In order to better understand the role of an individual species within a larger system, we must first gain a basic understanding of that system. An ecological system can be broken down into smaller component parts. The list below outlines a general breakdown of a natural system in descending order (large to small).
Biosphere – the Earth and the atmosphere
Biome - large areas of the Earth that display similar climatic and geographical features. Typically these features determine what type of organisms inhabit these areas.
Ecosystem – a general term used to describe an area and both its biotic (living) and abiotic (non-living) components.
Community – all of the living organisms that inhabit an ecosystem
Population - all members of a single species living in a community of organisms
Organism - a individual member of a population
Morphotype – any group of different types of individuals in any population
A species is a group of genetically similar individuals that can breed and produce viable offspring (capable of reproducing). All species have critical functional roles within their larger ecosystem. Sometimes this role is referred to as the species niche and the area where the species lives is known as its habitat.
Although every species has a functional role within its habitat or ecosystem, some species are critically important in the overall health of the ecosystem. Such species are known as keystone species. The removal of a keystone species would cause a series of changes within the system and could eventually cause the system to collapse.
Relationships between species are also critical in the functioning of a healthy ecosystem. These are known as symbiotic relationships. Generally speaking these relationships can be defined three ways:
Mutualism - a relationship between two species where both benefit from the relationship (+,+)
Parasitism - a relationship between two species where one species benefits and the other species is negatively impacted (+,-)
Commensalism - a relationship between two species where one species benefits and the other species experiences neither positive or negative gain (=,0)
Niche vs. Habitat
A long time ago I had a science teacher who made this distinction incredibly clear. I will paraphrase him here: An organisms niche is its occupation (what it does for a living, job) and its habitat is its home or address.
Niche = job (functional role), Habitat = address