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Factors Influencing the Survival of Sympatric Gorilla (Gorilla gorilla gorilla) and Chimpanzee (Pan troglodytes troglodytes) Nests

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Accurate and precise surveys of primate abundance provide the basis for understanding species ecology and essential information for conservation assessments. Owing to the elusive nature of wild apes and the vast region of dense forest they inhabit, population estimates of central chimpanzees (Pan troglodytes troglodytes) and western lowland gorillas (Gorilla gorilla gorilla) have largely relied on surveys of their nests. Specific information about the nesting behavior of apes permits the estimation of the number of nests built (nest creation rate). Similarly, information on nest characteristics and environmental factors can be used to estimate the time it takes nests to decay (nest decay rate). Nest creation and decay rates are then used to convert nest density estimates to absolute ape densities. Population estimates that use site-specific estimates of nest creation and decay rates are more accurate and precise. However, it is common practice to generalize these conversion factors across sites because of the additional cost of studies required to gather the information to estimate them. Over a 9-mo study period, we detected and monitored the time to decay of gorilla nests (N = 514) and chimpanzee nests (N = 521) in northern Republic of Congo. We investigated the influence of nest characteristics and environmental factors on nest survivorship and estimated the mean time to nest decay (or equivalently survival) using MARK. Key factors influencing nest decay rate included ape species, forest type, nest height, mean rainfall, nest structure, nest type, and primary aspects of nest construction. Our findings highlight the synergistic effect of behavior and environment on great ape nest degradation, as well as providing practical insights for improving measures to monitor remaining populations of these endangered species.
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Factors Influencing the Survival of Sympatric
Gorilla (Gorilla gorilla gorilla)andChimpanzee
(Pan troglodytes troglodytes)Nests
David Morgan
&Crickette Sanz
Jean Robert Onononga
&Samantha Strindberg
Received: 28 March 2016 /Accepted: 7 October 2016 / Published online: 12 December 2016
#Springer Science+Business Media New York 2016
Abstract Accurate and precise surveys of primate abundance provide the basis for
understanding species ecology and essential information for conservation assessments.
Owing to the elusive nature of wild apes and the vast region of dense forest they
inhabit, population estimates of central chimpanzees (Pan troglodytes troglodytes)and
western lowland gorillas (Gorilla gorilla gorilla) have largely relied on surveys of their
nests. Specific information about the nesting behavior of apes permits the estimation of
the number of nests built (nest creation rate). Similarly, information on nest character-
istics and environmental factors can be used to estimate the time it takes nests to decay
(nest decay rate). Nest creation and decay rates are then used to convert nest density
estimates to absolute ape densities. Population estimates that use site-specific
estimates of nest creation and decay rates are more accurate and precise. However, it is
common practice to generalize these conversion factors across sites because of the
additional cost of studies required to gather the information to estimate them. Over a 9-
mo study period, we detected and monitored the time to decay of gorilla nests (N= 514)
and chimpanzee nests (N= 521) in northern Republic of Congo. We investigated
the influence of nest characteristics and environmental factors on nest survivorship and
estimated the mean time to nest decay (or equivalently survival) using MARK. Key
factors influencing nest decay rate included ape species, forest type, nest height, mean
rainfall, nest structure, nest type, and primary aspects of nest construction. Our findings
Int J Primatol (2016) 37:718737
DOI 10.1007/s10764-016-9934-9
Handling Editor: Joanna M. Setchell
*David Morgan
Fisher Center for the Study and Conservation of Apes, Lincoln Park Zoo, Chicago, IL 60614, USA
Wildlife Conservation Society, Congo Program, B.P. 14537, Brazzaville, Republic of Congo
Department of Anthropology, Washington University in Saint Louis, Saint Louis, MO 63130, USA
Global Conservation Program, Wildlife Conservation Society, Bronx, NY 10460, USA
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... However, there are some disadvantages. Nest production rate [37][38][39] and nest decomposition or nest decay time [40,41] (i.e. conversion factors) are highly variable across species, space and time, but are required to scale down the number of counted nests to the number of apes [35], in order to permit estimations of great apes [42]. ...
... This is problematic, as nest decomposition may differ with differing climate conditions, leading to inaccurate population density estimates [56,63]. Nest-specific factors are known to drive nest decay; for example, nests built at the same time by members of the same group of apes exhibit different decomposition times [40]. However, rainfall is often reported as the most important variable affecting nest decomposition time, with lower rainfall resulting in longer decay times [38,40,41,64]. ...
... Nest-specific factors are known to drive nest decay; for example, nests built at the same time by members of the same group of apes exhibit different decomposition times [40]. However, rainfall is often reported as the most important variable affecting nest decomposition time, with lower rainfall resulting in longer decay times [38,40,41,64]. Therefore, a drier climate would be expected to increase the time for which great ape nests would remain visible in the forest [40]. ...
