PLOS ONE RESEARCH ARTICLE Long-term historical and projected herbivore population dynamics in Ngorongoro crater, Tanzania 1³ 2³ 2 3 Patricia D. Moehlman , Joseph O. OgutuID *, Hans-Peter Piepho , Victor A. Runyoro , Michael B. Coughenour4, Randall B. Boone5 1 IUCN/SSC Equid Specialist Group, Arusha, Arusha, Tanzania, 2 Institute for Crop Science-340, University of Hohenheim, Stuttgart, Germany, 3 Environmental Reliance Consultants Limited, Arusha, Tanzania, 4 Department of Ecosystem Science and Sustainability, Colorado State University, Fort Collins, Colorado, United States of America, 5 Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, a1111111111 Colorado, United States of America a1111111111 a1111111111 ³ PDM and JOO are joint first authors on this work. a1111111111 * [email protected] a1111111111 Abstract The Ngorongoro Crater is an intact caldera with an area of approximately 310 km2 located OPEN ACCESS within the Ngorongoro Conservation Area (NCA) in northern Tanzania. It is known for the Citation: Moehlman PD, Ogutu JO, Piepho H-P, abundance and diversity of its wildlife and is a UNESCO World Heritage Site and an Interna- Runyoro VA, Coughenour MB, Boone RB (2020) tional Biosphere Reserve. Long term records (1963±2012) on herbivore populations, vege- Long-term historical and projected herbivore population dynamics in Ngorongoro crater, tation and rainfall made it possible to analyze historic and project future herbivore population Tanzania. PLoS ONE 15(3): e0212530. https://doi. dynamics. NCA was established as a multiple use area in 1959. In 1974 there was a pertur- org/10.1371/journal.pone.0212530 bation in that resident Maasai and their livestock were removed from the Ngorongoro Crater. Editor: Floyd W. Weckerly, Texas State University, Thus, their pasture management that was a combination of livestock grazing and fire was UNITED STATES also removed and 'burning' stopped being a regular occurrence until it was resumed in 2001 Received: February 2, 2019 by NCA management. The Maasai pasture management would have selected for shorter Accepted: February 10, 2020 grasses and more palatable species. Vegetation mapping in 1966±1967 recorded predomi- nately short grasslands. Subsequent vegetation mapping in the crater in 1995 determined Published: March 10, 2020 that the grassland structure had changed such that mid and tall grasses were dominant. Copyright: © 2020 Moehlman et al. This is an open After removal of the Maasai pastoralists from the Ngorongoro Crater in 1974, there were sig- access article distributed under the terms of the Creative Commons Attribution License, which nificant changes in population trends for some herbivore species. Buffalo, elephant and permits unrestricted use, distribution, and ostrich numbers increased significantly during 1974±2012. The zebra population was stable reproduction in any medium, provided the original from 1963 to 2012 whereas population numbers of five species declined substantially author and source are credited. between 1974 and 2012 relative to their peak numbers during 1974±1976. Grant's and Data Availability Statement: All relevant data are Thomson's gazelles, eland, kongoni, and waterbuck (wet season only) declined significantly within the manuscript and its Supporting in the Crater in both seasons after 1974. In addition, some herbivore species were consis- Information files. tently more abundant inside the Crater during the wet than the dry season. This pattern was Funding: JOO was supported by a grant from the most evident for the large herbivore species requiring bulk forage, i.e., buffalo, eland, and German Research Foundation (Grant No. OG 83/1- 1). JOO was also supported by The Planning for elephant. Even with a change in grassland structure, total herbivore biomass remained rela- Resilience in East Africa through Policy, tively stable from 1963 to 2012, implying that the crater has a stable carrying capacity. Anal- Adaptation, Research, and Economic Development yses of rainfall indicated that there was a persistent cycle of 4.83 years for the annual (USAID PREPARED) project. This project has received funding from the European Union's component. Herbivore population size was correlated with rainfall in both the wet and dry PLOS ONE | https://doi.org/10.1371/journal.pone.0212530 March 10, 2020 1 / 39 PLOS ONE Herbivore population dynamics in Ngorongoro Crater, Tanzania Horizon 2020 research and innovation programme seasons. The relationships established between the time series of historic animal counts in under Grant Agreement No. 641918. the wet and dry seasons and lagged wet and dry season rainfall series were used to forecast Competing interests: The authors have declared the likely future trajectories of the wet and dry season