Using Stable Isotopes to Understand Trophic Relationships: Experimental Tests of Mass Balance Models

质子数相同但中子数不同的元素称为同位素。 这些同位素大多数是稳定的，它们不会发生放射性衰变，并且可以通过它们的质量来区分。生物体用来制造和转化有机分子的途径可以是同位素识别的。因此，包括生物组织在内的许多物质的同位素组成通常都包含一个标记，表明其产生的过程，生态学家和生理学家利用这些同位素标记来检测各种时空尺度上的过程印记。几十年来，植物生理学家、大气科学家和地球化学家一直依赖于对天然稳定同位素特征的测量，并从特征中得出有关过程的推论。动物生态学家是这个领域的后来者，但最近他们很活跃。在稳定同位素方法的帮助下，动物生态学中的许多问题已经得到解决。稳定同位素已被用于重建动物的饮食，以确定资源如何分配给繁殖，跟踪动物的运动，评估生态系统之间的物质流动，分配营养水平，并确定食物网的结构。稳定同位素方法已被科学家以几乎所有科学方法中最快的速度采用。稳定同位素分析在综合生态学中的作用类似于PCR（聚合酶链式反应）在分子生物学中的作用。在地球化学、植物生理学和植物生理生态学中，理论、实验室研究和实地研究的有力相互作用促进了稳定同位素应用的进展。由于动物生态学家对稳定同位素的使用采取了描述性的方法，因此创造同位素变化模式的机制仍未被探索。该项目的总体目标是描述和测试基于生理学原理的动物同位素生态学的理论框架。将测试的模型旨在解释1）控制同位素掺入时间过程的因素和2）动物同位素生态学中最广泛观察到的模式之一：在营养水平上观察到的重氮同位素（15 N）富集。简而言之，许多动物相对于其饮食富含15 N。这种15 N的富集非常有用，因为它为生态学家提供了一种估计动物营养位置的工具。例如，通过测量动物组织的氮同位素组成，可以确定动物是草食动物还是食肉动物。这些模型在经过充分研究的生理学观察结果与生态学家使用稳定同位素观察到的模式之间建立了联系。这些模型的预测将与三个物种的实验：鱼（尼罗罗非鱼，Orechromis niloticus），哺乳动物（实验室小鼠，小家鼠），和一只鸟（麻雀，Passer arteriticus）。也许这个项目的主要特点是，实验的目的是测试定性假设，并确定一个模型的充分性。构建该项目的模型对生态相关变量（如生长率，动物使用蛋白质的效率，动物的营养状态和食物的化学成分）与动物将13 C和15 N纳入其组织的速率之间的关系进行定量预测。他们还提供了机制假说来解释营养水平效应的大小变化，定义为动物组织和饮食中氮同位素组成的差异。该模型预测，营养效应将降低的速率和效率的氮掺入，但将增加动物在负氮平衡。本提案中描述的研究既相关又及时，因为它将为快速增长的数据提供机械基础。它还将允许建立限制生态学家可以从现场和实验室数据中做出的推论，也许更重要的是，它将带来稳定同位素模式的新推论。最终，该项目旨在为动物同位素生态学建立一个基于生理机制的预测框架。

Elements with the same number of protons but with a different number of neutrons are called isotopes. Most of these isotopes are stable, they do not undergo radioactive decay, and can be distinguished by their mass. The pathways that organisms use to manufacture and transform organic molecules can be isotopically discriminating. As a consequence, the isotopic composition of many materials, including the tissues of organisms, often contains a label of the process that created it. Ecologists and physiologists use these isotopic labels to detect the imprint of processes at a variety of temporal and spatial scales. Plant physiologists, atmospheric scientists, and geochemists have relied on the measurement of natural stable isotope signatures for decades, and have drawn inferences about processes from the signatures. Animal ecologists have been latecomers to the field, but recently they have been active. A large variety of questions in animal ecology have been solved with the aid of stable isotopic approaches. Stable isotopes have been used to reconstruct animal diets, to determine how resources are allocated to reproduction, to track animal movements, to assess the flow of materials between ecosystems, to assign trophic levels, and to determine the structure of food webs. Stable isotope methods have been adopted by scientists at one of the fastest rates of almost any scientific methodology. Stable isotope analyses play a role in integrative ecology analogous to the role that PCR (polymerase chain reaction) plays in molecular biology. In geochemistry, plant physiology, and plant physiological ecology, progress in the use of stable isotopes has been stimulated by the vigorous interaction of theory, laboratory research, and field study. Because animal ecologists have adopted a descriptive approach to the use of stable isotopes, the mechanisms that create isotope variation patterns remain unexplored. The overarching objective of this project is to describe and test a theoretical framework for the isotopic ecology of animals that is based on physiological principles. The models that will be tested aim to explain 1) the factors that govern the time course of isotopic incorporation and 2) one of the most widely observed patterns in animal isotopic ecology: the enrichment in the heavy nitrogen isotope (15N) observed across trophic levels. Briefly, many animals are enriched in 15N relative to their diet. This enrichment in 15N is very useful because it provides ecologists with a tool to estimate the trophic position of an animal. For example, by measuring the nitrogen isotope composition of an animal's tissues it can be determined whether the animal is a herbivore or a carnivore. The models establish connections between well-studied physiological observations and the patterns observed by ecologists that use stable isotopes. The predictions of these models will be examined with experiments on three species: a fish (Nile tilapia, Orechromis niloticus), a mammal (laboratory mouse, Mus musculus), and a bird (house sparrow, Passer domesticus). Perhaps the main distinguishing characteristic of this project is that experiments are designed to both test qualitative hypotheses and to determine the adequacy of a model. The models that structure this project make quantitative predictions about the relationship between variables of ecological relevance (such as growth rate, the efficiency with which animals use protein, an animal's nutritional state, and the chemical composition of food) and the rate of an animal's incorporation of 13C and 15N into its tissues. They also provide mechanistic hypotheses to explain variation in the magnitude of the trophic level effect, defined as the difference in nitrogen isotope composition of an animal's tissues and that of its diet. The models predict that the trophic effect will decrease with the rate and efficiency of nitrogen incorporation, but will increase in animals in negative nitrogen balance. The research described in this proposal is both relevant and timely because it will provide mechanistic grounding to a very rapidly growing body of data. It will also permit establishing limits to the inferences that ecologists can make from field and laboratory data, and perhaps more importantly, it will bring about novel inferences from stable isotope patterns. Ultimately, this project aims to create a predictive framework for animal isotopic ecology that is firmly grounded in physiological mechanisms.