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Since 1994, IUCN Red List assessments apply globally acknowledged standards to assess species distribution, abundance and trends. The extinction risk of a species has a major impact on conservation science and international funding mechanisms. Great ape species are listed as Endangered or Critically Endangered. Their populations are often assessed using their unique habit of constructing sleeping platforms, called nests. As nests rather than apes are counted, it is necessary to know the time it takes for nests to disappear to convert nest counts into ape numbers. However, nest decomposition is highly variable across sites and time and the factors involved are poorly understood. Here, we used 1,511 bonobo (Pan paniscus) nests and 15 years of climatic data (2003–2018) from the research site LuiKotale, Democratic Republic of the Congo, to investigate the effects of climate change and behavioural factors on nest decay time, using a Bayesian gamma survival model. We also tested the logistic regression method, a recommended time-efficient option for estimating nest decay time. Our climatic data showed a decreasing trend in precipitation across the 15 years of study. We found bonobo nests to have longer decay times in recent years. While the number of storms was the main factor driving nest decay time, nest construction type and tree species used were also important. We also found evidence for bonobo nesting behaviour being adapted to climatic conditions, namely strengthening the nest structure in response to unpredictable, harsh precipitation. By highlighting methodological caveats, we show that logistic regression is effective in estimating nest decay time under certain conditions. Our study reveals the impact of climate change on nest decay time in a tropical remote area. Failure to account for these changes would invalidate biomonitoring estimates of global significance, and subsequently jeopardize the conservation of great apes in the wild.
... Small-scale studies have looked at great ape densities in different forest types in the absence of logging and poaching. Detailed information on nesting patterns of both great apes allowed researchers to investigate the factors impacting nest decay (Morgan et al., 2016) and to develop a method to identify nest building species , which were subsequently applied in the landscape surveys of northern Congo . These studies found high gorilla densities in mixed species forest with open canopy (or forest dominated by terrestrial herbaceous vegetation: "Marantaceae forest") and nesting preference for chimpanzees in monodominant Gilbertiodendron forest Morgan et al., , 2019. ...
... This allowed us to investigate the impact of forest type in the absence of anthropogenic activities. We applied nest decay rates from previously published results from the Goualougo Triangle (Morgan et al., , 2016Sanz et al., 2007), which we assumed to be very similar to Mbeli Bai given the proximity of the sites and the similar survey periods. Similarly, we used a predictive model developed using the Goualougo Triangle nest data to assign the nests in our study to either chimpanzee or gorilla. ...
... We converted nest density into animal density using the conversion factors nest production and nest decay rate. We used a nest decay rate from the nearby Goualougo Triangle (146.4 ± 3 SE days; Morgan et al., 2016). We used published production rates (1.09 ± 0.05 SE nests per day; and applied them to both chimpanzees and gorillas. ...
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Small-scale comparisons help us to understand how habitat features and food availability impact primate abundance. This is particularly useful at sites without human impacts, as it allows for the investigation of the natural factors influencing nesting patterns and great ape abundance. We provide a small-scale study of sympatric great ape nests in an unlogged old-growth forest without poaching activities. We conducted a line transect survey (52 km total effort) around Mbeli Bai, a forest clearing in Nouabalé–Ndoki National Park, Congo, applying on-site nest decay rates and assigning nest builder using logistic regression. We found a high occurrence of monodominant Gilbertiodendron dewevrei at Mbeli Bai (34%) that correlates with low great ape densities at Mbeli Bai. Chimpanzees (Pan troglodytes troglodytes) showed a preference for nesting in trees in closed canopy monodominant forest. We found a low percentage of gorilla (Gorilla gorilla gorilla) nests in mixed species forest (35%) and a higher percentage in trees (64%) compared to other study sites. However, generalized additive models found higher gorilla nest encounter rates in mixed species forest with dense understory than in monodominant forest and open understory. We found no indication of higher gorilla densities close to Mbeli Bai than elsewhere, and line transect estimates were lower than the number of gorillas revealed from direct observations. There were substantial differences between our findings and those for nearby sites, demonstrating the utility of small-scale comparisons to further understand the factors determining chimpanzee and gorilla densities within and between sites and the limitations of nest surveys.
... We used a nest-building rate obtained through a long-term study (9). The model used to obtain spatially explicit nest decay rates came from a long-term study that accounted for habitat and nest characteristics, as well as variation in rainfall (10). Models themselves are also prone to biases due to mis-specification, as well as inaccurate or missing explanatory variables. ...
... This gave us a data set of more than 20,000 nests (7521 chimpanzee and 12,524 gorilla nests). During analyses of the survey data, we (i) modeled which factors-both environmental and human-were most likely to drive gorilla and chimpanzee distribution; (ii) used the resulting top-ranked models, which included the important environmental and human variables, to predict nest density surfaces (maps of nest density) for each taxon across their entire geographic range; (iii) applied a model of nest decay based on rainfall (10) to the nest density surfaces to obtain maps of gorilla and chimpanzee density and distribution across their range; and, on the basis of the animal density maps, (iv) estimated great ape abundance in each of the six countries in their range. ...
