Symbiosis - a relationship between populations of different species in close often intimate contact.
Recall - endosymbiontic evolution of mitochondria and chloroplasts
Mutualism - a symbiotic relationship where both  relationship species benefit.
There are numerous cases of mutualism in nature that have arisen through the process of coevolution. This is where 2 species have evolved in conjunction in a way that creates strong often obligatory codependence.

For example the bacteria in the gut of cows, they provide glucose from cellulose in the cow's diet and the cow supplies the food warmth and an anoxic environment. Cows as a result release significant volumes of methane contributing to global warming.
 

Lichens the leafy green plant like organism that grows on rocks is actually a fungus and an algae living together symbiotically. Neither the alga or the fungus can live with out each other
 

Coral polyps and algae are another good example and by the way if the coral algae dies the coral bleaches and the polyps  die soon later.
 
Cow

Lichen

Coral
Commensalism - relationship between 2 species  where only 1 species benefits and the other is not harmed.

Some ant birds follow the trails of tropical fire ants because the ants scare up other organisms upon which its preys.

Some Damsel Fish can hide among the normally stinging polyps of anemonae and coral polyps without being harmed and are protected by the polyps.
Antbird


Damselfish
Parasitism - parasites live in or on a host organism which supplies it with nutrition and habitat. A well evolved parasite does not cause the host great harm as it must count on the hosts survival for its own. Many diseases and parasites of humans originated and continue to originate in Africa where humans spent the most time coevolving with them. One of the benefits of human migration north to the inhospitable northern climes was the escape from these parasites and diseases although new and old diseases soon followed.

Microparasites - small protozoan or arthropod parasites that may live internally or externally. Plasmodium which causes disease malaria is an example of a protist parasite that is tiny enough to live in red blood cells.
 

Endoparasites - (inside of body) the hookworm lives in the small intestine attached to the intestinal wall. The Guinea worm lives inside the leg of a human host and protrudes out to lay eggs when the host contacts water. It must be removed by twisting on a twig over a period of and it is believed this action is displayed on the medical symbol the caduceus.
(Dracunculus medinensis)
Ectoparasites -   (outer, skin) Fleas, lice, ticks are all ectoparasites that live on the outside of its host. Even whales have ectoparasites such as isopods as well as endoparasites such as worms. A humpback whale has been observed to carry over 1000 kg of barnacle parasites.
In Ontario a nasty ectoparasite called the Deer Tick has a population in the UN biosphere reserve at Long Point a spit of land that stretches out into Lake Erie. This tick is a vector for the bacteria Borrelia burgdorferi that causes the Lyme disease (first found in Lyme, New Hampshire just as Norwalk flue was first found in Norwalk).  


Hookworm
endoparasite

 


Human Flea ectoparasite

 

 


parasite and disease vector