Symbiosis is a close and long-term interaction between two or more different biological species. These relationships can be crucial for the survival and evolution of the involved organisms and play a significant role in ecosystem dynamics.

Types of Symbiotic Relationships:

• Mutualism (+/+): A relationship where both interacting species benefit.
  • Example: The relationship between bees and flowering plants. Bees obtain nectar (food), and in the process, they transfer pollen, which helps the plants reproduce.
  • Example: Mycorrhizae are mutualistic associations between fungi and plant roots. The fungi enhance nutrient and water uptake by the plant, while the plant provides the fungi with carbohydrates.


• Commensalism (+/0): A relationship where one species benefits, and the other is neither harmed nor helped.
  • Example: Epiphytes (like orchids) growing on trees. The orchid benefits by getting better access to sunlight, while the tree is generally unaffected.
  • Example: Barnacles attaching to whales. The barnacles gain a mode of transportation and access to food in different locations, while the whale is usually not harmed.


• Parasitism (+/-): A relationship where one species (the parasite) benefits at the expense of the other species (the host), which is harmed. Parasites often live on or inside their hosts.
  • Example: Tapeworms living in the intestines of animals, absorbing nutrients from the host.
  • Example: Ticks feeding on the blood of mammals.
  • Note: Parasites usually do not kill their hosts immediately, as their survival depends on the host's survival, at least for a period.


• Amensalism (0/-): A relationship where one species is negatively affected by another, while the other is unaffected.
  • Example: The release of antibiotics by some microorganisms that inhibit the growth of other microorganisms.
  • Example: A large tree shading smaller plants, preventing them from getting enough sunlight.


• Neutralism (0/0): A relationship where neither species affects the other. This is difficult to prove in nature, as there might be indirect or unnoticed interactions.

Key Aspects of Symbiosis:

• Obligate Symbiosis: The symbiotic relationship is essential for the survival of one or both partners. They cannot survive independently.
  • Example: Lichens, which are a composite organism formed from a fungus and an alga or cyanobacterium. The fungus provides structure and protection, while the alga/cyanobacterium provides food through photosynthesis.


• Facultative Symbiosis: The symbiotic relationship is beneficial but not essential for the survival of the partners. They can survive independently.
  • Example: Cleaner fish that remove parasites from larger fish. Both benefit, but they can survive without each other.


• Ectosymbiosis: One symbiont lives on the surface of the other.
  • Example: Lice on a human head.


• Endosymbiosis: One symbiont lives inside the tissues of the other.
  • Example: Nitrogen-fixing bacteria (Rhizobium) living in the root nodules of leguminous plants.

Written by Kasiban Parthipan