BE/CNH: Homeostasis and Degradation in Fragile Tropical Agroecosystems

当人和自然环境都处于“边缘”时，自然资源基础的微小变化往往会对人们的生活产生重要影响。相反，人类活动的变化可能显著影响生态系统功能。这种响应性使得有必要研究人类行为、自然资本存量和生态系统服务流动之间的相互作用。农民在土地利用和改良、作物品种选择、牲畜管理策略、化学品应用和劳动力分配等方面做出许多决定。这些决定从根本上影响植物、牲畜和土壤生物的生长——简而言之，影响整个农业生态系统的功能。复杂的反馈回路向不同的子系统发送响应，从而产生动态反应。外生冲击，例如与气候变率和不断变化的经济激励有关的冲击，经常破坏这一复杂系统的过程。随着时间的推移，这些干扰在整个系统中传播，导致子系统的行为和性能的适应。了解肯尼亚小农与自然环境之间的相互作用是该项目的主要目标。在肯尼亚中部高地的Embu、肯尼亚西部半湿润地区维多利亚湖盆地的Siaya-Vihiga和裂谷上游的Baringo三个研究地点，一些农民已经调整了他们的农业实践，通过作物-牲畜结合和使用各种保护策略来维持土壤。在同一生态系统内，一些农民能够保持土壤肥力和农业生产力，而他们的邻居却陷入“自然资源贫困陷阱”，造成粮食不安全和农业生态系统退化。对能够维持体内平衡的系统以及无法从人为和自然冲击中恢复的系统的研究，为了解支撑热带农业生态系统功能的核心生物物理和社会经济过程提供了机会。研究人员将使用系统动力学方法开发一个模型，该模型包括经济（受社会学约束）、牲畜、作物和土壤子模型之间的复杂反馈循环。该模型将使用土壤状况（养分、有机物和微生物特征）、作物产量、植物生物量生产和牲畜生长以及生产和社会经济数据等数据进行校准和测试。人们日益认识到，土壤肥力的耗竭是非洲粮食产量下降的根本生物物理原因。除非土壤枯竭的原因得到补救，否则就不可能增加粮食产量以满足由于人口增长而产生的预计需求，也不可能确保撒哈拉以南37%的非洲人每天获得少于2200卡路里的营养。由于这个项目的研究地点因生态区和市场准入而异，它们代表了整个非洲和许多发展中国家的系统。这一研究项目将全面评估导致热带农业生态系统退化的社会经济和生物物理因素。该项目将增加对热带土壤微生物生态的有限了解，并增加关于土壤何时达到难以补充的阈值的知识。该项目还将有助于了解人们陷入贫困的过程。通过将对个别研究领域的贡献整合到一个动态模型中，可以探索这些脆弱系统对经济和生物物理冲击的反应，将使对这些研究领域的贡献的整合更加有用。该项目得到了2002年度环境生物复杂性特别竞赛的支持，该竞赛侧重于自然和人类系统耦合动力学。

When both people and their natural environment are "at the margin," small changes in the natural resource base often have important effects on people's lives. Conversely, modifications in human activities may significantly affect ecosystem functioning. This responsiveness makes it necessary to study the interactions between human behavior, natural capital stocks, and the flow of ecosystem services. Farmers make many decisions about land use and improvement, selection of crop varieties, livestock-management strategies, chemical applications, and labor allocation. These decisions fundamentally affect the growth of plants, livestock, and soil biology -- in short, the functioning of the entire agroecosystem. Complex feedback loops send responses to the different subsystems engendering dynamic reactions. Exogenous shocks, such as those associated with climate variability and changing economic incentives, regularly disrupt processes in this complex system. Over time, those disturbances propagate throughout the system, causing adaptation in behaviors and performance of the subsystems. Understanding the interplay between smallholder farmers in Kenya and their natural environment is the primary goal of this project. Within three Kenyan research sites, Embu in the central highlands, Siaya-Vihiga in the Lake Victoria basin in subhumid western Kenya, and upper Baringo in the Rift valley, some farmers have adapted their agricultural practices to sustain their soils through crop-livestock integration and use of a variety of conservation strategies. Within the same ecosystem, some farmers are able to maintain soil fertility and farm productivity, while their neighbors are caught in "natural resource poverty traps" that yield food insecurity and agroecosystem degradation. Studies of the systems that are able to maintain homeostasis as well as those that do not recover from human and natural shocks provide the opportunity to understand the central biophysical and socioeconomic processes that underpin agroecosystem functioning in the tropics. The investigators will develop a model using a system dynamics approach that includes the complex feedback loops among economic (conditioned by sociological constraints), livestock, crop, and soils submodels. The model will be calibrated and tested using data on soil status (nutrients, organic matter and microbial profiles), crop yields, plant biomass production, and livestock growth as well as production and socio-economic data.Depletion of soil fertility is increasingly acknowledged to be the fundamental biophysical cause of declining food production in Africa. Unless the causes for soil depletion are remedied, it will be impossible to increase food production to meet projected demand due to population growth and to ensure that the 37 percent of the sub-Saharan Africans receiving less that 2,200 calories per day are adequately nourished. Because this project's research sites vary by ecological zone and market access, they represent systems throughout Africa and much of the developing world. This research project will permit a comprehensive evaluation of socioeconomic and biophysical factors that contribute to degradation of tropical agroecosystems. The project will add to limited understanding of the microbial ecology of tropical soils, and to knowledge about when soils reach thresholds beyond which replenishment is difficult. The project also will contribute to knowledge of the processes through which people become trapped in poverty. The integration of the contributions to individual research areas will be made more useful through integration into a dynamic model that will permit exploration of the responses of these vulnerable systems to economic and biophysical shocks. This project is supported by an award resulting from the FY 2002 special competition in Biocomplexity in the Environment focusing on the Dynamics of Coupled Natural and Human Systems.