This occurs when two organisms use the same resource, such as food. Thus the concept of the niche is closely associated with the concept of competitive exclusion and competitive relationships among spices (see chapter 16).
Basically, Gause Principle or competitive exclusion theory states that no two species persist in the same niche. Either one becomes extinct, through natural selection; diverge into different niches. The latter phenomenon is called niche separation. Consider, for example, two species of plants that are able to grow in saline water, as in salt marsh.
Species A can grow successfully if the salt content of the soil solution is 0 2 per cent; and species B car grow successfully if the salinity of the soil solution is 1 —3 per cent. Individuals from both populations which are physiologically best adapted to survive under salinities of 1 2 percent lie in the area of overlap and are in direct competition with one another.
Those individuals of species A that favour a salinity lower than 1 per cent and those individuals of species B favoring a salinity greater than 2 per cent lie in the areas of non-overlap and are not in inter-specific competition. As a general rule, individuals from populations with overlapping niches but which themselves lie outside the area of overlap are likely to show greater survivorships and reproductive success, because they are not competing with individuals from the other species.
Thus natural selection tends to favour these individuals, and the non-overlapping portions of the population’s niche will tend to increase in relation to the overlapping portion. The reduction in the area of overlap is equivalent to separation of the niches.
The amount of niche overlap is usually proportional to the degree of competition for that resource (Note, competition involves only a resource that is in short supply). In extreme cases niches may be adjacent to one another with no overlap. In another extreme case the fundamental niche of one may be completely within the fundamental niche of another, as in the case of seedling Rumex and grass.
In such instances the Rumex seedling is competitively superior. In most cases, however, fundamental niches may partially overlap. Some niche space is shared and some is exclusive, enabling the organisms to coexist.
For example, in Natal (Africa), two species of rhinoceros, the black and white live in the same area but have different food niches. The black rhinoceros is a browser, feeding woody plants, while white rhinoceros is grazer, eating herbs and Grasse: so that they are not in competition for the same food resource.