Combining trends from disparate monitoring programs to inform Red List assessments: The case of the Cape Vulture (Gyps coprotheres)

Abstract

Conservation decisions are often informed by Red List assessments, which are substantially influenced by estimated population trends. Population trend estimates used by Red List assessments should therefore be as quantitative, comprehensive, and transparent as possible. We combine counts of breeding pairs of the Cape Vulture (Gyps coprotheres) that occur at separate breeding colonies and span differing time periods. The methodological concept is simple: transform time series into interannual rates of change, put those rates into a year-by-series matrix, then average that matrix to estimate the interannual population wide rate of change. Our analysis uses state-space models and basic arithmetic to estimate interannual rates of change per time series. Analyses are performed under a Bayesian framework to ensure that uncertainty is propagated into a composite index that estimates the percent global population reduction over three generations. Our results indicate that the global Cape Vulture population declined by 25 % (95 % CRI = 0.5 %–44 %) from 1977 to 2019. Such a decline suggests the species should be listed as Near Threatened under Criterion A2, instead of Vulnerable, as the species is currently listed. However, we performed a sensitivity analysis that suggested the species might indeed be Vulnerable if unmonitored colonies are in decline. Although the analysis for each species and time series should be customized, we suggest that the general practice of averaging the interannual rates of change for all available time series could improve qualitative estimates of population reduction used in many Red List assessments.