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Performance of IUCN proxies for generation length
Han Chi Fung1,2, Robin S Waples2
1Department of Applied and Computational Mathematical Sciences, University of Washington, Seattle, WA, 98105, U.S.A.
Summary
Estimating species generation length for conservation assessments is challenging. Proxies for generation length introduce biases, with an adjusted adult mortality proxy (T̂d(adj)) showing improved accuracy and reduced bias compared to other methods.
Area of Science:
- Ecology
- Conservation Biology
- Population Dynamics
Background:
- The International Union for Conservation of Nature (IUCN) uses generation length (T) to assess species threat status, requiring data on abundance decline over 3 generations or 10 years.
- Traditional calculation of T relies on detailed age-specific survival and fecundity data, which are frequently unavailable for many species.
- Proxies for generation length are often used due to data limitations, but these can introduce significant biases in conservation assessments.
Purpose of the Study:
- To evaluate the precision and bias of two commonly used generation length proxies (T̂d and T̂z) against true generation length.
- To assess the reliability of these proxies using published life table data from 78 diverse animal and plant populations.
- To identify potential improvements and limitations of existing methods for estimating generation length in conservation contexts.
Main Methods:
- Calculated true generation length (T) from published life tables for 78 populations.
- Compared T with two proxies: T̂d (based on adult mortality rate) and T̂z (based on reproductive life span).
- Evaluated an adjusted version of the adult mortality proxy, T̂d(adj) = T̂d - 1, for improved accuracy and reduced bias.
Main Results:
- Mean error rates for estimating T were 31% for T̂d and 20% for T̂z.
- The adjusted proxy T̂d(adj) reduced mean error rates to 16% and provided largely unbiased estimates.
- The performance of T̂z was inconsistent, and taxonomic relatedness did not reliably predict species comparability for proxy calibration.
Conclusions:
- Existing generation length proxies introduce substantial errors, highlighting the need for accurate life-history data.
- The adjusted adult mortality proxy (T̂d(adj)) offers a more reliable estimation method when detailed data are scarce.
- Collecting comprehensive life-history data remains crucial for robust conservation assessments, despite current data publication trends.
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