Ecological Interactions: Mutualism, Commensalism

Ecological interactions describe the ways organisms influence each other’s survival and reproduction. These include predation, parasitism, competition, mutualism, commensalism etc. These interactions shape ecosystem dynamics and can be broadly positive, negative or neutral. The key ecological interactions can be classified into Positive, Negative and Neutral Interactions.

Positive Interactions

Positive interactions occur between organisms when none of the species is harmed. Two key types are Mutualism and Commensalism.


In Mutualism, both species benefit in the relationship. Notable examples are as follows:

  • Bees obtain nectar from flowers while flowers are pollinated by bees carrying pollen.
  • Nitrogen fixing bacteria aid plant growth while living in them.
  • Gut flora in human as well as other animals are examples of mutualism whereby, they help us to keep healthy while living inside our body.
  • Acacia trees provide food and shelter to ants which in turn protect acacia trees from foragers like giraffes and other herbivores.
  • Clownfish and sea anemones offer protection to each other from potential predators.
  • Egyptian plovers eat parasites and debris from crocodiles’ teeth, benefiting both species.

In commensalism, one species benefits with the other being unaffected. Notable examples of commensalism are:

  • Epiphytes like orchids grow on trees gaining elevation but not harming trees.
  • Remora fish swim alongside sharks and rays to eat their leftovers without harming them.
  • Sea anemones are often found attached to hermit crabs’ shells. They benefit from food particles the crabs scavenge without hindering the crabs.
  • Once a lion has finished its meal, a vulture swoops down and finishes off the carcass. The relation between lion and vulture is commensalism.

Negative Interactions

Negative interactions happen when one species benefits at the expense of harming another. Some examples:

  • Predation – Individuals of one species (predator) kill and eat those of another species(prey). Lions prey on zebras and other herbivorous animals.
  • Herbivory – Animals feeding on plants exemplify herbivory. Deer and cattle browse on forest vegetation and grasses.
  • Parasitism – Parasites derive nutrients by harming host organisms. Ticks, lice and leeches are common parasites. Tapeworms parasitize vertebrate digestive systems.
  • Competition – Occurs when multiple species compete for limited resources like water, light, food, and mate. For example, trees in a dense forest compete for space and sunlight to grow. Less adapted competitors are negatively impacted. Interspecific competition is between different species (e.g. lions and hyenas) while intraspecific is between members of the same species.

Neutral Interactions

These interactions between species have no positive or negative impact on either organism. For instance, cows and sparrows may co-exist in a field without affecting each other.

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