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The Ecology and Conservation of the Critically Endangered Cross River Gorilla in Cameroon

  • Author(s): Sawyer, Sarah
  • Advisor(s): Brashares, Justin
  • et al.

The Cross River gorilla (Gorilla gorilla diehli; hereafter: CRG) is one of the world's most endangered and least studied primates. CRG exist only in a patchy distribution in the southern portion of the Cameroon-Nigeria border region and may have as few as 300 individuals remaining, divided into 14 fragmented subpopulations. Though Western gorillas (Gorilla gorilla spp) probably once inhabited much greater ranges throughout West Africa, today CRG represent the most northern and western distribution of all gorillas and are isolated from Western lowland gorilla populations by more than 250 km. CRG have proved challenging to study and protect, and many of the remaining subpopulations currently exist outside of protected areas. Very little is known about where the various subpopulations range on the landscape or why they occur in a patchy distribution within seemingly intact habitat. Active efforts are currently underway to identify critical habitat for landscape conservation efforts to protect the CRG in this biodiversity hotspot but, to date, a lack of understanding of the relationship between CRG ecology and available habitat has hampered conservation endeavors. This dissertation aims to improve our understanding of CRG ecology and distribution to inform conservation management decision making.

This research has four main components. First, I describe the plant diet of one CRG subpopulation (the Mone subpopulation). Successful habitat management for primate conservation requires understanding which plants are important and how these plants vary in availability across the landscape. Using feeding trail sign collected over a 10 month period, I record the components of the CRG diet, evaluate CRG selectivity among herbaceous food species, compare Mone CRG diet to that of another CRG subpopulation, and examine differences in herbaceous food availability in areas used and unused by the CRG. During the study period, the CRG ate 141 different plant parts from 102 different species, 23 of which were quantitatively important in the diet. Similarly to other Western lowland and CRG populations, Landolphia, Aframomum, and Palisota spp, were important staple species for the Mone CRG and Marantochloa purpurea played an important fallback role in their diet. By contrast, Araceae species, like Cercestis camerunensis, may be more important to the CRG at Mone than elsewhere. My results suggest that CRG in the Mone-Mt. Oko region prefer certain foods in their diet, and may also selectively use areas with higher availability of preferred foods.

Second, I estimate the Mone subpopulation range and assess both the effects of model choice on resulting range estimates and the conservation utility of various models. Measuring and characterizing the area utilized by a population or species is essential for evaluation of conservation status and for effective allocation of habitat to ensure population persistence. Models considered in this study range from basic traditional approaches (e.g. Minimum Convex Polygon) to newer home range techniques such as Local Convex Hull (LoCoH). I used overlap analysis comparing sub-sampled to complete data sets to evaluate the robustness of various modeling techniques to data limitations. I employed Likelihood Cross Validation Criterion to compare core range model performance. Results suggest that differing LoCoH models produce similar range estimates, are robust to data requirements, provide a good fit for core habitat estimation, and are best able to detect unused habitat within the subpopulation range. LoCoH methods may thus be useful for studies of habitat selection and factors limiting endangered species distributions. However, LoCoH models tend to overfit data, and Kernel methods may provide similar information about animal space use while supporting protection of larger swaths of critical habitat. Subpopulation range analyses for conservation/management planning should therefore explore multiple modeling techniques, and employ both qualitative and quantitative assessments to select the best models to inform decision making for species of conservation concern.

Third, I review current use of Least Cost Path modeling techniques for connectivity conservation, and highlight both weaknesses and ways to improve application for species like the Cross River gorilla. Promoting connectivity between areas utilized by isolated subpopulations is essential to maintain population viability in fragmented species like the CRG, where each subpopulation contains relatively few individuals. The most common approach to connectivity design is the Least Cost Path (LCP) analysis, which has been applied to the CRG landscape. This review highlights three weaknesses common in recent LCP analyses. First, LCP models typically rely on remotely-sensed habitat maps, but few studies assess whether such maps are suitable proxies for factors affecting animal movement or consider the effects of adjacent habitats. Secondly, many studies use expert opinion to assign costs associated with landscape features, yet few validate these costs with empirical data or assess model sensitivity to errors in cost assignment. Thirdly, studies that consider multiple, alternative movement paths often propose width or length requirements for linkages without justification. LCP modelling and similar approaches to linkage design guide connectivity planning, yet often lack a biological or empirical foundation. Ecologists must clarify the biological processes on which resistance values are based, explicitly justify cost schemes and scale (grain) of analysis, evaluate the effects of landscape context and sensitivity to cost schemes, and strive to optimize cost schemes with empirical data. Research relating species' fine-grain habitat use to movement across broad extents is desperately needed, as are methods to determine biologically relevant length and width restrictions for linkages. While data on such fine grain habitat use have to date been lacking for the Cross River gorilla, this dissertation research aims to improve our understanding of these variables.

Thus, finally, I use hierarchical resource selection functions (RSFs) to examine habitat selection and requirements of the CRG at multiple scales to inform connectivity modeling and conservation planning. Specifically, I employ generalized additive models at the scale of the subpopulation range and conditional logistic regression at the scale of individual movements. Understanding resource and habitat selection by endangered species will better inform conservation planning for protection of both critical habitat, and essential linkages between subpopulations. Results indicate that CRG habitat selection is highly scale dependent. Localized measures of habitat quality strongly influenced selection at the subpopulation or landscape scale, while human activity and food availability are the best predictors of selection at finer scales. Understanding why CRG do not occur in seemingly suitable habitat is crucial for designating critical habitat both within and between CRG subpopulations. My results indicate that conservation planning to maintain critical habitat and connectivity among CRG populations will require an integrative, multi-scale planning approach incorporating large-scale landscape characteristics, human use patterns and CRG food availability. Further fine-scale data collection across the landscape will be necessary to use RSF results in connectivity models to inform conservation of important linkages between subpopulations.

This research marks a significant addition to the current limited knowledge about the CRG dietary and spatial ecology and conservation biology. My study results complement past and ongoing research by other PhD students, conservation NGOs, and government officials, and compiling these various works will likely provide us with a more complete understanding of CRG ecology for effective conservation decision making

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