population size for each species that no competing interests exist under three alternative climate change scenarios. Introduction Biodiversity conservation is facing widespread and mounting challenges world-wide primarily due to human population expansion, land use changes, overexploitation of biodiversity, cli- mate change, invasive species and their interactions, leading to unprecedented biodiversity losses [1]. Effective biodiversity conservation therefore urgently requires an accurate under- standing of biodiversity dynamics and their primary drivers as a basis for developing sound conservation policies and management strategies. Rainfall primarily governs vegetation pro- duction and quality, surface water availability and quality in savannas [2±6] and therefore also the aggregate and species-specific biomass levels of large African savanna ungulates [7±9]. Consequently, it is important to first establish the influence of rainfall variation on large herbi- vore population dynamics before the contributions of other factors can be reliably established [10]. In the Serengeti-Mara ecosystem straddling Kenya and Tanzania, rainfall primarily drives population dynamics [11,12], aggregate population biomass [7, 8], recruitment dynamics [13], phenology, synchrony and prolificacy of calving [14,15,16], seasonal dispersal and migration of large herbivores [17,18]. However, increasing frequency and intensity of droughts [19,20], widening variation of river flows [21] and rising temperatures [19] hasten the need to advance our understanding of how anticipated climate changes will likely affect larger herbivore popu- lations. Such understanding can be gained by studying rainfall influences on large herbivore population dynamics using long-term monitoring data from protected areas with relatively lit- tle human influence, such as the Ngorongoro Crater, in northern Tanzania. The Ngorongoro Crater in Tanzania is known worldwide for the abundance and diversity of its wildlife [22]. It is situated in the Crater Highlands and is linked both to this area and the Serengeti Plains by the seasonal migration of several herbivores [22±24] and the emigration and immigration of large carnivores [25±29]. Earlier analyzes of large herbivore population dynamics in the crater [23,30,31] have described temporal trends but not formally related them to their putative drivers. Earlier stud- ies have identified droughts, disease, predation and their interactions and poaching as the main drivers of large herbivore population change in the crater. Severe drought years combined with disease can adversely impact herbivore populations. For example, in 2000 and 2001 there was significant mortality in buffalo (1500 animals), wilde- beest (250) and zebra (100) apparently due to nutritional stress resulting from the severe drought in the dry season in 2000 [23,32,33]. Poaching was the major impact on the Black rhino population from the 1970's to mid- 1980's. Since the early 1990's there has been limited poaching and the population is slowly recovering. Conservative population projections in 1995 [34] predicted that in the best scenario, i.e., no poaching, the population should be approximately 35 to 40 individuals by 2017. The Black rhino population was 59 individuals in 2018 (Pers comm, M. Musuha, 2018, NCAA). Since 1963, the herbivore population of Ngorongoro Crater has been monitored by the Ngorongoro Conservation Area Authority (NCAA), The College of African Wildlife PLOS ONE | https://doi.org/10.1371/journal.pone.0212530 March 10, 2020 2 / 39 PLOS ONE Herbivore population dynamics in Ngorongoro Crater, Tanzania Management and research scientists [23±24,30,35±37]. Since 1978, the Ngorongoro Ecological Monitoring Program has been responsible for conducting the wet and dry season systematic ground counts. The complete data set covers a period of 50 years (1963±2012). Here, we analyze the long-term population trends of the 12 most common large herbivore species as well as the aggregate biomass of the multi-species wild herbivore community and relate the trends quantitatively to historic rainfall and projected rainfall variation under three climate change scenarios and qualitatively to vegetation changes and burning. Relationships established between historic population abundance and historic rainfall are used to project the impacts of three different future rainfall scenarios on wild herbivore population dynamics to 2100. In particular, we test predictions of the following hypotheses. First, the eviction of Maasai, the removal of their livestock and cessation of grassland burning, resulted in changes in range- land management that generated complex changes in vegetation composition and structure and hence in wild herbivore population numbers