Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 8648
Title: Performance of IUCN proxies for generation length
Author: Han-Chi Fung, Robin S. Waples
Publication Year: 2017
Journal: Conservation Biology
Keywords: endangered species,risk assessment,population demography

A criterion commonly used to assess conservation risk by the International Union for Conservation of Nature (IUCN) is the rate of decline in abundance, calculated over 3 generations or 10 years, whichever is longer.  The traditional method for calculating generation length (T) uses age-specific survival and fecundity, but these data are rarely available.  Consequently, proxies that require less information are often used, which introduces potential biases.  Two proxies recommended by the IUCN are the adult-mortality proxy,  T^d = a + 1/d, and the reproductive-lifespan proxy,  T^z = a + z*RL, where a = age at first reproduction, d = adult mortality rate, RL = reproductive lifespan, and z is a coefficient derived from data for comparable species.  We evaluated performance of each proxy using published life tables for 78 animal and plant species.  Mean error rates in estimating T were 31% for T^d  and 20% for T^z , but error rates for the adult-mortality proxy dropped to 16% when it was adjusted by subtracting one year ( T^d(adj) = a – 1 + 1/d), as suggested by theory; T^d(adj) also provided largely unbiased estimates regardless of the true generation length.  Performance of  T^z depends on having detailed data for comparable species, but our results suggest taxonomy is not a reliable indicator of comparability.  All three proxies depend heavily on a reliable estimate of age at first reproduction, as we illustrated by creating mock conservation assessments for two species (desert tortoise and bocaccio rockfish).  The relatively large mean errors, even for T^d(adj), emphasize the importance of collecting the detailed life history information necessary to calculate true generation length in species of conservation concern.  Unfortunately, publication of such data is less common than it was several decades ago.  We identified generic patterns of age-specific change in vital rates that can be used to predict expected patterns of bias from applying T^d(adj). 

Theme: Recovery and rebuilding of marine and coastal species
Foci: Describe the relationships between human activities and species recovery, rebuilding and sustainability.
Characterize the population biology of species, and develop and improve methods for predicting the status of populations.
Official Citation:

Fung, H., R. S. Waples. 2017. Performance of IUCN proxies for generation length. Conservation Biology 31:883-893; DOI: 10.1111/cobi.12901).