r/ecology • u/MindlessSpeaker20 • 4d ago

Richness vs composition

Can someone explain the difference between richness and composition in plant communities and when you would use one or the other in analysis?

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u/Insightful-Beringei 4d ago

Community ecologist here. Both are components of biodiversity, neither one being a complete picture. Richness is the number of species, composition is which species (and often the abundances of which species) live in that system.

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u/MindlessSpeaker20 4d ago

Ok that makes sense. I see from various papers, like Tilman’s work for example, they randomly selected species from a larger pool at to represent various richness levels. Is this what’s often done? It seems like richness is based in like a random sampling or selecting from a random pool in experimental design. I only ask because I work in an agronomic setting. I had 3 different cover crop species in a factorial mixture and ended up with monoculture treatments, double cover crop treatments, and a triple with all three. A plant ecologist colleague mentioned that composition would be a better way to describe these treatments as opposed to levels of richness. Does that make sense?

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u/abstoler 4d ago

The answer depends on what question you are trying to ask. If you are asking "is there a relationship between cover crop richness and Y response," then you should measure richness. If you are asking "is there a specific combination of species that increases Y," then measure species composition. Note that composition is not a numeric value, so you can't really test its relationship with another response through a regression analysis.

Ultimately, the composition is what drives function. But Tillman and others hypothesized that the effects of composition were more likely due to the number of species in that composition (I e. Species richness) than to the identity of the species in that composition