Variation in herbivore space use: comparing two savanna ecosystems with different anthrax outbreak patterns in southern Africa

Background The distribution of resources can affect animal range sizes, which in turn may alter infectious disease dynamics in heterogenous environments. The risk of pathogen exposure or the spatial extent of outbreaks may vary with host range size. This study examined the range sizes of herbivorous anthrax host species in two ecosystems and relationships between spatial movement behavior and patterns of disease outbreaks for a multi-host environmentally transmitted pathogen. Methods We examined range sizes for seven host species and the spatial extent of anthrax outbreaks in Etosha National Park, Namibia and Kruger National Park, South Africa, where the main host species and outbreak sizes differ. We evaluated host range sizes using the local convex hull method at different temporal scales, within-individual temporal range overlap, and relationships between ranging behavior and species contributions to anthrax cases in each park. We estimated the spatial extent of annual anthrax mortalities and evaluated whether the extent was correlated with case numbers of a given host species. Results Range size differences among species were not linearly related to anthrax case numbers. In Kruger the main host species had small range sizes and high range overlap, which may heighten exposure when outbreaks occur within their ranges. However, different patterns were observed in Etosha, where the main host species had large range sizes and relatively little overlap. The spatial extent of anthrax mortalities was similar between parks but less variable in Etosha than Kruger. In Kruger outbreaks varied from small local clusters to large areas and the spatial extent correlated with case numbers and species affected. Secondary host species contributed relatively few cases to outbreaks; however, for these species with large range sizes, case numbers positively correlated with outbreak extent. Conclusions Our results provide new information on the spatiotemporal structuring of ranging movements of anthrax host species in two ecosystems. The results linking anthrax dynamics to host space use are correlative, yet suggest that, though partial and proximate, host range size and overlap may be contributing factors in outbreak characteristics for environmentally transmitted pathogens.研究背景 资源分布可影响动物活动范围大小，而异质环境中活动范围的变化又可能改变传染病的流行动态。病原体暴露风险或疫情的空间波及范围，可能随宿主活动范围大小发生改变。本研究聚焦两种生态系统下的草食性炭疽（anthrax）宿主物种的活动范围，并探究了多宿主环境传播性病原体的空间移动行为与疾病暴发模式之间的关联。研究方法 本研究针对纳米比亚埃托沙国家公园（Etosha National Park）与南非克鲁格国家公园（Kruger National Park）内的7种宿主物种的活动范围，以及炭疽暴发的空间波及范围展开分析——两处公园的主要宿主物种与暴发规模均存在差异。我们采用局部凸包法（local convex hull method）在不同时间尺度下评估宿主活动范围、个体内时间尺度活动范围重叠情况，以及各公园内宿主物种对炭疽病例数的贡献与活动行为之间的关联。此外，我们估算了年度炭疽致死病例的空间波及范围，并评估该范围是否与特定宿主物种的病例数存在相关性。研究结果 不同物种间的活动范围差异与炭疽病例数并非线性相关。在克鲁格国家公园，主要宿主物种的活动范围较小且范围重叠度较高，当疫情在其活动范围内暴发时，这种特征可能提升宿主的暴露风险。而埃托沙国家公园则呈现出截然不同的模式：其主要宿主物种活动范围较大，且重叠度相对较低。两处公园的炭疽致死病例空间波及范围整体相近，但埃托沙国家公园的该范围波动幅度小于克鲁格国家公园。克鲁格国家公园的疫情规模从局部小集群到大面积暴发不等，其空间波及范围与病例数及受影响物种均存在相关性。次要宿主物种对暴发的病例数贡献相对较少；然而，对于这类活动范围较大的次要宿主而言，其病例数与疫情波及范围呈正相关。研究结论 本研究结果为两种生态系统中炭疽宿主物种活动移动的时空结构提供了新的认知。将炭疽流行动态与宿主空间利用模式相关联的分析结果虽仅为相关性证据，但仍表明：尽管宿主活动范围大小与重叠度属于部分且间接的影响因素，但它们可能是环境传播性病原体暴发特征的潜在贡献因子之一。