Population interactions

The essentials

In nature, populations of different species interact. The interactions are classified by their effect on each species: beneficial (+), detrimental (−) or neutral (0).

• Mutualism (+, +): both benefit. Lichens (fungus and photosynthesising alga or cyanobacterium); mycorrhizae (fungi and roots); fig tree and its pollinating wasp; Mediterranean orchid Ophrys, which uses sexual deceit by resembling a female bee to get pollinated.
• Competition (−, −): both are harmed. Flamingoes and resident fishes compete for zooplankton in South American lakes. Competition can occur even between unrelated species. Gause's competitive exclusion principle states that two closely related species competing for the same resources cannot coexist indefinitely; the competitively inferior one is eliminated. Species may avoid competition by resource partitioning, as shown by MacArthur's five closely related warblers feeding in different parts of the same tree.
• Predation (+, −): the predator benefits, the prey is harmed. Predators transfer energy to higher trophic levels, keep prey populations under control and help maintain species diversity by reducing competition among prey. The prickly pear cactus spread in Australia until a cactus-feeding moth was introduced. Herbivores are also predators of plants; plants defend themselves with thorns (Acacia, cactus) and chemicals (Calotropis produces cardiac glycosides). The monarch butterfly becomes distasteful to predators by feeding on a poisonous weed as a caterpillar.
• Parasitism (+, −): the parasite benefits at the host's expense. Ectoparasites (lice on humans, ticks on dogs, Cuscuta on hedge plants) and endoparasites (liver fluke). Brood parasitism: the cuckoo lays eggs in the crow's nest.
• Commensalism (+, 0): one benefits, the other is unaffected. Orchid growing as an epiphyte on a mango branch; barnacles on a whale's back; cattle egret foraging near grazing cattle; sea anemone and clownfish.
• Amensalism (−, 0): one is harmed, the other is unaffected.

Common mistakes

• Predation and parasitism have the same sign pattern (+, −), but parasites usually do not kill the host quickly.
• Clownfish and sea anemone is commensalism in NCERT, not mutualism.