The occupation of different niches on the open plains and woodlands of Africa by the zebras and the wildebeests is a result of niche partitioning. The zebras and the wildebeests in a common niche would compete for food, water, shelter, and space. This competition would affect the population growth of the two species by negatively affecting their reproductive capabilities. Thus, in the same habitat, the two species choose to occupy two different niches such that the competition between them is minimised. Hence they end up in separate niches.
Oxygen, neccessary for cellular respiration, is the answer. As oxygen becomes scarce, your cells begin to aerobically produce very little ATP, in effect fermenting and producing lactic acid.
