Niches: How Species Coexist

Taken to its logical extreme, we could imagine an Earth with very few species, perhaps one green plant on the land, one herbivore to eat it, one carnivore, and one decomposer. If we added 4 species for the ocean and 4 for fresh waters, we would have only 12 species on our planet. Yet about 1.5 million species have been named and scientists speculate that many more millions may exist; the number is so great, we do not have even a good estimate of it. How can they all coexist? The answer is their niches—the species’ way of life or functional role in an ecosystem.Species that require the same resources can coexist by utilizing those resources under different environmental conditions. They are said to have different ecological niches.
An ecological niche takes into account all aspects of the organism’s existence—that is, all physical, chemical, and biological factors that the organism needs to survive, remain healthy, and reproduce—what it does for living: what it eats, what eats it, what organisms it competes with, and how it interacts with and is influenced by the abiotic components of the ecosystems in which it is found and the role it plays in the energy flow and matter recycling in an ecosystem ( its occupation ). The niche represents the totality of an organism’s adaptations, its use of resources, and the lifestyle to which it is suited. 

Fundamental Niche 

A species’ fundamental niche is the full potential range of physical, chemical, and biological conditions and resources it could theoretically use if there were no direct competition from other species. But in a particular ecosystem, species often compete with one another for one or more of the same resources. 
Realized Niche
The lifestyle that an organism actually pursues and the resources that it actually uses make up its realized niche.
Understanding a species’ niche can help us to prevent it from becoming prematurely extinct and to assess the impact of development or changes in land use (i.e., change in environment). Cutting a forest may drive away prey and eliminate the niche of the wolf.   

 
Symbiosis
Symbiosis is a close, long-lasting, physical relationship between two different species. In other words, the two species are usually in physical contact and at least one of them derives some sort of benefit from this contact.  Each partner in symbiosis is called a symbiont. Two species can benefit one another and enhance each other’s chance of persisting.
Symbiosis is widespread and common; most animals and plants have symbiotic relationships with other species. Symbiosis therefore affects biological diversity.                                      

 
Predation
Predation is the third way in which species interact. It occurs when one organism, known as a predator, kills and eats another, known as the prey. The predator benefits from killing and eating the prey and the prey is harmed. Predation can increase diversity of prey species by reducing the abundance of the dominant prey. In this way, predators can contribute to species evenness by keeping the dominant species from overwhelming others via competitive exclusion.                                    
Parasitism
Parasitism occurs when one species (the parasite) feeds on part of another organism (the host) by living on or in the host. In this relationship, the parasite benefits and the host is harmed. Parasitism can be viewed as a special form of predation. But unlike a conventional predator, a parasite usually is smaller than its host (prey). It also rarely kills its host, and remains closely associated with, draws nourishment from, and may gradually weaken its host over time. Parasitism also promotes biodiversity by helping prevent some species from becoming so plentiful that they eliminate other species through competition.                                                   


When one considers all the interactions among the organisms in a community, however, it is obvious that some species benefit at the expense of others. If one species declines during a disturbance such as a drought, other species that compete with it may flourish during that period. Thus, it appears that ecosystems with greater species richness are more likely to contain species that are resistant to any given disturbance.