Introduction
The Ecological and Societal Consequences of Biodiversity Loss

Michel LOREAU1, Andy HECTOR2, and Forest ISBELL3

1 Theoretical and Experimental Ecology Station, CNRS, Moulis, France

2 University of Oxford, UK

3 University of Minnesota, St. Paul, USA

One of the distinctive and fascinating features of ecological systems is their extraordinary complexity. An ecosystem is often composed of thousands of different species that interact in myriad ways at the scale of a single hectare. Each species is composed of many individuals that vary due to differences in their genetics and their particular experience of their local environment. These complex local systems are strongly connected to each other, and aggregate into larger and larger entities, from the landscape scale to that of the entire biosphere, where it becomes evident that they exert a major influence on the physical and chemical properties of our planet. How can such enormously complex systems be studied?

During the second half of the 20th century, two increasingly divergent approaches to ecological systems developed within ecology, which have gradually led to two largely distinct disciplines, community ecology and ecosystem ecology. A community is defined broadly as 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 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. But 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?

In a sense, community ecology provides a microscopic perspective on ecosystems because it analyzes their parts, while ecosystem ecology provides a macroscopic perspective on the same systems because it studies them as a whole. The distinction between micro- and macroscopic, however, does not necessarily apply to the spatial scales considered by the two disciplines. Although much of community ecology does consider species interactions at small scales, a growing fringe, known as macroecology, considers patterns of species diversity and species distributions at vast spatial scales. The focus on species - species distributions, species diversity, species interactions - is more central to the community approach than the spatial scale considered. Similarly, ecosystem ecology studies the fluxes of energy and materials at various spatial scales. What distinguishes the ecosystem approach is its focus on the system as a whole, often without considering the species that compose it.

At a time when humankind is rising to the status of a major global biogeochemical force and raising the prospect of a global ecological crisis, it is important to step back and ask whether individually studying communities and ecosystems is the best path to follow. Human environmental impacts include the destruction and fragmentation of natural habitats, pollution, climate change, overexploitation of biological resources, homogenization of biota, and biodiversity loss. These impacts affect species and ecosystems indistinctly. Moreover, they interact with each other, which may lead to synergistic effects. For instance, climate change is likely to cause massive additional biodiversity loss. Biodiversity loss in turn is likely to decrease the ability of ecosystems to resist the effects of climate change, with possible feedbacks on the climate system itself. Species, communities, and ecosystems have always been inextricably linked, but the major disruptions generated by humans in the current period make this reality plainly obvious. A synthetic approach to ecology, which integrates populations, communities, and ecosystems, is required to develop appropriate responses to the global ecological crisis we are entering.

Community ecology is a dynamic field of research in which knowledge has accumulated rapidly during the last 60 years or so based largely on a modern hypothetico-deductive approach. However, theories and hypothesis building have often outpaced empirical studies and hypothesis testing in community ecology, which hinders steady scientific progress. As a result, this subdiscipline has few "laws" or robust generalizations, except for some large-scale empirical patterns such as species-area relationships (Rosenzweig 1999). In contrast, ecosystem ecology is a subdiscipline that has traditionally had a strong empirical basis. Its theories are largely based on inductive generalizations from field measurements, with comparatively few theory-driven hypotheses and experimental tests. There is no doubt that the ecosystem approach has been instrumental in developing our understanding of the global biogeochemistry of the Earth system and of current global environmental changes. Yet, despite these successes, a number of authors have questioned its relatively static view of ecological systems and even its scientific relevance, calling for a fundamental rejuvenation of the discipline (O'Neill 2001). Strengthening theory, experimental tests and their interactions, and paying due attention to ecological dynamics and complexity are key ingredients of such a rejuvenation.

On balance, community ecology and ecosystem ecology provide two perspectives on complex ecological systems that have largely complementary strengths and weaknesses. Both disciplines have been called into question, and each would benefit from the perspective developed by the other. Developing theories about interactions between species and between these and their environment with the ultimate goal of predicting ecosystem functioning and ecosystem services would help to focus community ecology on issues that are both scientifically important and socially relevant. Incorporating the diversity, complexity, and dynamical nature of communities in its view of ecosystem functioning would help ecosystem ecology to be livelier and to provide more reliable, if probably more uncertain, predictions. It is becoming increasingly clear that merging the two perspectives is necessary both to ensure continued scientific progress and to provide society with the scientific means to face growing environmental challenges (Loreau 2010a, 2010b). A more integrative ecology also needs to include humans, not just as an external force that disrupts ecosystems, but as an integral part of the biosphere that interacts with its other components.

The new biodiversity and ecosystem functioning (BEF) research field that emerged in the 1990s and expanded over the last decades has greatly contributed to moving ecology forward in that direction. The idea that plant diversity enhances plant biomass production is arguably foundational in ecology, dating back to Darwin's Principle of Divergence (McNaughton 1993; Hector and Hooper 2002), but this idea did not catch on for more than a century because of the success of Liebig's law of the minimum and the lack of rigorous theory and experimental designs in agricultural sciences. Agriculture gradually shifted to a new model based on the industrial production of artificial fertilizers, the mechanization of agriculture, and the use of monocultures of artificially selected crop types. It is only when the detrimental ecological consequences of the modern industrial model, and in particular the threat of biodiversity loss, started to be widely recognized at the end of the last century that interest in the effects of biodiversity loss on ecosystem functioning emerged. This interest then spread rapidly, penetrated experimental and theoretical ecology, and led to the emergence of an entire new research field at the interface between community and ecosystem ecology (Loreau et al. 2001, 2002; Hooper et al. 2005; Naeem et al. 2009; Loreau 2010a; Cardinale et al. 2012; Tilman et al. 2014).

Interest in this issue grew largely out of practical concerns about the potential ecological consequences of current biodiversity loss caused by the increased impact of human activities on natural and managed ecosystems. There is growing recognition that the world's ecosystems provide society with a wide range of "ecosystem services" (Millennium Ecosystem Assessment 2005), or "nature's contributions to people" (Diaz et al. 2018), that are crucial to human well-being and sustainable development. These services or contributions are derived from the normal functioning of ecosystems, raising the important question whether impoverished ecosystems may in some way function less efficiently than the more species-rich systems from which they are derived, and hence gradually lose their ability to deliver ecosystem services to human societies. However, beyond this eminently practical motivation, the new BEF research field has had a much broader and deeper transformative role in ecology.

One of its main benefits has been to foster integration of community ecology and ecosystem ecology. Ecology has traditionally regarded,...