Facilitation and the Organization of Communities

by

Christopher J. Lortie

• LAST REVIEWED: 09 November 2022
• LAST MODIFIED: 23 May 2012
• DOI: 10.1093/obo/9780199830060-0048

Introduction

Species interactions are a cornerstone of ecological research wherein the effects of an individual of one species on another individual, frequently a different species, are studied. Within versus between species interactions are also commonly contrasted as a means to infer relative importance, but the majority of theory advances, at least at the community level, are associated with interactions between individuals of different species. Interactions can range from positive to negative, and effects are measured at all levels of development, or life history stages, of an organism. Positive interactions have been extensively studied in both population and community ecology. Facilitation, however, is a relatively specific term that has evolved primarily to describe positive plant–plant interactions (see Defining Facilitation). Facilitation, or positive interactions, is a relatively recent subset of these species interactions in general, including related processes, such as competition, mutualism, and parasitism. Facilitation is best viewed as the antithesis of the plant competition literature, as it shares many of the main attributes, both in terms of scope and approach, and arose as a comparator to this research. Facilitation studies mainly refer to positive plant–plant interactions, as the term was proposed in the plant literature and extensively used to describe interactions that include a positive effect of one species on another. Mutualism and parasitism research is often plant–insect based and formally identifies the reciprocal effect in the interaction, that is, (+, +) in mutualism and (+,−) in parasitism, whereas facilitation studies are generally (+,0) or (+,?), with the second effect often unreported. Interactions that include at least one negative interaction are usually described as competition in the plant literature and do not apply the term facilitation (although the frequency of both being discussed concomitantly is increasing). Hence, the term facilitation, owing to historical use, describes the subset of interactions that are (+,0) and is mostly specific to within plants, although its usage is expanding. The research on facilitation has most likely peaked, similar to plant competition studies, in that facilitation has been clearly established as an important process in the formation of plant communities. Additional studies simply demonstrating facilitation are increasing unlikely to be present in the literature. That said, the implications to theory and other, more nuanced aspects of interaction, such as context dependence, shifting balances, and importance of the environment, as they relate to facilitation, are still largely unexplored. In the early 21st century the most contentious debates, with respect to facilitation, center on either disagreement concerning what a community is and whether research should be conducted at this scale or on how to use environmental gradients (i.e., stress) most effectively. Both of these topics are described herein, with readings also included on Historical Background, Experimental and Analytical Approaches, Evolution, other taxa, and Applications.

General Overviews

Facilitation is a subset of the possible interactions associated with the organization of community, ranging from positive, to neutral, to negative. Hence, facilitation, in the broadest sense, is often included in many works describing plant communities. There are several excellent overviews, both books and peer-reviewed articles, that formally discuss facilitation and that are ideal launching points for further reading. Callaway 1995 is the classic and the first review that promoted the term facilitation. A more comprehensive treatise on the subject by Callaway followed that original paper (Callaway 2007). Although the general focus and theme are implicitly community based, this book focuses more directly on facilitation itself, with chapters on direct and indirect mechanisms of facilitation, interactions with competition, and species specificity. The final chapter is, however, on community organization. A second book on this topic, written by a subset of the leaders of the field, including Callaway, and edited by Pugnaire, was published in the following year (Pugnaire 2010). This book is explicitly focused on the community, with chapters on indices, diversity, biodiversity, mycorrhizae, climate change, and stress gradients, and less directly on the specifics of facilitation. Callaway 2007 is the most thorough reader available on plant facilitation and addresses virtually every aspect of its study, whereas the latter text is a broader and more diverse treatment of facilitation, as it relates to community ecology. Two overviews on facilitation have also been published in the Journal of Ecology and another, in Biology Letters. The first Journal of Ecology paper (Brooker, et al. 2008) articulates previous research and illuminates future directions whereas the second (Brooker and Callaway 2009) introduces a special section on a symposium on facilitation held at the University of Aberdeen, Scotland, from 20 to 22 April 2009. The final overview, in Biology Letters (Pakeman, et al. 2009), also discusses this symposium and describes the conceptual status of facilitation in the early 21st century, including future directions.

• Brooker, Rob W., and Ragan M. Callaway. 2009. Facilitation in the conceptual melting pot. Journal of Ecology 97.6: 1117–1120.

  DOI: 10.1111/j.1365-2745.2009.01580.x

  This is a short, synthetic piece that offers a clear overview of the broad significance of facilitation research and its importance to theory.

• Brooker, Rob W., Fernando T. Maestre, Ragan M. Callaway, et al. 2008. Facilitation in plant communities: The past, the present, and the future. Journal of Ecology 96.1: 18–34.

  DOI: 10.1111/j.1365-2745.2007.01295.x

  This is a highly cited paper for future directions and clearly describes the advances and key developments from the published empirical work. The first table provided is an excellent resource for quick identification of readings associated with seminal works. As an excellent, comprehensive starting point that is a single, short read, this is likely the best place to begin for an overview—particularly for future directions.

• Callaway, Ragan M. 1995. Positive interactions among plants. Botanical Review 61.4: 306–349.

  DOI: 10.1007/BF02912621

  Callaway thoroughly examines all the literature published up to that time on most aspects of facilitation. This paper is a perfect starting point for assessing the biology and biological mechanisms associated with positive interactions in plants. As a seminal citation on facilitation, this publication is extremely useful. Also, the paper is an excellent source for the critical physiological studies that underpin modern facilitation research. Available online for purchase or by subscription.

• Callaway, Ragan M. 2007. Positive interactions and interdependence in plant communities. Dordrecht, The Netherlands: Springer.

  This book provides the reader with a comprehensive treatment of direct versus indirect mechanisms of facilitation and examination of the evidence for whether positive effects are species specific. Every aspect of facilitation is covered in detail, and if the reader is interested in mechanisms, this is the best source for a full understanding of the scope of effects studied in plant facilitation experiments.

• Pakeman, Robin J., Francisco I. Pugnaire, Richard Michalet, et al. 2009. Is the cask of facilitation ready for bottling? A symposium on the connectivity and future directions of positive plant interactions. Biology Letters 5.5: 577–579.

  DOI: 10.1098/rsbl.2009.0384

  This is a short and accessible essay on the status of facilitation in the early 21st century, in terms of conceptual refinement, evolution, community-level effects, and restoration, and on future directions in general for the field. This is likely the quickest read on the topic, serving as a brief and broad overview. Topical issues are identified and future directions, explained.

• Pugnaire, Francisco I., ed. 2010. Positive plant interactions and community dynamics. Boca Raton, FL: CRC.

  DOI: 10.1201/9781439824955

  A collection of essays focused on facilitation and the plant community. This is not so much a book about the biology of facilitations as one on the derived or advanced aspects of facilitation, in that it addresses implications at the community level. This source is an excellent community ecology book mainly focused on positive plant interactions, with less detail than Callaway 2007 but with more focus on the community. A logical pipeline for the reader would be to begin with Callaway 2007, for an understanding of the mechanisms, and then move on to this book for community-level implications.