... Range-wide predictions of nest decay rate were derived from the species-specific averages of each of these variables, except for the Gilbertio variable (set to zero, as nests of this type decay more slowly and are uncommon) and rainfall (average rainfall was calculated using Global Climate Data for precipitation at a 2.5 arc min resolution). The model was assumed to asymptote for average rainfall values above 6 mm and below 3 mm per day to account for saturation at the upper extremes of daily average rainfall and other factors superseding rainfall at the lower extremes (10). The forested portion of WEA straddles the equator between 5.99°N and 5.76°S ranging across a longitude of 8.71°E to 20.49°E, where mean average daily rainfall is 4.7 mm (2.3 to 8.7 mm). ...
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We present a range-wide assessment of sympatric western lowland gorillas Gorilla gorilla gorilla and central chimpanzees Pan troglodytes troglodytes using the largest survey data set ever assembled for these taxa: 59 sites in five countries surveyed between 2003 and 2013, totaling 61,000 person-days of fieldwork. We used spatial modeling to investigate major drivers of great ape distribution and population trends. We predicted density across each taxon’s geographic range, allowing us to estimate overall abundance: 361,900 gorillas and 128,700 chimpanzees in Western Equatorial Africa—substantially higher than previous estimates. These two subspecies represent close to 99% of all gorillas and one-third of all chimpanzees. Annual population decline of gorillas was estimated at 2.7%, maintaining them as Critically Endangered on the International Union for Conservation of Nature and Natural Resources (IUCN) Red List. We quantified the threats to each taxon, of which the three greatest were poaching, disease, and habitat degradation. Gorillas and chimpanzees are found at higher densities where forest is intact, wildlife laws are enforced, human influence is low, and disease impacts have been low. Strategic use of the results of these analyses could conserve the majority of gorillas and chimpanzees. With around 80% of both subspecies occurring outside protected areas, their conservation requires reinforcement of anti-poaching efforts both inside and outside protected areas (particularly where habitat quality is high and human impact is low), diligent disease control measures (including training, advocacy, and research into Ebola virus disease), and the preservation of high-quality habitat through integrated land-use planning and implementation of best practices by the extractive and agricultural industries.
... To convert sign density to animal density, the sign density is divided by its estimated production rate and time to decay (Hedges, 2012;Laing et al., 2003). When correctly designed, these methods work well; however, these multipliers, introduce additional sources of error into density estimates when decay rates from non-representative sites or seasons are used (Morgan et al., 2016). Unfortunately, because of financial or time constraints, estimates from other sites or seasons are occasionally used in lieu of obtaining study-specific degradation rates. ...
... While we focused on forest elephants, these methods could improve indirect transect surveys for other difficult to observe species (e.g. deer, duikers, great apes; Koster and Hart, 1988;Rovero and Marshall, 2004) and could be applied to other types of degrading sign, such as ape nests (Stokes et al., 2010) especially when studies showing environmental influence on decay already exist (Morgan et al., 2016). ...
Accurate and ecologically relevant wildlife population estimates are critical for species management. One of the most common survey methods for forest mammals – line transects for animal sign with distance sampling – has assumptions regarding conversion factors that, if violated, can induce substantial bias in abundance estimates. Specifically, for sign (e.g. nests, dung) surveys, a single number representing total time for decay is used as a multiplier to convert estimated sign density into animal density. This multiplier is likely inaccurate if not derived from a study reflecting the spatiotemporal variation in decay times. Using dung decay observations from three protected areas in Gabon, and a previous study in Nouabalé‐Ndoki National Park (Congo), we developed Weibull survival models to adaptively predict forest elephant (Loxodonta cyclotis) dung decay based on environmental variables from field collected and remotely sensed data. Seasonal decay models based on remotely sensed covariates explained 86% of the variation for the wet season and 79% for the dry season. These models included canopy cover, cloud cover, humidity, vegetation complexity and slope as factors influencing dung decay. With these models, we assessed sensitivity of elephant density estimates to spatiotemporal environmental heterogeneity, showing that our methods work best for large‐scale studies >50 km2. We simulated decay studies with and without these variables in four Gabonese national parks and reanalyzed two previous surveys of elephants in Minkébé National Park, Gabon. Disregarding spatial and temporal variation in decay rate biased population estimates up to 1.6 and 6.9 times. Our reassessment of surveys in Minkébé National Park showed an expected loss of 78% of forest elephants over ten years, but the elephant abundance was 222% higher than previously estimated. Our models incorporate field or remotely sensed variables to provide an ecological context essential for accurate population estimates while reducing need for expensive decay field studies. Population abundance for elusive species are estimated from sign counts and multipliers of sign production and decay rates. To combat the improper use of single decay rates often obtained from an unrepresentative site, we observed and modeled forest elephant dung decay rates and created adaptive models which incorporate spatiotemporal environmental variables. We showed that disregarding spatial and temporal variation in decay rate can bias population estimates by a factor of up to 1.6 and 6.9 times.
