This is usually a yearly probability of persistence.
Realistically, what are the chances of a local population of species y go functionally extinct over x years if not managed?
An expert may think - or an analysis may show - there is a 10% chance that the local population will go functionally extinct over 10 years without management: $p(y=extinct, period= 10 years)= 0.10$
This extinction probability can then be expressed as a persistence probability over 10 years: $p(y=persist, period= 10 years)=0.9$
Which can then be expressed as a yearly probability: $p(y=persist, period= 1 year)= (0.9)^{\frac{1}{10}} = 0.98951$
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