stable isotope ratios act as naturally-occurring tracers for organic matter, making possible, under certain conditions, the quantification of coastal-offshore exchanges. In general, organic matter has isotope ratios characteristic of its origin (e. g. plants with different modes of photosynthesis and different growth conditions, anthropogenic compounds). These ratios are maintained as the organic matter moves through the biosphere and geosphere. A mixture of organic matter from two sources has isotope ratios intermediate between those of the two sources, in proportion to the fraction of material from each source. Isotope ratios are one of the few methods which can trace organic matter as it moves through natural ecosystems. Ratios can be measured on both the total organic matter and on particular chemical fractions or compounds. When used on organisms, isotope ratios provide information of organic matter actually assimilated into body tissues, not just material ingested. As with all tools, this method has certain limitations which must be borne in mind when interpreting its results. Firstly, specific environmental conditions must be met. This generally means an ecosystem with a limited and known number of sources of organic matter having different isotope ratios. Two sources with different isotope ratios are ideal; additional sources with other isotope ratios complicate interpretation. Secondly, the difference in isotope ratios of the two sources should be large compared with analytical variability. Thirdly, the ratios within each source should vary as little as possible.

What potential problems does biodiversity loss create for humankind? What basis is there for biologists' concern about what has been described as the sixth mass extinction on our planet? The Biodiversity Programme of the Royal Swedish Academy of Sciences' Beijer Institute brought together eminent economists and ecologists to consider these and other questions about the nature and significance of the problem of biodiversity loss. This volume reports key findings from that programme. In encouraging collaborative interdisciplinary work between the closely related disciplines of economics and ecology, programme participants hoped to shed new light on the concept of diversity, the implications of biological diversity for the functioning of ecosystems, the driving forces behind biodiversity loss, and the options for promoting biodiversity conservation. The results of the programme are surprising. They indicate that the main costs of biodiversity loss may not be the loss of genetic material, but the loss of ecosystem resilience and the insurance it provides against the uncertain environmental effects of economic and population growth. Because this is as much a local as a global problem, biodiversity conservation offers both local and global benefits. Since the causes of biodiversity loss lie in the incentives to local users, that is where reform must begin if the problem is to be tackled successfully.

This volume reports key findings of the Biodiversity Program of the Royal Swedish Academy of Sciences' Beijer Institute. The program brought together a number of eminent ecologists and economists to consider the nature and significance of the biodiversity problem. In encouraging collaborative work between these closely related disciplines it sought to shed new light on the concept of diversity; the implications of biological diversity for the functioning of ecosystems; the driving forces behind biodiversity loss; and the options for promoting biodiversity conservation. The results of the program are surprising. It is shown that the core of the biodiversity problem is a loss of ecosystem resilience and the insurance it provides against the uncertain environmental effects of economic and population growth. This is as much a local as a global problem, implying that biodiversity conservation offers benefits that are as much local as global. The solutions as well as the causes of biodiversity loss lie in incentives to local users.