The association of the two terms “ecology” and “chemistry” has recently become evident for researchers, biologists and chemists alike, working at the interface of biology and chemistry. Chemical ecology is now an entire area of research. It is a recent discipline, born during the 1970s/80s, and its development was associated with major progress in analytical chemistry during the same period. This discipline has greatly deepened our understanding of semiochemicals emitted by microorganisms, plants and animals.

To survive and adapt, all living things, from the simplest to the most complex, must intercept the information emitted in their perimeter of perception. The majority of living species communicate among themselves by molecules and chemical signals that we may term “chemical mediators”. In effect, any ecosystem is a dynamic assembly promoted by interactions that are, therefore, essentially founded on trophic exchanges such as molecular exchanges, involving complex substances that often transmit simple messages.

The chemical language, using semiochemicals much like words, is, in nature, a kind of universal language and appears to be indispensable for maintaining terrestrial and aquatic ecosystems. Chemical communication is by far the most frequently used mode of communication in the living world.

In an attempt to understand this language of nature, ecologists and chemists are confronted with the complexity and the creativity of organisms. Studying an ecosystem – its structure and functioning, the interactions of organisms within it, among themselves and with their physico-chemical environment – requires a multi- and pluridisciplinary approach, an approach indispensable in chemical ecology, which is a natural interface between the two sciences.

An increasingly large amount of data on organisms and the chemicals that mediate interactions between them, in both terrestrial and aquatic environments, continually reinforce our understanding of the biodiversity and chemobiodiversity of living things. The most innovative aspects are linked to the evolution of the species and the mediation of complex interactions in their multitrophic environment, considering each species as an integral part of a community, not as an entity unto itself (Chapter 1). Characterized semiochemicals, either attractants or repellents, selected over thousands of years of evolution and co-evolution for their efficiency, generally have very specific effects on the target organism. Interactions between organisms involve multiple scales; research on chemical ecology therefore relies on a wide range of experimental approaches. Furthermore, given the current loss of biodiversity and ongoing climate change, it is important to understand the functioning of ecosystems and the interaction between their microbial, plant and animal components before considering the effects of human disturbances on these ecosystems (Chapter 2). Work on sociality has allowed the elucidation of a complex evolutionary history of chemical communication in animal behavior, particularly in social species that we think of as microsmatic (i.e. having limited olfactory sense) such as human and non-human primates. In these species, the chemical composition of body odor can reflect individual characteristics. In addition, the use of natural substances by animals for self-medication, which has been shown in arthropods and vertebrates, including non-human primates as well as humans, emerges as an important evolutionary theme. Semiochemicals can, therefore, be considered as a central element of the organization of most animal societies (Chapter 3). Likewise, recent advances in the chemical ecology of the microscopic living world, a theme that was long largely neglected, have in effect modified an overly simplified image of interactions. Microorganisms – prokaryotes (bacteria, cyanobacteria, archaea) or eukaryotes (fungi, protists) – live in communities where intense competition occurs. In response to particular environmental constraints, these microorganisms produce an entire arsenal of molecules. Understanding the mechanisms by which these molecules are produced, and their effects on other organisms, is indispensable to the understanding of the interactions in microbial communities. The study of how microorganisms adapt in sometimes extremely hostile environments often has applications in the field of biotechnology (Chapter 4). The interactions between components of ecosystems can be disturbed by human activities. The active biological and chemical interactions of components in and with the elements of soil, air or water are called ecogeochemical. Ecogeochemistry thus proposes to analyze, by integrative approaches, the complexity of ecological systems and the mechanisms by which the biotic and abiotic components of the ecosystem interact. It complements the classic “biogeochemical” approach of functional ecology by addressing in a single conceptual context the organisms and components of their abiotic environment, particularly the chemical compounds in interaction with these organisms (Chapter 5). For several years, chemical ecology has benefited from the progress achieved in genomics, transcriptomics, proteomics and metabolomics; chemical ecology has thus entered the era of “omics”. “Omics” regularly lead to new tools that are very useful for shedding new light on evolutionary mechanisms. “Omic” approaches in chemical ecology vary greatly and are based on a range of biological models, from the simplest to the most complex (Chapter 6). Metabolomics is the most recent of the “omic” sciences. Metabolomics can be applied without an a priori approach, aiming to analyze the largest possible portion of the metabolome. It can also be applied in a priori approaches targeting a family of metabolites that belong to a particular path of biosynthesis. Metabolomics provides essential information to clarify the key roles played by semiochemicals in the interactions between organisms and their environment, and the mechanisms regulating these interactions. The increasingly powerful analytical, mathematical and statistical tools made available to biologists and chemists thus enable the consideration of increasingly detailed characterization of metabolomes (Chapter 7). The characterization of mediators by increasingly perfected chemical tools, and the new techniques of genome sequencing, have together allowed all these innovative approaches to contribute to better understanding of the living world and its language. Improvements in instrumentation, with gains in sensitivity and resolution, have made it possible to obtain increasingly precise and detailed analyses of primary or secondary metabolites. These approaches generate masses of data, making indispensable automatic comparison with online databases (Chapter 8). The characterization of a chemical mediator of ecological interactions can lead to multiple applications in the fields of applied research such as medicinal chemistry, pharmacology and phytopharmacy. Characterization of the biological target of a semiochemical can lead to the discovery of new biological receptors. In certain cases, nature can adapt to the presence of high levels of pollutants. In ecosystems seriously affected by pollutants, a combined study of the chemistry and ecology of the plants that are able to develop despite pollution can lead to the development of procedures to decontaminate soils, purify water or air (phytoremediation) and restore ecosystem functioning (green engineering) (Chapter 9). At the end of the book (Conclusion), we address the questions that remain unanswered in this constantly changing discipline.

The scientists, including biologists, ecologists, biochemists, chemists and biostatisticians, who have contributed to this book are interested in both continental and marine environments, both temperate and tropical ecosystems, and in living things ranging from microorganisms to mammals, all of which are covered in their analyses of chemical ecology and of its perspectives. The work presented herein illustrates the most advanced and varied aspects of this rapidly expanding discipline. Compared with other books available on related themes, which for the most part deal with relatively simple systems, covering pairwise or tritrophic interactions and comprising a small number of model organisms and semiochemicals, our book offers a holistic vision of chemical ecology.

As a final remark, we wish to pay homage to Murray S. Blum, who just recently passed away and who was one of the first to demonstrate the importance of chemical mediation in the living world. Among other things, Murray brought the notion of parsimony into chemical ecology, along with his smile, which he distributed without parsimony.

Introduction written by Anne-Geneviève BAGNÈRES and Martine HOSSAERT-MCKEY.