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III Communities and Ecosystems Michel Loreau Ecology is the science of the interactions between living organisms and their environment. What makes ecology so fascinating, and at the same time so disturbing for the layperson, is the extraordinary diversity and complexity of these interactions, which create a wide range of nested complex systems from the scale of a droplet of water to that of the entire planet. Anything that happens here and now is almost certain to have an effect elsewhere and later. And this also concerns us as humans. Just as any other species, we transform the world around us by the mere act of living—consuming resources, releasing waste products, changing land and sea for our purposes. But are the effects elsewhere and later predictable? Can we make sense of this complexity , or is it better to just ignore it and to attend to our affairs without worrying about their consequences? Questions of this type are crucial, both for ecology as a science to understand the world in which we live and for society at large to cope with the unforeseen consequences of its past and current actions. Ecology has approached complex ecological systems from two different angles, which have gradually led to two distinct subdisciplines, community ecology and ecosystem ecology. A community is a set of species that live together in some place. The focus in community ecology has traditionally been on species diversity: What exogenous and endogenous forces lead to more or less diverse communities? How do species interactions constrain the number of species that can coexist? What patterns emerge from these species interactions? An ecosystem is the entire system of biotic and abiotic components that interact in some place. The ecosystem concept is broader than the community concept because it includes a wide range of biological, physical, and chemical processes that connect organisms and their environment. However, the focus in ecosystem ecology has traditionally been on the overall functioning of ecosystems as distinct entities: How is energy captured, transferred, and ultimately dissipated in different ecosystems? How are limiting nutrients recycled, thereby ensuring the renewal of the material elements necessary for growth? What factors and processes control energy and material flows, from local to global scales? Community ecology and ecosystem ecology, then, provide two complementary perspectives on complex ecological systems. Community ecology has been a very lively subdiscipline during the last decades because of a growing interest in biodiversity and the stability of complex systems. The first chapters in this section summarize some of the main findings and debates in this area. Robert Colwell (chapter III.1) begins by defining the concept of biodiversity and examining how it can be measured. Biodiversity is a relatively new term that has gained wide popularity in the general public because of current concerns about its loss as a result of growing human environmental impact; but the concept of biological diversity from which it arose is much older and has been studied abundantly in ecology since its inception . Measuring species diversity from local to regional scales is a much more challenging task than might appear at first sight. Robert Colwell provides a very accessible digest of decades of research on this issue. Jérôme Chave (chapter III.2), Robert Holt (chapter III.3), and Ragan Callaway (chapter III.4) then move from patterns to processes and explore the respective roles of the three types of elementary species interactions that hold communities together. Competition has long been regarded as the primary factor that governs species coexistence and, hence, species diversity and other community-wide patterns. Recent research has both confirmed its prevalence and relativized its role in the organization of natural communities (chapter III.2). Predation is another interaction that affects species persistence and diversity. But it can either enhance or hamper prey species coexistence depending on the details of species interactions, which argues for a thorough investigation of its mechanics if we are to predict its impacts on natural communities (chapter III.3). Positive species interactions, such as facilitation and mutualism, have received comparatively much less attention until recently. Yet there is ample evidence that facilitation is widespread and plays an important role in the organization of plant communities (chapter III.4). Competition, predation, and facilitation are simple pairwise species interactions that can be viewed as the building blocks of more complex communities. But whenever three or more species are engaged in such interactions, we see the emergence of indirect effects in which one species affects another through...


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