REVIEW Chinese Science Bulletin Vol. 49 No. 22 November 2004 2339 Chinese Science Bulletin 2004 Vol. 49 No. 22 2339—2347 Applications of stable isotopes to study plant-animal relationships in terrestrial ecosystems WANG Jianzhu 1 , LIN Guanghui 1,2 , HUANG Jianhui 1 & HAN Xingguo 1 1. Key Laboratory of Quantitative Vegetation Sciences and Research Center for Plant Ecology and Biodiversity Conservation, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China; 2. Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA 94305, USA Correspondence should be addressed to Lin Guanghui (e-mail: ghlin@ibcas.ac.cn , glin@globalecology.stanford.edu ) Abstract As natural tracers, stable isotopes have been extensively used in plant physiological, ecological and envi- ronmental research. Recently, animal physiological ecologists have also applied stable isotope techniques to study plant- animal relationships. The isotopic compositions of animal body generally reflect and integrate their diets over a time period ranging from hours to years to the lifetime of an indi- vidual. When animal living habitat changes or animals move to a new environment, the animal isotopic compositions will shift accordingly. Thus, stable isotope signatures of an ani- mal can truly reflect its food sources, habitat, distribution and movement patterns during a given time period. More- over, by analyzing animal-tissue isotopic compositions at different temporal scales, we can improve our understanding of animal adaptation to environmental changes. Stable iso- tope technique also provides an ideal tool to study animal foodweb relationship and community structure because of isotopic fractionation during the processes of nutrient as- similation by animals. Stable isotope technique can continu- ously measure animal trophic position in a foodweb, which can eventually reveal the predator-prey relationship and its role in determining matter balance and energy flow in the entire ecosystem. Stable isotope technique has been one of the most important and efficient tools in studying plant-animal relationship. In this paper, we first review recent advances in the application of stable isotope techniques to plant-animal relationship research then evaluate their advantages and disadvantages, and finally discuss some future directions associated with stable isotope applications to plant-animal relationship research. Keywords: stable isotopes, plant-animal relationship, food sources, trophic level, animal migration. DOI: 10.1360/982004-112 In the past two decades, the application of stable isotope technique in the fields of ecology and environ- mental research has grown steadily. In plant physiological ecology [1] , stable isotopes have been used to evaluate the photosynthetic pathways [2] , nutrient absorption [3] , water sources [4] , water balance, and water use efficiencies of plants [5] . They also permitted ecologists to study ecosys- tem gas exchanges [6] , ecosystem functions, their responses to global change [7] , etc. Stable isotope technique has been one of the most powerful tools in the research of ecology and environmental sciences [1] . In animal ecology, stable isotopes ( 2 H, 13 C, 15 N and 18 O) have also been greatly used to investigate animal food sources [8,9] , food chain, foodweb [10] , community structure [11] and their migratory behavior [12] . In this regard, stable isotope technique has many advantages in studying plant-animal relationships. It has resolved many difficult problems intractable using traditional methods, such as food sources of small invertebrate animals, complicated food web structure, and stopover locations of birds during their migration. Stable isotope technique has been one of the most important tools for studying plant-animal rela- tionship. However, the applications of stable isotope tech- niques to ecological and environmental sciences in China are just beginning [13] and their applications to study the plant-animal relationships in terrestrial ecosystems are still rare [14] . In this review, we highlight recent advances in the applications of stable isotopes to plant-animal rela- tionship research, including animal food source, tropic relationship and movement patterns. By evaluating the advantages and disadvantages in using this technique, we hope we can offer some inspirations and provide useful references to Chinese animal ecologists, and promote ap- plications of this technique to this field research in China. 1 Stable isotopes and measurements Stable isotopes are naturally occurring stable forms of elements with different nuclear masses. Elements that include the same number of atoms and protons, but dif- ferent number of neutrons are called isotopes to each other. It is their mass differences, which confer disparate physical properties, that cause different isotopes of an element to behave differently in biogeochemical processes (such as their conductance in gas, molecular bond energy, rates of biochemical process and rates of decomposition). Thus differential chemical behavior of those isotopes in- volved in a reaction, which results in their differential representation in the reactants and products of a reaction, makes stable isotope techniques new useful methods in the field of ecology and environmental science [15] . Isotope effect refers to the phenomenon that differ- ence in the physiochemical properties of isotopes result in different isotope compositions in the reactants and the products. The magnitude of isotope effect is often ex- pressed by the degree of isotope fractionation or isotope discrimination. However, absolute abundance of the heavy isotopes is usually very low in nature, so it is very difficult to express them using their absolute abundance. We gen- erally express the stable isotope composition of a particu- lar material or substance as a ratio of heavy to light iso-