... For example, conditions of the local environment are a commonly acknowledged influence on probability of sign encounter, and heterogeneity is common in metrics of sign de-cay rates across locations (e.g., (Bessone et al., 2021;Kuehl et al., 2007;Walsh White, 2005). In both dung and nestcount surveying, sign decay is affected by climatic seasonality, especially in rainfall, as well as other contextual factors such as construction material or dung matrix, storm frequency, and sun exposure (Bessone et al., 2021;Kamgang et al., 2020;Kouakou et al., 2009;Laing et al., 2003;Morgan et al., 2016;Nchanji Plumptre, 2001;Plumptre, 2000). Because of these environmental influences, it is commonly recommended that local measures of decay rates must accompany surveying, as failure to do so may result in imprecise measurement and hinder validity of inter-site comparisons (e.g., (Bessone et al., 2021;Kühl, 2008;Laing et al., 2003;Mohneke Fruth, 2008). ...
Wildlife population monitoring depends on accurate counts of individual animals or artefacts of behavior (e.g., nests or dung), but also must account for potential biases in the likelihood to encounter these animals or artefacts. In indirect surveying, which depends largely upon artefacts of behavior, likelihood to encounter indirect signs of a species is derived from both artefact production and decay. Although environmental context as well as behavior contribute to artefact abundance, variability in behaviors relevant to artefact abundance is rarely considered in population estimation. Here we demonstrate how ignoring behavioral variability contributes to overestimation of population size of a highly endangered great ape endemic only to the Democratic Republic of the Congo, the bonobo (Pan paniscus). Variability in decay of signs of bonobo presence (i.e., nests) is well documented and linked to environmental determinants. Conversely, a single metric of sign production (i.e., nest construction) is commonly used to estimate bonobo density, assumed to be representative of bonobo nest behavior across all contexts. We estimated nest construction rates from three bonobo groups within the Kokolopori Bonobo Reserve and found that nest construction rates in bonobos to be highly variable across populations as well as seasonal within populations. Failure to account for behavioral variability in nest construction leads to potentially severe degradation in accuracy of bonobo population estimates of abundance, accounting for a likely overestimation of bonobo numbers by 34%, and in the worst cases as high as 80% overestimation. Using bonobo nesting as an example, we demonstrate that failure to account for inter- and intra-population behavioral variation compromises our ability to monitor population change or reliably compare contributors to population decline or persistence. We argue that variation in sign production is but one of several potential ways that behavioral variability can affect conservation monitoring, should be measured across contexts whenever possible, and must be considered in population estimation confidence intervals. With increasing attention to behavioral variability as a potential tool for conservation, conservationists must also account for the impact that behavioral variability across time, space, individuals, and populations can play upon precision and accuracy of wildlife population estimation.
... For example, the decay rates of gorilla and chimpanzee nests depend on forest type, nest height, and structure, and above all, precipitation . The decay rates of signs should thus be estimated in the same survey area and season (Laing et al., 2003;Morgan et al., 2016), which may involve substantial effort and costs (Kuehl et al., 2007). Production rates of signs are less variable, hence estimates from similar or nearby sites can be used F I G U R E 1 This review is structured along four key questions that we believe need to be considered when choosing a monitoring method T A B L E 1 An overview of how observations by humans, camera traps, and passive acoustic sensors relate to the characteristics of interest for the four questions discussed in this article (e.g., Theuerkauf & Gula, 2010). ...
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Wildlife monitoring is essential for conservation science and data‐driven decision‐making. Tropical forests pose a particularly challenging environment for monitoring wildlife due to the dense vegetation, and diverse and cryptic species with relatively low abundances. The most commonly used monitoring methods in tropical forests are observations made by humans (visual or acoustic), camera traps, or passive acoustic sensors. These methods come with trade‐offs in terms of species coverage, accuracy and precision of population metrics, available technical expertise, and costs. Yet, there are no reviews that compare the characteristics of these methods in detail. Here, we comprehensively review the advantages and limitations of the three mentioned methods, by asking four key questions that are always important in relation to wildlife monitoring: (1) What are the target species?; (2) Which population metrics are desirable and attainable?; (3) What expertise, tools, and effort are required for species identification?; and (4) Which financial and human resources are required for data collection and processing? Given the diversity of monitoring objectives and circumstances, we do not aim to conclusively prescribe particular methods for all situations. Neither do we claim that any one method is superior to others. Rather, our review aims to support scientists and conservation practitioners in understanding the options and criteria that must be considered in choosing the appropriate method, given the objectives of their wildlife monitoring efforts and resources available. We focus on tropical forests because of their high conservation priority, although the information put forward is also relevant for other biomes. Wildlife monitoring is essential for conservation science and data‐driven decision‐making. We compare three commonly used monitoring methods by following four questions that are always important in relation to wildlife monitoring.
... As such, we used nest data as the only available ecologically sound and unbiased information on abundance distribution. Nest decay rate was shown to vary between gorilla and chimpanzee, and is also influenced by habitat types, nesting material and seasons (Morgan et al., 2016). It is evident that such variable nest decay rate may have influenced our observed nesting site distribution. ...
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The ongoing global decline in mammal populations has led researchers and conservationists to question which factors drive their abundance and distribution. Specifically, there is an urgent need to understand how the ranging behaviour of mammals determines their response to human-induced environmental changes. Threats such as hunting and habitat fragmentation and degradation through agricultural expansion and logging have received considerable attention. However, a potential threat resulting from the non-consumptive use of natural systems by humans has often been overlooked. The ecological and anthropogenic factors influencing the abundance and distribution of mammal populations in tropical forests were evaluated using great apes (Gorilla gorilla gorilla and Pan troglodytes troglodytes) as focal species in order to improve understanding of the drivers of local extinction of species. To achieve this goal, data on diet, fruit phenology, botany, and nest abundance and distribution were collected in a design involving a sampling grid and line transects. Data were analysed using modelling techniques in R and ArcGIS. The preferred fruiting plants for both gorillas and chimpanzees were more abundant in chimpanzee-preferred nesting habitats, while their fallback fruits were more abundant in gorilla-preferred nesting habitats. The patterns of habitat use by both gorillas and chimpanzees varied seasonally. In the absence of human disturbance, the distribution of gorilla nests was predicted by the availability of their preferred nesting habitats, while the distribution of chimpanzee nests was predicted by elevation and their preferred nesting habitats. However, when considering the research camp and human settlements, the distribution of gorilla nests was predicted first by the distribution of human settlements and then by their preferred nesting habitats, while chimpanzee nests remained predicted by elevation and their preferred nesting habitats. The long-term monitoring of great ape nests in the research site revealed a decline in both gorilla and chimpanzee populations resulting from an increase in hunting activities in the site. Results suggest that in the absence of human disturbance, ecological factors (habitat preference, seasonal patterns of fruit availability, fruit preference, and spatial distribution of habitat types) may be responsible for seasonal changes in mammal population abundance and distribution. Animal species traits (body size, terrestrial/arboreal, level of specialization/generalization, and competitive inferiority/superiority) have a profound influence on the response of mammals to human activities. Due to their spatial flexibility and their reliance on more available fallback food sources when preferred fruits are scarce, gorillas may vacate areas disturbed by hunting and non-hunting human activities and related noise. Chimpanzees, on the other hand, persist in their preferred nesting habitats despite human disturbance due to their high level of specialisation in fruit consumption. Additionally, the competitive dominance of chimpanzees over gorillas facilitated by their grouping patterns may allow them to cope with human disturbance better than gorillas. Human impacts other than direct killing of animals may influence the abundance and distribution of great ape populations and may account for the long-term decrease in population size. As habitat and resource heterogeneity facilitate the local coexistence of gorillas and chimpanzees, preserving both preferred and fallback fruiting plant species is crucial. Patterns observed in great apes may be an indication that human disturbance is also negatively influencing other mammals. However, species may respond differently to human disturbance, depending on their interaction with other sympatric mammals, their level of dietary specialisation, and their interaction with their physical environment. Further research is required to assess how these biological traits affect mammal response to anthropogenic disturbance. Furthermore, future studies should investigate the threshold beyond which the non-consumptive use of natural systems by humans becomes detrimental to species and their habitats.
... ). Grâce à la méthode d'échantillonnage à distance, nous avons pu calculer l'abondance et la densité des populations des deux espèces avec le logiciel DISTANCE 7.0 qui prend en compte les taux de production et de décomposition des nids.Etant dans la difficulté de garder une équipe sur le terrain pendant suffisamment longtemps pour calculer les taux de décomposition des nids de gorilles de Grauer et de chimpanzés spécifiques à la zone d'étude, nous avons utilisé les taux de décomposition présentés dans le Tableau 4, calculés dans le Parc National de Nouabalé-Ndoki en République du Congo pour les gorilles des plaines de l'ouest (Gorilla gorilla gorilla) et les chimpanzés communs (Pan troglodytes troglodytes)(Morgan et al., 2016).Tableau 4. Valeurs des taux de production et de décomposition des nids utilisées pour estimer l'abondance des gorilles et des chimpanzés dans le PNKB.Des signes de 26 espèces de grands mammifères ont été observés dans le parc (Tableau 5).Tableau 5. Liste des espèces de grands mammifères observées dans le parc. ...
Technical Report
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Afin de planifier adéquatement les actions de conservation à entreprendre dans les aires protégées, il est nécessaire de connaître la biodiversité qui s’y trouve. Cela nécessite donc de dresser une liste d’espèces et de connaître la distribution et l’abondance des populations cibles de conservation de l’aire protégée. Entre 2014 et 2017, la Wildlife Conservation Society (WCS) a ainsi mené le tout premier inventaire par transects standardisé complet qui ait été planifié dans le Parc National de Kahuzi-Biega. La situation sécuritaire n’a pas permis d’accéder à tout le parc tel qu’il était prévu initialement, mais 61% du parc a pu être couvert. Au total, 26 espèces de grands mammifères, 22 espèces de petits mammifères, 153 espèces d’oiseaux, 28 espèces d’amphibiens, 8 espèces de reptiles, et 1088 espèces de plantes ont été recensées. Les résultats des inventaires effectués dans le parc montrent qu’il abrite d’importantes populations de gorilles, de chimpanzés, de petits singes et d’ongulés, y compris de bongos, de buffles et de céphalophes. Les calculs de densités et d’abondances ont permis d’estimer le nombre de gorilles dans toute la zone echantillonée à 1262 individus (intervalle de confiance (IC) à 95% : 638 – 2493 ; coefficient de variation (CV) : 35%), avec une densité de 0,31 individu/km² (IC à 95% : 0,16 – 0,61), et le nombre de chimpanzés à 955 individus (IC à 95% : 671 – 1360 ; CV : 18%), avec une densité de 0,23 individu/km² (IC à 95% : 0,16 – 0,33). Le secteur de haute altitude du parc (Tshivanga) abrite des densités plus importantes de gorilles et de chimpanzés que la zone de basse altitude (secteurs de Kasese, Itebero, Nzovu-ouest et Nzovu-est), ainsi qu’une composition d’espèces d’oiseaux, d’amphibiens, de reptiles, et de plantes significativement différente de celles des secteurs de basse altitude. Des signes d’activités humaines, dont la chasse (présence de pièges, camps et douilles) et la présence de nombreux villages et de sites miniers, ont été recensés dans tout le parc et représentent une menace permanente du fait de l’installation de la population exploitant les ressources naturelles du parc. Le secteur de Nzovu-est est dans un état de conservation critique, avec un déclin avéré des populations de faune sauvage et la présence de nombreuses habitations humaines qui menacent l’importance de conservation de ce secteur. Des actions clés à mettre en oeuvre pour conserver la biodiversité du parc sont : - Débarrasser le parc et ses alentours des groupes et individus armés ; - Démilitariser et fermer les carrières minières qui se trouvent dans le parc ; - Mener des séances de sensibilisation et de concertation avec les villageois installés dans le parc pour les amener à se relocaliser volontairement en dehors du parc ; - Développer des projets de conservation communautaire ciblés, comprenant notamment des initiatives liés aux moyens de subsistance durables ; - Elargir la couverture du parc par les éco-gardes de l’ICCN pour renforcer la mise en application des lois de conservation dans le parc ; - Démarquer le secteur de Nzovu-est qui contient de nombreux villages et pourrait attirer davantage d’immigration ; - Mettre en oeuvre des stratégies de conservation ciblées pour assurer la protection des espèces clés de conservation dans les secteurs de Kasese et d’Itebero ; - Améliorer les relations entre l’ICCN et les populations vivant dans ou autour du parc.
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Herbivorous mammals are important for natural ecosystems even today, but how much stronger would there effects be without human-linked extinctions and extirpations? The ranges of many mammal species have contracted and numerous species have gone extinct due to human pressures, so herbivore impacts in even seemingly natural ecosystems likely deviate from their pre-anthropogenic state. However, such effects remain poorly understood and often unrecognized. To address this issue, we here quantified and mapped plant consumption by all terrestrial mammals in natural areas based on both current and estimated natural ranges. We then compared the estimated consumption rates to current plant net primary productivity, and summarised the results for global ecosystem types both broadly and in the wildest remaining natural areas around the world (the Last of the Wild). We found that wild mammals consume 7.3% (95% interquantile range: 0.85% - 26%) of net primary productivity in current natural areas, and that this would be much higher in the absence of extinctions and extirpations, namely 13% (95% interquantile range: 1.7% - 40%), i.e., a >50% higher consumption rate. Marked human-linked declines in herbivory were seen even in the wildest remaining natural areas, where mammals now consume a mean of 9% (95% interquantile range: 2.2% - 26%) of plant primary productivity, which is only 60% of no-extinction level. Our results show that mammalian herbivores naturally play an important part in ecosystems at a global scale, but that this effect has been strongly reduced by extinctions and extirpations.
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The tropical forests of Western Equatorial Africa are home to extraordinary biodiversity, including sympatric chimpanzees (Pan troglodytes troglodytes) and western lowland gorillas (Gorilla gorilla gorilla). The region is also comprised of significant stands of Intact Forest Landscapes (IFL) that are in rapid decline. As part of a regional monitoring effort, we partnered with local government officials, conservation NGOs, and the timber company working in the region to assess ape abundances in relation to habitat characteristics and anthropogenic disturbances and compare IFL and non-IFL areas in the Sangha Trinational landscape, Republic of Congo. We found that chimpanzees and gorillas occur at high densities in IFL, as well as non-IFL. To better understand how selective logging changes floristic factors, we compared herb and tree densities from botanical surveys conducted in IFL and non-IFL. IFL had higher tree stem densities and less terrestrial herbs than logged habitats. However, few ape resources were logged in this extraction cycle and areas with tree stems removed subsequently had higher abundances of terrestrial herbs preferred by apes, which may contribute to the elevated ape abundance estimates. Floristic differences in logged forest were identified to coincide with differences in ape resource use. The chimpanzee tree nesting niche was reduced in non-IFL as night nests were constructed significantly closer to the ground than in IFL. Whereas, gorilla nest height locations did not differ significantly between IFL and non-IFL. To identify other potential anthropogenic impacts, we assessed direct and indirect impacts of road expansion and illegal hunting on wildlife in these remote areas. Increased access to IFL that facilitates illegal hunting raises concern for protecting wildlife across Western Equatorial Africa. We urge that the results of biodiversity assessments and strategic aspects of long-term protection should be taken into account when identifying conservation set-asides and maintaining diverse states of modified forests. Finally, the results of our monitoring efforts are provided as evidence of the value of long-term collaborations among local stakeholders, government officials, conservation agencies, and industrial partners to improve the implementation of certification standards and biodiversity conservation initiatives.
