Value of combining transect counts and telemetry data to determine short‐term population trends in a globally threatened species

Abstract

In order to evaluate conservation interventions, it is necessary to obtain reliable population trends for short (<10 years) time scales. Telemetry can be used to estimate survival rates, and has become a common tool for assessing population trends, but can be limited by high costs, weight and technology constraints that limit duration of function and number of fixes, and can be biased to specific behavioral traits of tagged individuals. Encounter rates calculated from transects can be useful for assessing changes across multiple species, but can have large confidence intervals and can be affected by variation in survey conditions. Declines of African vultures, largely due to poisoning, have been well‐documented over the last several decades, but our understanding of more recent trends is lacking. We used two methods to assess population trends in three protected areas in southern Tanzania over 6–8 years: telemetry data primarily from White‐backed vultures to estimate survival rates, with corresponding estimated population trajectories based on population models, and transect counts for six scavenging raptors. Both methods suggest significant declines for White‐backed vultures in Ruaha and Nyerere National Parks, while only telemetry estimates suggested significant declines in Katavi National Park. Encounter rates calculated from transects also showed declines in Nyerere National Park for Lappet‐faced vultures and Bateleurs and in Ruaha National Park for White‐headed vultures. Mortalities recorded and inferred from telemetry suggest that poisoning is prevalent while highlighting the challenges of determining cause of death when working across large landscapes. Despite these declines, southern Tanzania remains an important stronghold for African vultures with higher current encounter rates than elsewhere in East Africa. However, preventing further declines will heavily depend on the ability to mitigate the threat of poisoning. This study demonstrates the value of using multiple techniques when attempting to understand population trends over short time periods.This article is protected by copyright. All rights reserved