Final answer:
The phenomenon where similar species coexist by each using only part of the available resources is due to niche partitioning, which allows them to minimize direct competition and occupy different microniches.
Step-by-step explanation:
When similar species coexist and each species uses only part of the available resources, this phenomenon is most likely due to niche partitioning. In this process, species evolve to utilize different resources or different aspects of the same resource, which allows them to coexist by minimizing direct competition.
Competitive exclusion is a principle that says two species cannot occupy the same niche in a habitat: if they are competing for all the same resources, one will outcompete the other. However, through niche partitioning, species avoid this by occupying different microniches. For instance, they might use different resources, areas of the habitat, or feed during different times of the day.
Symbiosis refers to close interactions between species, but not all symbiotic relationships involve niche partitioning. In the case of mutualism, both species benefit from the interaction, but this is a different concept from what allows multiple species to efficiently coexist through minimizing direct competition for the same resources.