Functional redundancy is being considered as a key component of biodiversity, potentially associated with the ability of ecosystems to buffer environmental perturbations.
Redundancy is possible when species are similar in some features but dissimilar in others. For instance the loss or decrease of nitrogen-fixing species, e.g. due to environmental perturbations, can be partially compensated by the presence of other nitrogen fixers, that are more tolerant to perturbation. This effect would be impossible if there was only one nitrogen-fixing species or if there were several nitrogen fixers with exactly the same response or sensitivity to perturbations. Clearly the presence of multiple species within each effect group increases the likelihood that they do not share the same sensitivity to perturbations.
Ecosystem stability should be maximized when species similar in some traits are dissimilar in others and such redundancy can allow both coexistence and stability in ecosystem functioning.