Long‐Term Temporal and Spatial Dynamics of Food Availability for Endangered Mountain Gorillas in Volcanoes National Park, Rwanda

Abstract

Monitoring temporal and spatial changes in the resource availability of endangered species contributes to their conservation. The number of critically endangered mountain gorillas (Gorilla beringei beringei) in the Virunga Volcano population has doubled over the past three decades, but no studies have examined how food availability has changed during that period. First, we assessed if the plant species consumed by the gorillas have changed in abundance and distribution during the past two decades. In 2009-2010, we replicated a study conducted in 1988-1989 by measuring the frequency, density, and biomass of plant species consumed by the gorillas in 496 plots (ca. 6 km(2)) in the Karisoke study area in Volcanoes National Park, Rwanda. We expected to observe a decreased presence of major gorilla food plants as a likely result of density-dependent overharvesting by gorillas. Among the five most frequently consumed species (composing approximately 70% of the gorilla's diet, excluding bamboo), two have decreased in availability and abundance, while three have increased. Some species have undergone shifts in their altitudinal distribution, possibly due to regional climatic changes. Second, we made baseline measurements of food availability in a larger area currently utilized by the gorillas. In the extended sampling (n = 473 plots) area (ca. 25 km(2) ), of the five most frequently consumed species, two were not significantly different in frequency from the re-sampled area, while two occurred significantly less frequently, and one occurred significantly more frequently. We discuss the potential impact of gorilla-induced herbivory on changes of vegetation abundance. The changes in the species most commonly consumed by the gorillas could affect their nutrient intake and stresses the importance of monitoring the interrelation among plant population dynamics, species density, and resource use.

Quadratic relationships between group size and foraging efficiency in a herbivorous primate

“…In which case, its higher energy intake rate could be a result of being in the right place at the right time rather than outcompeting smaller groups. Rubus has increased in biomass over the last 20 years (from being present in 1% to 26% of sampled plots), which may also help to explain why our results do not match previous studies 35,45,73 . Rubus does not seem to entirely explain our quadratic relationships, however, because the results for daily travel distance remained significant even when we excluded the largest group.…”

“…Home range sizes have remained unchanged 45,52 . The biomass density declined by more than 50% for two of their most frequently consumed foods, but it increased for three of their other top-five foods 73 . Some of these apparent changes could be due to differences in methodology, and collectively the results do not indicate an immediate threat to continued population growth, but they highlight areas that warrant continued monitoring for this critically endangered species 52,65,73 .…”

“…The biomass density declined by more than 50% for two of their most frequently consumed foods, but it increased for three of their other top-five foods 73 . Some of these apparent changes could be due to differences in methodology, and collectively the results do not indicate an immediate threat to continued population growth, but they highlight areas that warrant continued monitoring for this critically endangered species 52,65,73 . The changes in population density and food density could also intensify both intragroup feeding competition and intergroup contest competition, thereby contributing to the quadratic relationships that were detected in this study.…”

“…The Virunga Volcano region contains a range of habitats that vary along an altitudinal gradient from the bamboo zone below 2800 meters to afro-montane vegetation above 3600 meters 36,39,45,73 . Between those two extremes, the additional habitats are the mixed species montane forest with an understory of Mimulopsis spp.…”

Age‐related changes in molar topography and shearing crest length in a wild population of mountain Gorillas from Volcanoes National Park, Rwanda

“…The majority of their diet (93%) consists of leaves and stems, with smaller contributions of pith, root epithelium, bark, flowers, and fruit (Watts, 1984). Mountain gorillas inhabit high-altitude forests that boast an abundance of these foods, which are available year-round for their consumption (Schaller, 1963;Fossey and Harcourt, 1977;Watts, 1984;McNeilage, 2001;Grueter et al, 2013). The mechanical properties of mountain gorilla foods have been measured at Mgahinga Gorilla National Park, Uganda, Bwindi-Impenetrable National Park, Uganda, and Volcanoes National Park, Rwanda.…”