Primates in Human-Modified and Fragmented Landscapes: The Conservation Relevance of Modelling Habitat and Disturbance Factors in Density Estimation

Abstract

Accurate density estimations of threatened animal populations is essential for management and conservation. This is particularly critical for species living in patchy and altered landscapes, as is the case for most tropical forest primates. In this study, we used a hierarchical modelling approach that incorporates the effect of environmental covariates on both the detection (i.e. observation) and the state (i.e. abundance) processes of distance sampling. We applied this method to already published data on three arboreal primates of the Udzungwa Mountains of Tanzania, including the endangered and endemic Udzungwa red colobus (Procolobus gordonorum). The area is a primate hotspot at continental level. Compared to previous, ‘canonical’ density estimates, we found that the inclusion of covariates in the modelling makes the inference process more informative, as it takes in full account the contrasting habitat and protection levels among forest blocks. The correction of density estimates for imperfect detection was especially critical where animal detectability was low. Relative to our approach, density was underestimated by the canonical distance sampling, particularly in the less protected forest. Group size had an effect on detectability, determining how the observation process varies depending on the socio-ecology of the target species. Lastly, as the inference on density is spatially-explicit to the scale of the covariates used in the modelling, we could confirm that primate densities are highest in low-to-mid elevations, where human disturbance tend to be greater, indicating a considerable resilience by target monkeys in disturbed habitats. However, the marked trend of lower densities in unprotected forests urgently calls for effective forest protection.

Effects of anthropogenic disturbance on primate density at the landscape scale

“…The negative effect of elevation on 3 species and preferences for areas with climber coverage are consistent with Barelli et al. () and Cavada et al.’s () results. Our approach, however, provides more robust and wider inferences.…”

“…; Cavada et al. ). For all species, group size varied considerably among forests and among transects within each forest, and given the difficulties of counting individuals during censuses, we did not use group size to derive individual densities.…”

“…The fit for the half‐normal key function was tested on the same detection data in previous studies (Cavada et al. , ). However, we visually inspected the fit of the hazard rate and compared it with the half normal (Supporting Information).…”

Free and open source software for geospatial applications (FOSS4G) to support Future Earth

“…It has been also used to simulate and predict the long-term evolution of the Paneveggio forest (northern Italy) based on changes in human activities (Tattoni et al 2011) and to correlate changes in the agro-forest landscape with the loss of traditional ecological knowledge (Ianni et al 2015). GRASS GIS and R have been used to monitor the presence of primates in relation to human disturbances (Cavada et al 2016 , jointly developed by the European Commission and the UN, combines Web-based geoprocessing using ZOO WPS and GRASS GIS to alert the international community in case of major sudden disasters. Finally, ZOO WPS was used to create DSS for rainfall modelling as part of a FOSS4G architecture to build a geoportal for North Vietnam (Brovelli et al 2013b).…”

High density of white‐faced capuchins (Cebus capucinus) and habitat quality in the Taboga Forest of Costa Rica

“…The majority of non‐human primates (hereafter, “primates”) across the globe are either under threat of extinction or experiencing population declines (Estrada et al, 2018). Primate densities tend to decrease in unprotected areas, yet primates can nevertheless still flourish in areas of human activity, suggesting both a vulnerability and resilience to anthropogenic disturbance (Cavada, Barelli, Ciolli, & Rovero, 2016). Some species show remarkable behavioral flexibility and quickly adjust to new circumstances while others get pushed closer to extinction (Kulp & Heymann, 2015; Laurance et al, 2007; Ries, Fletcher, Battin, & Sisk, 2004).…”