Mutualism and Parasitism

Over the last three weeks, I have been introduced to the world of Abiotic and Biotic interactions.  I have an interest in the different behaviours and relationships of organisms so found these particular topics highly fascinating.


Association between individuals of two species for mutual benefit.

Now, I have seen many examples of mutualism documented on television, but up until now have never really researched this relationship.  There are countless mammals, birds, insects, plants, fungi, bacteria etc which interact with one another symbiotically.  One of the most obvious mutualistic relationships is the interaction between pollinators and flowering plants.  The plants gain by having their pollen transferred from one flower to another and the pollinators gain a reward of nectar.

An example of mutualism that I found through my research is between the Oxpecker bird and Rhino or Zebra.  The Oxpecker will land and sit on the Rhino or Zebra, where it will eat ticks and parasites off their skin.  The bird gains food and the mammal gains a personal ‘pest-controller’.  Another interesting point is that the Oxpecker will also fly upwards and call a warning sound if in danger, which of course also benefits the Rhino or Zebra.

Mutualism is a +/+ relationship and Ecologists describe Biotic relationships in terms of who benefits and who loses…

+ signifies that one party benefits

– signifies that one party loses

0 signifies that one party is unaffected


A relationship between two organisms where a parasite gains from a relationship with its host.

Whereas mutualism is a +/+ relationship, parasitism tends to be +/- or +/0 relationship.  Initially the host may not be affected, however as a parasite continues it’s aim of gaining nutrients, a favourable environment or even the use of biochemical processes, the host inevitably suffers and dies once the parasite has reproduced.

One particular +/0/- interactions that I find to be the perfect mix of both amazing and disturbing is the Green-banded Broodsac, Snail and Bird cycle!  The ultimate goal of a Green-banded Broodsac parasite is to live inside the warm interior of a bird and feed off it. The larvae of this parasite live in bird droppings, which is eaten by snails. Once inside, the parasite grows and moves through the snail, protrudes out through the eyes, develops green bands like a caterpillar and pulsates in a similar way to a caterpillar. The parasite even manages to control the brain, making the snail move to somewhere noticeable.

Birds are consequently fooled into seeing two caterpillars and thinking they are getting a double snack, eat the snails’ eyestalks and therefore the parasite. The larva moves into the birds interior, lives and breeds… the bird then passes out the eggs in it’s droppings and the cycle continues.

Green-banded Broodsacs benefit – they achieve the goal of a warm interior and food for reproduction.  Birds gain a small meal but nothing much else (they apparently do not mind the parasite’s presence).  Snails infected with this parasite often live longer than snails that do not have it, but other than gaining a meal, the snails lose as they are ultimately turned into ‘zombies’.

Parasitic plants

As defined earlier, parasitism is a relationship between two organisms, and this of course includes plants.  There are over 4,000 species of parasitic flowering plants in the world – the most famous probably being Mistletoe, but after some interesting reading, I discovered the ‘Corpse flower’.  It is the largest individual flower in the world and is found in the forests of Malaysia and Indonesia.  It is nearly 1 meter in diameter and weighs up to 11 kg. As the name suggests, the corpse flower smells like rotting flesh in order to attract carrion-feeding flies as pollinators, whilst it’s sticky fruit is spread by rodents.  It cannot photosynthesize on its own, so being a parasite, steals it’s nutrients from the roots of neighbouring vines!

Do you know of any other interesting parasites or mutualistic relationships?


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