David Tilman's study of two diatom species, each with differing abilities to utilize different levels of multiple resources, showed that coexistence is only possible when both resource levels are high. coexistence is only possible when both resource levels are low. coexistence is not possible. coexistence is possible if each species is more efficient at using a different resource than the other species.

coexistence is possible if each species is more efficient at using a different resource than the other species.

Explanation:

Competition is an ecological and evolutive process very common in nature. Competition might be intra- or interspecific. Competition between different species in a community or ecosystem might be due to the same resource use, or the same territory, shelter, etcetera. When a resource is useful for two or more species, and limited, they compete to gain it.

The principle of competitive exclusion states that different species with the same requirements sharing the niche can not coexist indefinitely based on the same limited resource. When two competing species coexist, this is because of niche partitioning or niche differentiation.

Differentiation of effective niche is closely related to character displacement.

Character displacement is the result of interspecific competition, in which two or more species that live in the same habitat manage to avoid competition by developing different traits. Morphological divergence, or any adaptative trait development, fixated genetically, is the product of niche segregation. Species tend to differentiate morphologically in the presence of strong competitors. Traits divergence favors coexistence in the same place.

In the exposed example, both species have different abilities to utilize different levels of multiple resources. If both species live in the same place, this ability is significant to avoid competition and competitive exclusion.  

explanation: