Dian Fossey Gorilla Fund International - Demography, population dynamics and movement projects for the Karisoke Mountain Gorilla Project

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The dynamics of wild populations are determined by the population’s demographic rates (e.g. survival, fecundity, and migration) and by the impact of environmental factors on these rates (Boyce, Haridas & Lee 2006; Morris et al. 2011; Colchero et al. 2019). Abrupt changes in environmental conditions, such as those resulting from human activities, can greatly influence the risk of extinction of small populations by reducing survival and fecundity and by increasing their variability, which in turn reduces key demographic variables such as population growth rates (Boyce et al. 2006; Pearson et al. 2014; Palmer et al. 2017). The mountain gorilla population of the Virunga Massif has historically faced constant human pressure in the form of habitat reduction, poaching, and other types of human-wildlife conflicts (Kalpers et al. 2003; Mehlman 2008; Meder 2015). Long-term conservation efforts have allowed the population to increase from less than 300 individuals in the late 1970s (Weber & Vedder 1983), to almost 500 individuals in the early 2010s (Gray et al. 2013) and over 600 in 2016 (Hickey et al. 2018). Despite this encouraging recovery, ensuring the survival of this population is still a pressing issue, and thus efforts to estimate its long-term viability are constantly needed. Several projects were carried out in the 1980s and 1990s to estimate the long-term persistence of the population (Weber & Vedder 1983; Harcourt 1995). Although the results were encouraging, as Harcourt (1995) stated, the models used were limited in that they remained unrealistically simple, ignoring the effect of environmental stochasticity and interactions between demographic rates and environmental variables. Furthermore, and due to limitations in the available data, models were constructed assuming constant demographic rates within age classes (e.g. juvenile, immature, adult). As we recently showed (Colchero et al. 2019), this oversimplification can greatly distort long-term predictions of population dynamics, population growth rates and extinction probability. More recent studies have shed important insights into the current dynamics of the population. For instance, in a forthcoming paper, Caillaud et al (under review) have found that the recovery of the population coupled with the encroachment of the habitat have exacerbated the overlap between group home ranges, which in turn have increased antagonistic interactions such as infanticide and male mortality. The end result is that population growth rates are declining.
Effective start/end date01/10/201930/09/2020