Abstract:
To evaluate conservation interventions, it is necessary to obtain reliable population trends
for short (<10 years) time scales. Telemetry can be used to estimate short-term survival
rates and is a common tool for assessing population trends, but it has limitations and can
be biased toward specific behavioral traits of tagged individuals. Encounter rates calculated
from transects can be useful for assessing changes across multiple species, but they
can have large confidence intervals and be affected by variations in survey conditions. The
decline of African vultures has been well-documented, but understanding of recent trends
is lacking. To examine population trends, we used survival estimates from telemetry data
collected over 6 years (primarily for white-backed vultures [Gyps africanus]) and transect
counts conducted over 8 years (for 7 scavenging raptors) in 3 large protected areas in Tanzania.
Population trends were estimated using survival analysis combined with the Leslie
Lefkovitch matrix model from the telemetry data and using Bayesian mixed effects generalized
linear regression models from the transect data. Both methods showed significant
declines for white-backed vultures in Ruaha and Nyerere National Parks. 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
(38% annual declines) and Bateleurs (18%) and in Ruaha National Park for white-headed
vultures (Trigonoceps occipitalis) (19%). Mortality rates recorded and inferred from telemetry
suggested that poisoning is prevalent. However, only 6 mortalities of the 26 presumed
mortalities were confirmed to be caused by poisoning, highlighting the challenges of determining
the cause of death when working across large landscapes. Despite declines, our data
provide evidence that southern Tanzania has higher current encounter rates of African
vultures than elsewhere in East Africa. Preventing further declines will depend greatly on
mitigating poisoning. Based on our results, we suggest that the use of multiple techniques
improves understanding of population trends over the short term.
