Most of the interactions between species involve food:
These interactions are often brief. There are many cases, however, where two species live in close association for long periods. Such associations are called symbiotic ("living together").
In symbiosis, at least one member of the pair benefits from the relationship. The other member may be:
Mutualism: Both species benefit. The
two organisms help each other. An example would be a honey bee and a dandelion. The honey
bee gets to eat the pollen from the flower. The dandelion uses the bee to spread its
pollen to another flower.
Commensalism: One species benefits. The other species is unaffected. A common example is an animal using a plant for shelter. An American Robin benefits by building its nest in a Red Maple tree. The tree is unaffected.
Parasitism: Ones species benefits. The other species is harmed. An example would be a deer tick and a White-tailed Deer. The tick gets food from the deer without killing it. The deer is harmed by losing blood to the tick, and possibly by getting an infected wound.