is unaffected.

• Example: The fungus Penicillium producing penicillin, which inhibits or kills the growth of neighboring bacteria without itself being benefited or harmed.