Shaffer and Stein broadly define the biological principles of representation, resiliency, and redundancy as “saving some of everything,” and “saving enough to last.”

In their application to Yellowstone bison:

Representation is saving populations “in an array of different environments,” and “the ecological and evolutionary patterns and processes” that allow for natural selection, adaptation and reproduction in the wild.

Redundancy is the ability to withstand catastrophic events by “having essential backups” elsewhere “as a hedge against the failure of any individual population” in the wild.

Resiliency is the ability to withstand disturbances and adverse events in protected habitats large enough to accommodate population dispersal and recovery in the wild.

Mark L. Shaffer & Bruce A. Stein, Safeguarding our Precious Heritage, (2000).

“Together, the 3Rs, and their core autecological parameters of abundance, distribution and diversity comprise the key characteristics that contribute to a species’ ability to sustain populations in the wild over time. When combined across populations, they measure the health of the species as a whole.” U.S. Fish & Wildlife Service Guidance on Responding to Petitions and Conducting Status Reviews under the Endangered Species Act (August, 2016, PDF).