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食草哺乳动物（如大象、角马、水牛、羚羊和斑马）数量的减少对非洲稀树草原栖息地的影响。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，失去这些物种可能会对植被产生深远的影响。研究人员将测试两种大型食草动物-植物相互作用频率的下降和随之而来的变化是如何改变热带草原植被的。为了确定依赖大型哺乳动物种子传播的树种数量是否减少，研究小组将在莫桑比克戈龙戈萨国家公园进行实地调查。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，研究人员将测试最近，人类导致的非洲保护区大象数量的减少是否会导致树木覆盖面积的增加，并可能因此威胁到适应非洲稀树草原历史上稀疏树木覆盖的物种。大象是稀树草原上特别重要的食草动物，因为它们会推倒树木，可能会维持稀树草原上开放的栖息地。许多美国生态系统最近也经历了哺乳动物食草压力的巨大变化，因为鹿的数量激增，这项研究将促进我们对生态系统对变化的食草制度的反应的理解。项目成果将被纳入生态学和自然保护的本科课程。研究结果将通过科学文章、公园及其附属网站的博客以及E.O.威尔逊生物多样性基金会制作的在线视频传播。由于火灾制度的改变和气候变化，非洲热带稀树草原上的植被结构和组成正在发生变化，但食草动物种群数量的减少对植被变化的影响程度尚不清楚。在戈龙戈萨的田野调查将包括比较在大型哺乳动物减少之前和之后招募的林地斑块的植被，以测试新的林地群落是否相对缺乏哺乳动物分散的植物。这种模式可以从传播种子的哺乳动物减少的热带森林中了解到，但稀树草原树木在多大程度上依赖大型哺乳动物进行传播尚不清楚。为了验证最近大象数量的减少导致稀树草原树木覆盖面积增加的假设是否属实，研究人员将建立统计模型，根据已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面积的变化。

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.