Symbiosis is the term that scientists use to describe the relationship between two or more species living in close association with one another.  Many of these interactions involve one species living on or within another species.  In this way, most, if not all, larger organisms are actually composite organisms:  multiple organisms living symbiotically with one another.  Consider, for a moment, the fact that the  bacterial cells on and within your body outnumber your own cells 10:1!  In some species, such as termites and cockroaches, symbiotic microorganisms make up 30-50% of their body weight.  In many cases, symbioses between species have evolved to the point that each species is critically dependent on the other:  eliminate one partner and the other will be eliminated also. 

    Symbioses are generally described as falling into one of three categories: parasitism, commensalism and mutualism.  These categories are further divided into facultative and obligate relationships.  In a facultative symbiotic relationship, the symbionts are not dependent upon the relationship and therefore can exist independently of one another under natural conditions.  In an obligate relationship, at least one of the symbionts have become entirely dependent upon the relationship for survival. 

     We live in a magnificently diverse world. In the 14 mile thick blanket of life that covers the Earth's surface reside an estimated 100 million species.  Regardless of where we look, from the driest deserts to the deepest depths, we find an incredibly diverse assemblage of living things, and across all geographical boundaries and taxonomic categories we find symbiosis. Of course, the vast majority of life on Earth is microscopic: virtually unnoticed until the mid-17th century, and arguably under appreciated even today.  But, the truth is, it is upon this microscopic world and its symbiotic relationships that the diversity of life depends.