Doctoral Dissertation Research: Regional Ecogeographical Impacts of Large Herbivores on Savanna Ecosystems

本博士论文研究改进项目通过量化大型食草动物（100公斤大型食草动物），特别是大象的大尺度生态和气候影响，重点研究热带稀树草原生态系统的地理格局和过程。稀树草原和大草原覆盖了地球陆地表面的五分之一，是超过5亿人的家园，并不成比例地影响着全球碳循环的年际变化。然而，人们对造成和维持稀树草原低木本覆盖（树木和灌木）的因素了解有限。几十年来，研究人员已经清楚地记录了大型食草动物如何抑制当地稀树草原的树木覆盖，而大规模的影响仍然不清楚。通过量化大型食草动物对生态系统的影响，本论文将加强对热带稀树草原形成因素及其对全球气候影响的理解。这样的认识对全世界大型食草动物的保护至关重要。这项工作将建立一个实地站点，这将有利于研究人员的早期学术生涯，并与一位成熟的热带草原研究人员建立合作伙伴关系。该项目将通过学生实习和为期一周的学生和教师编程课程来培养能力。这个博士论文研究改进奖将提供支持，使有前途的学生建立一个独立的研究生涯。尽管进行了数十年的研究，但对稀树草原和大草原木材覆盖决定因素的科学理解仍落后于其他生物群系。为此目的，越来越多针对特定地点的研究发现，一些保护区的树木死亡率主要由大型食草动物控制。然而，目前尚不清楚这些影响是否会显著影响更大景观和区域的木材覆盖总量。如果确实如此，这将有力地支持越来越多的观点，即生物群的范围和持久性是食草性的直接产物。这个项目的重点是量化非洲稀树草原象（Loxodonta africana）密度和稀树草原树木覆盖之间的关系，这些保护区分布在东非大象活动范围内的12个保护区。然后，它询问大象的影响是否会改变全球气候。为了回答这些问题，研究人员使用卫星图像绘制了每个研究地点的树木覆盖情况。国家科学基金会的资金将用于通过在选定地点收集地面真实数据来评估地图的准确性。通过将这些数据纳入一个全球网络，并计划在未来监测实地地点，研究人员将建立重要的基线数据，这些数据将提供给研究人员用于未来的研究。在全球范围内，世界巨型动物种群数量持续减少，它们所支撑的生态系统、国家收入和当地生计面临不利后果。虽然这个研究项目将集中在东非的稀树草原生态系统上，但这项研究将为包括美国在内的许多其他国家的大型食草动物保护提供新的见解和方法。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This doctoral dissertation research improvement project focuses on geographical patterns and processes of savanna ecosystems by quantifying the broad-scale ecological and climatological impacts of the megaherbivores (large herbivores 100 kg), particularly elephants. Savannas and prairies cover a fifth of Earth's land surface, are home to over a half a billion people, and disproportionately affect interannual variability of the global carbon cycle. Yet, there is limited understanding of the factors that create and maintain the low woody cover (trees and bushes) of savannas. For decades, researchers have clearly documented how large herbivores suppress local savanna woody cover, while the large-scale effects continue to remain unclear. By quantifying a large herbivore's ecosystem-wide impacts, this dissertation will enhance understanding of the factors shaping savannas and their influence on global climate. Such understanding is critical for large herbivore conservation worldwide. The work will establish a field site that will benefit the early academic careers of the researchers and create a collaborative partnership with an established savanna researcher. The project will build capacity through a student internship and a week-long programming course for students and faculty. This Doctoral Dissertation Research Improvement award will provide support to enable a promising student to establish an independent research career.Despite decades of research, the scientific understanding of the determinants of woody cover in savannas and prairies lags behind that of other biomes. To this end, a growing number of site-specific studies have found that tree mortality rates in some protected areas are principally controlled by large herbivores. However, it is not known whether these impacts significantly affect the total woody cover of the larger landscapes and region. If they do, it will lend significant support to the growing argument that biome extent and persistence can be a direct product of herbivory. This project focuses on quantifying the relationship between African savanna elephant (Loxodonta africana) densities and savanna woody cover in twelve protected areas across elephants' eastern African range. It then asks whether elephants' impacts alter global climate. To answer these questions, the researchers used satellite imagery to map the woody cover in each of the study sites. NSF funding will be used to assess the accuracy of the maps through the collection of ground truth data at a select site. By incorporating the data into a global network, with plans to monitor the field site into the future, the researchers will establish important baseline data which will be made available to researchers for future studies. Globally, the world's megafauna populations continue to decline and the ecosystems, national revenues, and local livelihoods they support face detrimental consequences. Although this research project will focus on savanna ecosystems in eastern Africa, the research will provide new insights and approaches for megaherbivore conservation in many other countries, including the United States.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.