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The daily construction of a sleeping platform or "nest" is a universal behavior among large-bodied hominoids. Among chimpanzees, most populations consistently select particular tree species for nesting, yet the principles that guide species preferences are poorly understood. At Semliki, Cynometra alexandri constitutes only 9.6% of all trees in the gallery forest in which the study populations ranges, but it was selected for 73.6% of the 1,844 chimpanzee night beds we sampled. To determine whether physical properties influence nesting site selection, we measured the physical characteristics of seven common tree species at the Toro-Semliki Wildlife Reserve, Uganda. We determined stiffness and bending strength for a sample of 326 branches from the seven most commonly used tree species. We selected test-branches with diameters typically used for nest construction. We measured internode distance, calculated mean leaf surface area (cm2) and assigned a tree architecture category to each of the seven species. C. alexandri fell at the extreme of the sample for all four variables and shared a tree architecture with only one other of the most commonly selected species. C. alexandri was the stiffest and had the greatest bending strength; it had the smallest internode distance and the smallest leaf surface area. C. alexandri and the second most commonly selected species, Cola gigantea, share a 'Model of Koriba' tree architecture. We conclude that chimpanzees are aware of the structural properties of C. alexandri branches and choose it because its properties afford chimpanzees sleeping platforms that are firm, stable and resilient.
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A topic of major interest in socio-ecology is the comparison of chimpanzees and bonobos' grouping patterns. Numerous studies have highlighted the impact of social and environmental factors on the different evolution in group cohesion seen in these sister species. We are still lacking, however, key information about bonobo social traits across their habitat range, in order to make accurate inter-species comparisons. In this study we investigated bonobo social cohesiveness at nesting sites depending on fruit availability in the forest-savannah mosaic of western Democratic Republic of Congo (DRC), a bonobo habitat which has received little attention from researchers and is characterized by high food resource variation within years. We collected data on two bonobo communities. Nest counts at nesting sites were used as a proxy for night grouping patterns and were analysed with regard to fruit availability. We also modelled bonobo population density at the site in order to investigate yearly variation. We found that one community density varied across the three years of surveys, suggesting that this bonobo community has significant variability in use of its home range. This finding highlights the importance of forest connectivity, a likely prerequisite for the ability of bonobos to adapt their ranging patterns to fruit availability changes. We found no influence of overall fruit availability on bonobo cohesiveness. Only fruit availability at the nesting sites showed a positive influence, indicating that bonobos favour food 'hot spots' as sleeping sites. Our findings have confirmed the results obtained from previous studies carried out in the dense tropical forests of DRC. Nevertheless, in order to clarify the impact of environmental variability on bonobo social cohesiveness, we will need to make direct observations of the apes in the forest-savannah mosaic as well as make comparisons across the entirety of the bonobos' range using systematic methodology.
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The construction of nests (or beds) for sleeping is a chimpanzee universal, yet little is known about the adaptive function of nest-building. We present an in-depth study of nest-building by unhabituated chimpanzees at the Seringbara study site in the Nimba Mountains, Guinea, West Africa. We recorded 1520 chimpanzee nests over 28 mo during three study periods between 2003 and 2008. We investigated where chimpanzees built their nests, both across the home range and in nest trees, and assessed how altitude and habitat type affected nest site selectivity. We examined whether or not chimpanzees were selective in nest tree choice regarding physical tree characteristics and tree species and assessed plant species preference for both tree- and ground-nesting. We tested three, nonmutually exclusive, hypotheses for the function of arboreal nest-building. We assessed whether selectivity for nest tree characteristics reflected an antipredator strategy, examined whether nesting patterns (both arboreal and terrestrial) and nesting height were influenced by variation in climatic conditions (temperature, humidity, wind), and measured mosquito densities at ground level and in trees at 10 m and related mosquito densities to nesting patterns. Chimpanzees preferred to nest above 1000 m and nested mainly in primary forest. They preferred relatively large trees with a low first branch, dense canopy, and small leaves and showed preference for particular tree species, which was stable across years, whereas plant choice for ground-nesting was largely based on plant availability. We found no support for the antipredation hypothesis, nor did mosquito densities explain arboreal nest-building. The thermoregulation hypothesis was supported, as both nesting patterns and nest-height variation across seasons reflected a humidity-avoidance strategy. Chimpanzees nested higher in trees and at higher altitudes in the wet season. In sum, chimpanzees were selective in their choice of nest sites, locations, and materials, and tree-nesting patterns at Seringbara were best explained by a thermoregulation strategy of humidity avoidance.
