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 a shared, intermediary species. Oswald Schmitz (chapter III.5) explores some of the myriad fascinating ways that indirect effects emerge in, and affect, communities and ecosystems. An indirect effect of special interest is that which emerges from a simple food chain in which carnivores eat herbivores, and herbivores eat plants. Predation by carnivores tends to suppress herbivore populations, which in turn tends to release plants from control by herbivores. This cascade of effects as a result of top-down control of the food chain, which is known as the trophic cascade, can strongly affect the biomass of the various trophic levels and ecosystem functioning. Elizabeth Borer and Dan Gruner (chapter III.6) assess its importance relative to bottom-up processes in the regulation of ecosystems. When a large number of species prey on each other, however, they do not always fall into well-defined trophic levels. They then constitute a food web rather than a food chain. The persistence and stability of these enormously complex food webs are one of the main enigmas of ecology. Kevin McCann (chapter III.7) reviews the history of thought on this issue and suggests that the solution to this enigma may lie in the stabilizing role of variability in space and time. Spatial dynamics has received increasing attention in ecology during the last decades. Many of the patterns and processes within local communities are strongly influenced by movements of organisms and materials at the landscape or regional scale. Mathew Leibold (chapter III.8) examines the consequences of these movements on the maintenance of biodiversity in metacommunities, i.e., in sets of communities connected by dispersal.
Compared with community ecology, ecosystem ecology generally offers a more macroscopic, integrated view of ecological systems. A second set of chapters in this section summarizes current knowledge on the flows of energy and materials in ecosystems, from local to global scales. Julien Lartigue and Just Cebrian (chapter III.9) provide a comparative analysis of patterns of carbon flows and productivity across a wide range of ecosystems, both terrestrial and aquatic. In particular, they examine the factors that determine the movements of carbon through the activity of herbivores and decomposers, two of the main agents of carbon flows in ecosystems. Carbon flows are usually closely associated with energy flows: both energy and carbon are typically captured by primary producers through photosynthesis and ultimately released through respiration. In contrast, other elements that often limit primary production, such as nitrogen and phosphorus, are tightly recycled within ecosystems. Peter Vitousek and Pamela Matson (chapter III.10) present an overview of nutrient cycling in ecosystems, the factors that control it, and its implications for global biogeochemical cycles. Although much research has focused on the global cycle of carbon, in part because of the importance of carbon dioxide in the climate system, they show that humanity has altered the cycles of other elements to a much greater extent than that of carbon. The following three chapters by Richard Houghton (chapter III.11), Darren Bade (chapter III.12), and Paul Falkowski (chapter III.13) examine the biogeochemical cycles of carbon and other elements in more detail in the three great types of ecosystems on Earth, i.e., terrestrial, freshwater, and marine ecosystems. Collectively, these chapters summarize the fundamental ecological knowledge that is necessary to understand and predict human impacts on the biosphere.
Although community ecology and ecosystem ecology provide different perspectives on ecological systems, these perspectives should ultimately be compatible with each other and inform each other. A recent trend in ecology has been the emergence of unifying approaches that seek to lay bridges among these subdisciplines through common principles or topics that lie at their interface. One of these topics, which has attracted a great deal of attention in the last decade, is the relationship between biodiversity and ecosystem functioning. Interest in this issue arose from the realization that biodiversity is currently being lost at an accelerating rate globally. But the impacts of biodiversity loss on the functioning of ecosystems and on their ability to provide ecosystem services to human societies were largely unknown until recently. Andy Hector and Andy Wilby (chapter III.14) summarize the progress achieved by more than a decade of active research in this area. Linking biodiversity and ecosystem functioning clearly requires working at the intersection between community ecology and ecosystem ecology. But there are other ways to work toward integration across subdisciplines. Ecological stoichiometry examines how the nutrient content of organisms shapes their ecology. Robert Sterner and Jim Elser (chapter III.15) show how stoichiometric constraints play a role in a wide range of ecological phenomena, from the growth rate of animals through population dynamics to the rates of recycling of elements in food webs, thus providing a unifying theme through ecology. Macroecology is another emergent research program in ecology, which Pablo Marquet reviews in chapter III.16. Macroecology studies ecological patterns and processes at large spatial and temporal scales. One of its goals is to identify statistical regularities that might reflect the operation of elementary principles underlying both community structure and ecosystem functioning.
As do all biological systems, communities and ecosystems change through time as a result of exogenous environmental changes, endogenous ecological interactions, and evolution. Dynamic and evolutionary constraints play a significant role in shaping ecological systems, whether these are viewed from a community or an ecosystem perspective. In particular, complex systems can have tipping points, where the slightest disturbance can trigger rapid change until they reach an alternative stable state. Marten Scheffer (chapter III.17) presents the theory of these regime shifts and the empirical evidence that supports them. He also discusses how insights into regime shifts can be used in ecosystem management. Erika Zavaleta and Nicole Heller (chapter III.18) examine how communities and ecosystems respond to current global environmental changes driven by human activities, in particular in the Earth’s climate, atmosphere, and biogeochemistry. A picture emerges of natural systems altered drastically by the accumulating effects of multiple global changes. They conclude that safeguarding the capacity of ecosystems to adapt to change is a minimum requirement to preserve options for the future. Evolution is increasingly recognized as an important determinant of community structure and ecosystem functioning. Nicolas Loeuille (chapter III.19) summarizes its effects on community and ecosystem properties along a gradient of increasing evolutionary complexity, from evolution of single species to coevolution of a large number of interacting species. He shows how complex adaptive systems emerge from the evolutionary dynamics of organisms that compose them.
The chapters in this section vividly illustrate the vital importance of ecology as a science that provides a rigorous body of knowledge on the complex natural systems of which we humans are part. They show that the diversity and complexity of the interactions between living organisms and their environment can be understood. Even though ecological systems are intrinsically too complex to be amenable to a complete description by a few simple laws yielding simple predictions, some principles, rules, and general trends do emerge and can usefully guide the way we manage ecosystems and cope with environmental changes. Ecology provides a unique perspective on diversity and complexity that will be critically important as humankind faces the most formidable environmental challenges of its history.