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D. Both competition and environmental filtering shape community structure.
Step-by-step explanation:
Based on competition theory, two organisms occupying the same ecological niche can only coexist provided there is spatial or conditioning niche partitioning.This prevent competitive exclusion , ensuring survival of the two species in their niche. Since the two species developed greater variability in (over-dispersed traits) to resist the selection pressure,(competition).Many overdisperesed traits is present in the communities.
However environmental filtering, is concerned with selection of species with natural ability or triats to survive in a given communities,Thus if two species of organism meet the same ecologocal requirements in a community, environmental filtering, fitters them and ensure their survival ahead of others who do not meet the requirements, and could not survive.
Thus the simultaneous operation of theses two opposing selection pressures, structured communities ensuring coexistence and favoured ability of organism to survive.