Primates Decline Rapidly in Unprotected Forests: Evidence from a Monitoring Program with Data Constraints

Abstract

Growing threats to primates in tropical forests make robust and long-term population abundance assessments increasingly important for conservation. Concomitantly, monitoring becomes particularly relevant in countries with primate habitat. Yet monitoring schemes in these countries often suffer from logistic constraints and/or poor rigor in data collection, and a lack of consideration of sources of bias in analysis. To address the need for feasible monitoring schemes and flexible analytical tools for robust trend estimates, we analyzed data collected by local technicians on abundance of three species of arboreal monkey in the Udzungwa Mountains of Tanzania (two Colobus species and one Cercopithecus), an area of international importance for primate endemism and conservation. We counted primate social groups along eight line transects in two forest blocks in the area, one protected and one unprotected, over a span of 11 years. We applied a recently proposed open metapopulation model to estimate abundance trends while controlling for confounding effects of observer, site, and season. Primate populations were stable in the protected forest, while the colobines, including the endemic Udzungwa red colobus, declined severely in the unprotected forest. Targeted hunting pressure at this second site is the most plausible explanation for the trend observed. The unexplained variability in detection probability among transects was greater than the variability due to observers, indicating consistency in data collection among observers. There were no significant differences in both primate abundance and detectability between wet and dry seasons, supporting the choice of sampling during the dry season only based on minimizing practical constraints. Results show that simple monitoring routines implemented by trained local technicians can effectively detect changes in primate populations in tropical countries. The hierarchical Bayesian model formulation adopted provides a flexible tool to determine temporal trends with full account for any imbalance in the data set and for imperfect detection.

Estimating the population size of the Sanje mangabey (Cercocebus sanjei) using acoustic distance sampling

“…Considering population trends from other primates in the same forests, it is likely that there may have been a decline and that the population in the Uzungwa Scarp continues to be at risk of further decline. Populations of primates in Mwanihana have shown to be stable in recent surveys and the active protection measures to be efficient (Beaudrot et al, ; Rovero et al, , ), and although Rovero et al () detected a potential decline in mangabey abundance between surveys in 2004–2005 and 2007–2008 and a survey in 2009, the visual line transect method used was highlighted as inefficient for the mangabey and results to be taken with caution. For the Uzungwa Scarp however, studies report a decline for several primate species (Rovero et al, , ).…”

Impending extinction crisis of the world’s primates: Why primates matter

“…Despite these trends, protected areas can and do provide long-term sanctuary for wildlife. For example, a study in the Udzungwa mountains of Tanzania showed that colobine primate populations were stable in the protected areas but declined severely in the unprotected forests ( 120 ). An 8-year study (2007–2014) using camera traps to annually monitor terrestrial mammals and birds in 15 protected areas in the Neotropics, Africa, and Southeast Asia showed strong evidence of stability and even increases in populations, including those of 23 primate species (19 cercopithecines in Africa and Asia, 3 African apes, and 1 lemur) ( 121 ).…”

“…It includes taking advantage of global telecommunication systems and wireless Internet, satellite- and airborne-based imagery, drone technology, ever more powerful handheld devices (for example, smart phones and tablets), and camera traps (Supplementary Text) ( 120 , 128 ). Combined with geographic information system and ground surveys, some of this technology has been used in evaluating sustainable land-use spatial planning and human-primate conflicts [for example, Javan gibbons ( Hylobates moloch )] ( 129 ) and in providing case-by-case assessments of species vulnerability to climate change, as shown for Borneo’s orangutans (Supplementary Text) ( 80 ).…”

“…There are relatively few multigeneration studies of primate population dynamics, largely because primates are long-living animals and population changes occur over time periods longer than the duration of most studies and research grants. However, a recent study from the Udzungwa Mountains of Tanzania combined locally based monitoring routines and advanced statistical approaches to investigate population abundance, even when information was missing for some monitoring periods ( 120 ). Another study showed the value of basic field procedures (transect surveys) in monitoring Sumatran orangutan populations, with results that doubled the estimated population from 6600 to 14,613, even though the population was still assessed as declining rapidly due to deforestation ( 138 ).…”

Peer Review #2 of "Primates in peril: the significance of Brazil, Madagascar, Indonesia and the Democratic Republic of the Congo for global primate conservation (v0.1)"

“…3, Fig. 9) (Mascia et al, 2014;Rovero et al, 2015;Spracklen et al, 2015). Due to illegal 806 activity in the Brazilian Amazon, natural resource reduction is pervasive.…”