Symbiosis is the notion that two organisms of different species have a relationship. The relationships play a great part in communities, and are generally categorised as either mutualism, commensalism, or parasitism.
MUTUALISM is a relationship in which both species benefit from each other. For example in a cows stomach certain bacteria survive by breaking down cellulose eaten by the cow. They obtain energy this way, and allow the cow to absorb nutrients from the plant matter which would otherwise be undigestable. Similarly, the clown fish and the anemone exist in a mutual relationship. The clown fish are not harmed by the anemone’s secretions unlike most organisms, meaning it provides protection. The fish also drop food on the anemone when eating, providing it with some nutrients and energy.
COMMENSALISM is the relationship in which one organism benefits whilst the other is unharmed. In the marine environment barnacles attach themselves to other shellfish. This provides them with a habitat, but they do not harm the shellfish in any way.
PARASITISM is when one organism benefits and the other is harmed. This includes worms which can live in the human intestinal tract (they use up nutrients). Flea’s, ticks and lice are similar, as they take nutrients from their host organisms.