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How can two communities have the same number of species but different diversity patterns?

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Diagram showing energy flow in a food chain from a tree to a caterpillar, then to a bird, and finally to microbes, with red arrows illustrating energy transfer and heat loss at each stage. — community ecology Energy transfer and heat loss along a food chain.

In community ecology, two biological communities can possess the same number of species—a measure known as species richness—yet exhibit different species diversity patterns due to variations in species abundance and the evenness of distribution among those species.123

While species richness simply counts the total number of unique species, species diversity also accounts for the relative abundance of individuals within those species. For example, if two communities each contain 300 individuals across 5 species, they have identical richness. However, their diversity patterns will differ significantly if:13

Evenness: In one community, the 300 individuals are distributed equally, with 60 individuals per species.3
Dominance: In the second community, one species significantly outnumbers the other four, creating an uneven distribution.3

Because species diversity combines both the number of unique species and the population size of each, a community where individuals are spread more evenly among species is considered to have a different, and often more stable, diversity pattern than one dominated by a single species.423