Dissertation Research: Testing the effects of large mammalian herbivores on savanna dynamics and community structure with regional- and continent-scale natural experiments

该项目的目标是确定大型哺乳动物食草动物(例如大象、角马、水牛、羚羊和斑马)的数量下降如何影响非洲稀树草原的栖息地。小规模的实验研究表明，由于这些大型食草动物消耗大量的植物材料，这些物种的丧失可能会对植被产生深远的影响。研究人员将测试种群数量下降以及由此导致的两种大型食草动物-植物相互作用频率的变化如何改变稀树草原植被。为了确定依赖大型哺乳动物种子传播的树种是否大量减少，研究小组将在莫桑比克戈龙戈萨国家公园进行田野调查，1977-1992年内战期间，那里的偷猎活动几乎消灭了所有大型哺乳动物。其次，通过分析历史和现代卫星图像，研究人员将测试非洲保护区内大象数量最近由人类驱动的下降是否会导致树木覆盖率增加，以及可能随之而来的对适应非洲大草原历史上稀疏树木覆盖的物种的威胁。大象是特别重要的稀树草原食草动物，因为它们会推倒树木，可能会维持稀树草原的开阔栖息地。随着鹿数量的激增，许多美国生态系统最近也经历了哺乳动物食草动物压力的戏剧性变化，这项研究将促进我们对生态系统对变化的草食制度的反应的理解。项目成果将被纳入一门关于生态与保护的本科课程。研究结果将通过科学文章、公园和附属网站上的博客以及E.O.威尔逊生物多样性基金会制作的在线视频进行传播。由于火灾制度和气候变化，非洲大草原的植被结构和组成正在发生变化，但草食动物种群的减少在多大程度上导致了植被的变化尚不清楚。戈龙戈萨的实地考察工作将包括对大型哺乳动物减少之前和之后招募的林地斑块中的植被进行比较，以测试新的林地群落是否相对缺乏哺乳动物分布的植物。这种模式是从热带森林中得知的，那里传播种子的哺乳动物已经减少，但稀树草原树依赖大型哺乳动物进行传播的程度尚不清楚。为了测试对最近大象数量下降导致稀树草原树木覆盖率增加的假设的支持，研究人员将建立统计模型，根据公布的大象密度估计以及遥感火灾和降雨数据预测非洲公园遥感树木覆盖率的变化。

The goal of this project is to determine how population declines of large mammalian herbivores (e.g., elephants, wildebeest, buffalo, antelopes, and zebra) affect African savanna habitats. Small-scale experimental studies have shown that because these large herbivores consume huge quantities of plant material, losing these species is likely to profoundly affect vegetation. The researchers will test how population declines and consequent changes in the frequency of two types of large herbivore-plant interactions alter savanna vegetation. In order to determine whether tree species reliant on large-mammal seed dispersal have declined in abundance, the research team will conduct fieldwork in Gorongosa National Park, Mozambique, where poaching during the 1977-1992 civil war nearly eliminated all large mammals. Second, by analyzing historical and modern satellite imagery, the researchers will test whether recent, human-driven declines in elephant populations across African protected areas lead to increased tree cover and possibly consequent threats to species adapted to historically sparse tree cover in African savannas. Elephants are particularly important savanna herbivores because they topple trees, potentially maintaining open savanna habitats. Many U.S. ecosystems have also experienced recent, dramatic changes in mammalian herbivore pressure as deer populations have exploded, and this research will advance our understanding of ecosystem responses to changed herbivory regimes. Project results will be incorporated into an undergraduate course on ecology and conservation. Results will be disseminated through scientific articles, blogs on park and affiliated websites, and online videos produced by the E.O. Wilson Biodiversity Foundation.Vegetation structure and composition is shifting in savannas across Africa due to altered fire regimes and climate change, but the extent to which declining herbivore populations are responsible for changing vegetation is unclear. Fieldwork in Gorongosa will include comparisons of vegetation in woodland patches that recruited prior to and since large mammal declines to test whether new woodland communities are relatively devoid of mammal-dispersed plants. This pattern is known from tropical forests where seed-dispersing mammals have declined, but the extent to which savanna trees depend on large mammals for dispersal is unknown. To test support for the hypothesis that recent declines in elephant populations lead to increased savanna tree cover, the researchers will build statistical models predicting changes in remotely sensed tree cover for African parks from published estimates of elephant density along with remotely sensed fire and rainfall data.