Survival Analysis Using S: Analysis of Time-to-Event Data is designed as a text for a one-semester or one-quarter course in survival analysis for upper-level or graduate students in statistics, biostatistics, and epidemiology. Prerequisites are a standard pre-calculus first course in probability and statistics, and a course in applied linear regression models. No prior knowledge of S or R is assumed. A wide choice of exercises is included, some intended for more advanced students with a first course in mathematical statistics. The authors emphasize parametric log-linear models, while also detailing nonparametric procedures along with model building and data diagnostics. Medical and public health researchers will find the discussion of cut point analysis with bootstrap validation, competing risks and the cumulative incidence estimator, and the analysis of left-truncated and right-censored data invaluable. The bootstrap procedure checks robustness of cut point analysis and determines cut point(s). In a chapter written by Stephen Portnoy, censored regression quantiles - a new nonparametric regression methodology (2003) - is developed to identify important forms of population heterogeneity and to detect departures from traditional Cox models. By generalizing the Kaplan-Meier estimator to regression models for conditional quantiles, this methods provides a valuable complement to traditional Cox proportional hazards approaches.
This chapter gives results from some illustrative exploration of the performance of information-theoretic criteria for model selection and methods to quantify precision when there is model selection uncertainty. The methods given in Chapter 4 are illustrated and additional insights are provided based on simulation and real data. Section 5.2 utilizes a chain binomial survival model for some Monte Carlo evaluation of unconditional sampling variance estimation, confidence intervals, and model averaging. For this simulation the generating process is known and can be of relatively high dimension. The generating model and the models used for data analysis in this chain binomial simulation are easy to understand and have no nuisance parameters. We give some comparisons of AIC versus BIC selection and use achieved confidence interval coverage as an integrating metric to judge the success of various approaches to inference.
A census was made of gorilla and chimpanzee populations throughout Gabon between December 1980 and February 1983. The aim of the census was to estimate the total numbers of both species and describe their distributions. The method was based on nest counts from line transects which allowed the calculation of population densities of all individuals except suckling infants. Fifteen types of habitat were recognized and defined in terms of their structural features. In the initial phase of the study we did transects in each habitat-type and computed mean densities for each species in each habitat-type. In the second phase of the study we estimated the sizes of gorilla and chimpanzee populations throughout the country by extrapolation from these population density values. We did transects in all areas of the country and conducted interviews to check the accuracy of the population totals obtained by extrapolation. Corrections were made to the extrapolated totals to take into account different levels of hunting pressure and other human activities found to modify ape population densities. Total populations of 34,764 gorillas and 64,173 chimpanzees were estimated. An error of ± 20% was associated with the estimated population totals, which allows the conclusion that Gabon contains 35,000 ± 7,000 gorillas and 64,000 ± 13,000 chimpanzees. The figure for gorillas is much larger than previous estimates. This seems to be because (1) gorillas occur in almost all types of forest and are not restricted to man-made secondary forest as had been though; and (2) the geographical distribution of gorillas in Gabon is wider than previously believed. Gabon's large areas of undisturbed primary forest offer exceptional potential for conservation, not only of gorillas and chimpanzees, but also of the intact tropical rain forest ecosystems which they inhabit.
We collected nesting data from 512 fresh nest sites, including 3725 individual nests, of western gorillas at the Mondika Research Site, Central African Republic and Republic of Congo from 1996 through mid-1999. The mean count of nests of weaned individuals is 7.4 per nest site. Nest types included bare earth with no construction (45% of total), partial to full ground construction (34%), and arboreal (21%). Females, blackbacks, and juveniles as a combined age-sex class built significantly more arboreal nests (21% of total) than silverbacks did (2%). Proximate rainfall (independent of temperature) is significantly correlated with nest construction, i.e., as rainfall increased, silverbacks built more ground nests, and non-silverbacks built more ground and arboreal nests. Maximum daily temperature (independent of rainfall) is significantly negatively correlated with nest construction, i.e., as temperature increased, gorillas slept more often on bare earth without constructing a nest. Accordingly, we conclude that although nest building in gorillas may have innate components shared with other great apes, it is a flexible behavioral pattern that in some western populations is often not exhibited. It appears that when gorillas in this population build nests, they do so in response to both wet and cool conditions, and independently of diet, ranging, or group size.
Information on the densities of threatened species in non-protected areas is crucial for assessing the degree of isolation of adjacent protected areas and consequently their potential for preserving species from extinction. Relatively few studies, however, provide such information. We present the results of a survey of the densities of two great ape species, the gorilla Gorillagorillagorilla and chimpanzee Pantroglodytestroglodytes, in a non-protected area on the northern periphery of Dja Faunal Reserve, Cameroon. Densities of chimpanzees and gorillas were estimated to be 1.1 and 3.8 weaned individuals per km2, respectively. The results confirm that gorillas prefer building nests in vegetation types with limited visibility, and that within preferred vegetation types for nesting, gorillas select patches that are the most difficult to penetrate, resulting in less conspicuous nests. Although the opposite tendencies were exhibited by chimpanzees, no firm conclusions could be drawn from our data. Despite its non-protected status and past and ongoing logging activities in the area, the densities of gorillas and chimpanzees on the northern periphery of Dja Faunal Reserve are comparable to those found within the reserve itself, indicating the need for developing alternative conservation action to protect these important populations. The creation of a Communal Wildlife Zone in this area is legislatively possible, and could be an effective conservation tool because it has to originate from the local people.