Introduction

The central role of plant secondary metabolites (PSMs) in mediating the ecological interactions between plants and other organisms is both well known and well studied, particularly in the case of the defensive responses of plants against attack by herbivores or pathogens (Dangl & Jones, 2001; Kessler & Baldwin, 2002). Furthermore, because plants face many simultaneous threats (Maleck & Dietrich, 1999; Paul et al., 2000), the chemical changes within plants in response to one attacking organism can influence the behaviour and performance of many others (Thaler et al., 2002; Biere et al., 2004). Thus, chemically mediated plant-based interactions have significant consequences for individual species, ecological communities and ecosystem function, so gaining an in-depth understanding of the chemical basis of these interactions is vital for ecologists (van der Putten, 2003; Dicke, 2006; Schuman & Baldwin, Chapter 15; Dicke et al., Chapter 16).

Recent advances in ecological genomics have demonstrated the complexity of plant responses to biotic challenges at the molecular level: hundreds of genes are now known to be up- or down-regulated in response to herbivore or pathogen attack (Zheng & Dicke, 2008). This has been of great benefit in understanding the molecular basis of plant defence, but microarray data alone cannot unravel the complexity and variability in plant responses, many of which are specific to particular types of natural enemy, and/or vary according to environmental conditions (Kant & Baldwin, 2007). Genomic analysis needs to be supported by manipulative experiments which assess all the metabolic responses of plants to environmental challenges as well as the molecular ones – so-called metabolomic approaches. Metabolomics is the systematic analysis of the set of metabolites synthesised by an organism at a particular ‘snapshot’ in time and can be described as providing the link between genotypes and phenotypes (Fiehn, 2002; Macel et al., 2010). Assuming that this set of metabolites reflects the interactions the plant is having with its abiotic and biotic environment, chemical ecologists can use this technique to study the mechanisms underpinning these interactions (Bundy et al., 2009), including those between plants and other organisms such as herbivores and pathogens (Allwood et al., 2008).

Preface

Although chemical interactions between plants and other organisms had been documented many years previously (e.g. Stahl, 1888; Verschaffelt, 1910), it was the seminal work of Fraenkel published in 1959 that placed the ecological function of plant secondary metabolites (PSMs) onto the agenda of modern ecology. He recognised that PSMs are not merely a repository for plants’ waste products, but rather they had a primary function: they could act as defences against enemies such as phytophagous insects. Crucially, he also realised that these enemies exert a selection pressure on the plants to defend themselves. Since then, numerous ecological roles of PSMs have been elucidated, notably as defences against a broad range of herbivores and pathogens, as mediators of interactions with competitors and mutualists, and as defence against abiotic stress. Recently, emerging developments have taken us well beyond consideration of PSMs in the context of simple interactions between pairs of species. Our view of plant secondary metabolites has shifted significantly in the past 50 years and we now understand the subtlety and scale of their effects, which cross trophic levels, spread throughout ecosystems, and even affect global processes. At the same time, methodological developments, particularly in the ‘-omics’ technologies, have led to a greater understanding of the synthesis and regulation of PSMs. These methodological developments now also facilitate unique tools for the targeted manipulation of both the synthesis of PSMs and their ecological function independently of other phenotypic effects. Consequently, we are now in a position to assess the extent to which PSMs and their effects traverse natural systems from genes upwards, and, in the spirit of Fraenkel, the reciprocal effects of the biotic and abiotic environment on those genes. This is, therefore, the ideal time to take stock of our current understanding of the function, ecology and evolution of PSMs, in order to focus our future efforts to use this knowledge to best effect in science and its application.

It is well known that tropical forests and savannas house a significant proportion of global biodiversity. However, an appreciation of the diversity of interactions among organisms in tropical ecosystems is only just emerging. Interactions among species are important because they affect the growth, survival and reproduction of individuals, but also because they have a key role in structuring communities and in the functioning of ecosystems. A sound knowledge of these interactions is therefore fundamental to understanding how tropical ecosystems work, as well as informing important practical concerns such as conservation, management and carbon sequestration. The aim of this book, and the meeting from which it derives, is to synthesize the current state of knowledge of biotic interactions in terrestrial communities in the tropics. Each of the 22 chapters of this volume provides a review or a case study of interactions among organisms from tropical ecosystems, with a perspective drawn from the organisms and sites with which the individual authors work. Our aim was to draw on research conducted in both Old and New World tropics and to include biotic interactions among taxa at all trophic levels. Most authors have taken plants (typically trees) as their starting point, but taken together the chapters consider interactions of plants with other plants, with micro-organisms and with animals, and the inter-relationships of human-induced disturbance with interactions among species.

Introduction

Understanding the processes underpinning high biodiversity in tropical rain forests is a major goal for ecologists. There has been a long-standing interest in the role of herbivory in the tropics, in part because it has been proposed as a mechanism contributing to the maintenance of this diversity for tree species (Janzen 1970; Connell 1971; Marquis, Chapter 13, this volume). The Janzen–Connell hypothesis predicts that herbivore damage may act in a density-, distance- or frequency-dependent manner and, if the amounts of damage are sufficient to cause changes in growth rates or mortality of seedlings (or seeds), then this could lead to the distancing of conspecifics and hence promote species diversity. In areas of high conspecific density, increased herbivory could kill or reduce the growth of more common species, allowing rarer species to persist. This hypothesis relies on the ability of herbivory to significantly affect seedling growth or mortality, and although many studies have found variation in damage levels between species (Coley 1983; Coley & Barone 1996; Blundell & Peart 1998; Howlett & Davidson 2001), few studies have found conclusive effects on seedling growth, much less on mortality (Aide & Zimmerman 1990; Webb & Peart 1999; Howlett & Davidson 2001; Horvitz & Schemske 2002; Hyatt et al. 2003; Pearson et al. 2003b). Of the studies that have demonstrated differences in growth rates or mortality between species caused by herbivory (Marquis 1984; Coley 1986; Bazzaz et al. 1987; Sork 1987; Osunkoya et al. 1993; Terborgh & Wright 1994; Sagers & Coley 1995; Whitmore & Brown 1996; Ickes et al. 2001; Wallin & Raffa 2001), the majority are attributable to mammalian herbivores and seed predators (Sork 1987; Osunkoya et al. 1993; Terborgh & Wright 1994; Ickes et al. 2001).