Calibrating occupancy to density estimations to assess abundance and vulnerability of a threatened primate in Tanzania

The current decline of mammals worldwide makes quantitative population assessments crucial, especially for range‐restricted and threatened species. However, robust abundance estimations are challenging for elusive or otherwise difficult to detect species. Alternative metrics requiring only presence/absence data, that is, occupancy, are possible but calibration with independent density estimates should be foreseen, although rarely performed. Here, we calibrated density estimates from acoustic surveys to occupancy estimates from camera‐trapping detections to derive the abundance of the endangered Sanje mangabey (Cercocebus sanjei) across its entire range in the Udzungwa Mountains of Tanzania. We found marked occupancy–density relationships for the two forest blocks where this primate occurs and used them to derive spatially explicit density estimates. Occupancy increased in montane forest zones at mid‐elevation but decreased slightly with proximity to forest borders. We predicted an average density (±SE) of 0.26 ± 0.05 groups/km2 in the national park and 0.24 ± 0.06 in the nature reserve. Accordingly, and given the much larger area of the reserve, the average predicted individual abundance was 1555 ± 325 and 2471 ± 571 in the national park and nature reserve, respectively. We found higher density and abundance in the nature reserve compared with previous studies. Given the past disturbance and poorer protection in the nature reserve relative to the national park, our results instill optimism for the status of the species, although occupancy analysis highlighted the potential vulnerability of this primate to human disturbance. Our approach appears valuable for spatially explicit density estimations of elusive species, and provides robust assessments of vulnerability and identification of priority areas for conservation of threatened populations

Hunting or habitat degradation? Decline of primate populations in Udzungwa Mountains, Tanzania:an analysis of threats

Biological ConservationHunting and habitat degradation are universal threats to primates across the tropics, thus deciphering the relative impact of threats on population relative abundance is critical to predicting extinction risk and providing conservation recommendations. We studied diurnal primates over a period of nearly 6 years in the Udzungwa Mountains of Tanzania, a site of global importance for primate conservation. We assessed how population relative abundance of five species (of which two are endemic and IUCNEndangered) differed between two forest blocks that are similar in size and habitat types but contrast strongly in protection level, and how abundance changed during 2004–2009. We also measured habitat and disturbance parameters and, in the unprotected forest, evaluated hunting practices. We found significant differences in primates’ abundance between protected and unprotected forests, with the greater contrast being the lower abundance of colobine monkeys (Udzungwa red colobus and Angolan colobus) in the unprotected forest. At this site moreover, colobines declined to near-extinction over the study period. In contrast, two cercopithecines (Sanje mangabey and Sykes’ monkey) showed slightly higher abundance in the unprotected forest and did not decline significantly. We argue that escalating hunting in the unprotected forest has specifically impacted the canopy-dwelling colobus monkeys, although habitat degradation may also have reduced their abundance. In contrast, cercopithecines did not seem affected by the current hunting, and their greater ecological adaptability may explain the relatively higher abundance in the unprotected forest. We provide recommendations towards the long-term protection of the area

Primates decline rapidly in unprotected forests: evidence from a monitoring program with data constraints

<p>Group count data for the study species: see supplementary model script for further details.</p

Prior (dashed line) and posterior (solid lines) distributions of the parameters for abundance trends, referred to Mwanihana (<i>γ</i>_<i>MW</i>, gray solid line) and Uzungwa Scarp (<i>γ</i>_<i>US</i>, black solid line).

<p>Results are for (A) Angolan colobus, (B) Udzungwa red colobus, and (C) Sykes' monkey. A uniform (-5, 5) prior is used in all cases. The vertical dotted line indicates population stability.</p

Map of the study area, the Udzungwa Mountains of Tanzania (location in the top left inset).

<p>The map shows the forest blocks among which are Mwanihana forest (MW) to the northeast and Uzungwa Scarp (US) to the southwest. The four line transects used to count primates are shown as white lines in each of these two forests. The background layer is a Digital Elevation Model (dark is lower elevation). The borders of the Udzungwa Mountains National Park (UMNP) are also indicated.</p

Posterior summary of model parameters for count data of three primate species (Angolan colobus, Udzungwa red colobus, and Sykes' monkey) for a protected (Mwanihana, MW) and a non protected forest (Uzungwa scarp, US) in Tanzania.

<p>Posterior summary of model parameters for count data of three primate species (Angolan colobus, Udzungwa red colobus, and Sykes' monkey) for a protected (Mwanihana, MW) and a non protected forest (Uzungwa scarp, US) in Tanzania.</p

Results of observed and estimated primate abundance from line transect counts in the Udzungwa Mountains of Tanzania.

<p>Number of primate groups per km of transect at Mwanihana (solid symbols) and Uzungwa Scarp (open symbols) for Angolan colobus (A,B), Udzungwa red colobus (C,D), and Sykes' monkey (E,F). (A), (C), and (E) show the mean observed counts per km (with 1 SE); (B), (D), and (F) show the estimated total number of groups per km (posterior median and 95% CRI). The predicted values for years with no surveys are shown in gray.</p

Details of line transects, sampling effort, and observers involved in counting primate groups in a protected (Mwanihana, MW) and unprotected forest (Uzungwa Scarp, US) in the Udzungwa Mountains of Tanzania during the dry season of 2002–2012.

<p>Details of line transects, sampling effort, and observers involved in counting primate groups in a protected (Mwanihana, MW) and unprotected forest (Uzungwa Scarp, US) in the Udzungwa Mountains of Tanzania during the dry season of 2002–2012.</p

Greco_et_al_Ecosphere

Dataset for Greco et al 2023 "Calibrating occupancy to density estimations to assess abundance and vulnerability of a threatened primate in Tanzania". Ecosphere.  File 1 has calibration data with site-specific densities and occupancy values; file 2 is the detection/non-detection data of the Sanje mangabey from camera traps across the 2 forests in Tanzania where it is endemic.   </p