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How do species richness and relative abundance contribute to species diversity?

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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, species diversity is defined by two primary components: species richness and relative abundance.1

Species richness is the total count of unique species present within a specific biological community or area. It provides a simple measure of the variety of life but does not account for the population sizes of those species.2
Relative abundance refers to the evenness of the distribution of individuals among the species in a community. It measures how common or rare each species is in relation to others.1

These two factors work together to determine the overall diversity of an ecosystem. For instance, two communities might have the same number of species (equal richness), but they will differ in diversity if one community has a more even distribution of individuals across those species than the other, where one species might significantly outnumber the rest. Understanding both components is essential for assessing the health, stability, and resilience of biological communities in response to environmental changes or disturbances.13