Positive plant interactions and community dynamics

Abstract

The idea that positive interactions among plants may be as important as competition has emerged as a captivating theme in community ecology over the past decade or so, and growing attention is being paid to the manner by which both positive and negative interactions respond to environmental conditions and feedback on the structure and functioning of ecosystems. Such progress demands review and synthesis, and Positive plant interactions and community dynamics makes important strides in meeting this demand. The editor Francisco Pugnaire draws on the experience of leading researchers and presents a concise but comprehensive exploration of roles for positive plant interactions in the community context. The slim volume covers a surprisingly broad range of topics pertinent to the incorporation of positive interactions into community theory, reviewing key contributions to the primary literature and providing original synthesis wherever helpful, including new data and meta-analyses of published studies. The international panel of authors, representing eight countries and four continents, provides a global perspective on the subject, and the balance of relatively light technical detail with strong consideration of general and conceptual relevance makes this book of interest to a fairly wide audience of ecologists. Callaway leads off the first of the book's seven chapters with a convincing survey of recent and classic literature to answer the question of whether positive plant–plant interactions ‘matter’ to the larger ecological world. The answer is clearly affirmative. This section introduces readers to the foundations of plant interactions but quickly proceeds to explore some of the most interesting questions and implications arising from our growing understanding of positive interactions, such as impacts of facilitation on species' evolutionary trajectories. Employing an engaging narrative and brief reviews of the best studies to date, the evidence is allowed to make the case for itself in key research areas that are revisited more thoroughly in subsequent chapters, including interaction-based explanations for species' invasions and relationships between biodiversity and ecosystem functioning. Chapter 2 dives directly into the challenge of measuring species' interactions, and while it does get somewhat technical it may actually be the most helpful chapter in the book for practicing ecologists. Authors Kikvidze and Armas succinctly review the gamut of plant interaction indices that have developed, illustrating how interactions must be investigated differently depending on the number of focal species and whether intensity or importance of interactions are of greater interest. Chapter 3 initially returns to concepts, exploring how facilitation by stress-ameliorating nurse species may determine community diversity at multiple spatial and organizational scales. However, Cavieres and Badano go on to demonstrates these principles in action with new analyses of their own data from alpine cushion plant communities along the Andes of southern South America. Interestingly, the variation in interaction strength and sign along the large latitudinal gradient studied is consistent with the stress-gradient hypothesis (SGH) that positive interactions are more frequent in more stressful environments. This controversial but increasingly supported hypothesis is revisited in Chapter 4, where Michalet and Touzard elegantly integrate the SGH with classic ‘humpback’ diversity models to show how shifting balances of interactions along environmental severity gradients can explain relationships between biodiversity and community productivity. Chapter 5 steps beyond the plants-only focus to consider how fungal symbiotes of plants (specifically, arbuscular mycorrhizal fungi, or AMF) influence plant–plant interactions. Moora and Zobel present a new meta-analysis of published studies investigating how suppression of plants by their competitors changes in response to AMF, revealing that AMF tend to intensify competition within species but reduce it among species. Chapter 6 explorers how the nature of plant–plant interactions may mediate the impacts of climate change on plant communities, with Brooker analysing plant communities as functions of species' pools and environmental filters (including neighbour interactions) that may be altered by climate change in specific ways. The intriguing conclusion is reached that certain locations may become less stressful and thus more limited by competition, while other locations will become more stressful and thus more sensitive to facilitation. In the final chapter, Lortie sheds new light on both values and limitations of the stress-gradient hypothesis by deconstructing what exactly is meant by ‘stress’ and ‘gradient’ in ecology, and reformulating a clear, testable SGH that accounts for potential sources of confusion. Meta-analysis of the ten highest-quality SGH studies is used to test this, yielding a significant signal for stress-induced shifts in the directionality of plant–plant interactions. Clear lessons from the analysis are distilled into a useful ‘checklist’ of best practices for testing and reporting SGH experiments. The presentation of materials is clean and efficient, with a good balance of relevant tables, graphs, conceptual illustrations, and colour photographs that successfully enhance clarity and understanding. The references are extensive but not excessive, and adequately capture foundational works as well as the large growth in the field over the past decade. The most comparable work in recent years to this one is Callaway's Positive interactions and interdependence in plant communities (Springer, 2007), but the two works are considerably different in their scope and the level of detail presented. Whereas Callaway's tome exhaustively reviews a broader literature and digs further into the physiological mechanisms of positive interactions, Pugnaire's volume is a relatively light read that touches on many interesting issues and questions but leaves much of the technical detail to the cited primary literature. Indeed, such brevity may leave some readers dissatisfied, but the cost is probably negligible next to the benefits of keeping the work accessible. One minor weakness of the book is that for a work concerned with ecological dynamics and responses to global change, consideration of plant interactions related to community resistance, resilience and temporal constancy is conspicuously absent. It remains unexplored how interactions directly related to stress amelioration may influence stability and biodiversity–stability relationships, although some intriguing and important relationships certainly seem possible. Overall, this work is of high value because it both reviews and integrates a decade's worth of growth in one of ecology's most exciting fields, incorporating new analyses and results that nicely demonstrate the principles discussed and point to directions for future research. The book is refreshingly focussed on how positive and negative plant interactions are important beyond the population level, influencing the structure of communities and the functioning of ecosystems, with implications for understanding processes as diverse as succession, invasion, restoration, global environmental change and evolution. While too brief to extensively mine any of these topics, the concise and pointed explorations of each make this work useful to students of plant interactions but also potentially influential to a broader audience interested in the fundamentals of community ecology.