Use of abundance data to detect species at risk of extinction may fail to detect initial declines in abundance

Variability in Population Abundance and the Classification of Extinction Risk

HOWARD B. WILSON, BRUCE E. KENDALL, HUGH P. POSSINGHAM

Conservation Biology, Volume 25, Issue 4, pages 747–757, August 2011

In conservation practice it is standard practice to Classify species according to their risk of extinction. The reliability of the classification depends on the accuracy of threat categorizations.  When it comes to errors in the process we have very limited info. It involves getting information from many sources. Understanding the quality of each source is de rigueur to evaluate the overall status of the species.

One common criterion used to classify extinction risk is a decline in abundance. Abundance is a direct measure of conservation status. So counts of individuals are generally the preferred method of evaluating whether populations are declining.

Using the thresholds from criterion A of the International Union for Conservation of Nature (IUCN) Red List (critically endangered, decline in abundance of >80% over 10 years or 3 generations; endangered, decline in abundance of 50–80%; vulnerable, decline in abundance of 30–50%; least concern or near threatened, decline in abundance of 0–30%), the researchers assessed 3 methods used to detect declines solely from estimates of abundance

1)      Use of just 2 estimates of abundance

2)       Use of linear regression on a time series of abundance;

3)      Use of state-space models on a time series of abundance.

The researchers generated simulation data from empirical estimates of the typical variability in abundance and assessed the 3 methods for classification errors.

The estimates of the proportion of falsely detected declines for linear regression and the state-space models were low (maximum 3–14%), but 33–75% of small declines (30–50% over 15 years) were not detected. Ignoring uncertainty in estimates of abundance (with just 2 estimates of abundance) allowed more power to detect small declines (95%), but there was a high percentage (50%) of false detections. For all 3 methods, the proportion of declines estimated to be >80% was higher than the true proportion

The researchers conclude that use of abundance data to detect species at risk of extinction may either fail to detect initial declines in abundance or have a high error rate.