Ecological Networks

by

Paulo R. Guimarães Jr.

• LAST REVIEWED: 05 January 2022
• LAST MODIFIED: 25 June 2013
• DOI: 10.1093/obo/9780199830060-0091

Introduction

In any given ecological community individuals of hundreds of different species interact in multiple ways, forming networks of interacting species. A network is defined by a set of elements connected by links between some of the elements. An ecological network is a network in which the elements are often species and the links represent ecological interactions. Until the end of the 1990s, most of the studies on ecological networks focused on food webs, the trophic interactions between species within an ecological community. Nowadays, the notion that a variety of ecological interactions form networks of species—from predation to mutualism, from parasitism to competition—has become more pervasive. The structure of these ecological networks may provide information on the ecological and evolutionary processes generating and shaping biodiversity. Moreover, the structure of ecological networks may also tell us about the fragility of ecological communities to different kinds of perturbations, from species extinctions to the invasion of alien species, from climate change to the poaching of keystone species. The perception that species form networks of interacting species is not new, and the study of ecological networks cannot be separated from some of the long-lasting, unsolved questions in ecology and evolution. Examples of these questions are: Are static representations of feeding interactions useful to infer ecological dynamics? What is the relationship between complexity and stability of ecological communities? Are assemblages of interacting species coevolving in specific ways? Analysis of the network structure has suggested that we can infer about ecological organization using information on feeding interactions (see Pattern Description and Biological Correlates), challenged the long-lasting view that complex communities are intrinsically more stable (see Ecological Dynamics), and provided insights on how to understand the evolution and coevolution of large interacting assemblages of species (see Evolutionary Dynamics). The field of ecological networks blossomed in the turn of the 21st century, fueled by the appearance of large databases, the development of new approaches and tools, and the finding that disparate complex systems such as human societies, biochemical pathways, ecological interactions, and the Internet share similar network organization. This bibliography focuses on the study of networks formed by ecological interactions among individuals of different species. The suggested readings explore different aspects of this topic, including references on networks studied in other scientific fields, classical works on ecological networks, descriptions of main types of ecological networks, studies exploring the underlying processes shaping ecological networks, and the evolutionary, ecological, and conservationist consequences of the network organization of species interactions. The term ecological networks has been used to describe not only the organization of multispecies assemblages, but also the spatial networks formed by natural habitats and/or reserves connected by migration. This review focuses on the networks formed by species interactions.

Foundational Works

The study of ecological networks is deeply rooted in some of the long-lasting questions in ecological science. To represent feeding interactions among species as webs or networks is not new, as seen in Pascual and Dunne 2006 (cited under Types of Ecological Networks), and the perception that ecological interactions connect species directly or indirectly led to the definition of the term food chain, introduced in Elton 2001. The view that ecological communities are formed by interacting assemblages of species led to studies on the role of different species in organizing biological diversity. Empirical manipulations of a particular species, as in Paine 1966, illustrated the importance of patterns of interaction in shaping biodiversity and the remarkable role that species such as top predators can have in shaping this organization. At the same time, the study of how energy and mass flow across the elements of the ecosystems in Odum 1960 and the search for generalizations on the structure and dynamics of ecological systems in Margalef 1963 pointed out the importance of these patterns of interaction for ecological dynamics and community stability. Cohen 1978 shows how the use of mathematical tools led to a quantitative characterization of the organization of ecological communities. Moreover, the analysis of community matrices depicting the effects of each species on populations of interacting species led to the complexity–stability debate; the view that stability is an inherent property of diverse ecological communities is challenged in May 2001. An additional major advance was the realization that simple assembly models derived from random graph theory were able to reproduce the organization of food webs, which are the networks describing trophic interactions among species, as reviewed in Pimm 2002. The seminal paper Jordano 1987 generalized ideas first used for characterizing food webs to mutualisms, anticipating the exploration of the network structure of different kinds of species interactions (see Types of Ecological Networks). All these studies illustrate the previous work that, together with the explosion of complex network theory (see Fundamentals of Networks) and the development of computational approaches allowing us to handle large datasets, allowed the emergence of the field of ecological networks at the turn of 21st century.

• Cohen, Joel E. 1978. Food webs and niche space. Princeton, NJ: Princeton Univ. Press.

  The main elements of current ecological network analysis are present in this foundational book, including the aim to contribute to a central question in ecology (how many dimensions are needed to characterize food webs?), the compilation of a large database of empirical networks, the use of graph theory concepts such as intervality to characterize empirical networks, and the conclusion that similar patterns occur in several disparate ecological communities.

• Elton, Charles S. 2001. Animal ecology. Chicago: Univ. of Chicago Press.

  This book introduces the idea of food chains together with the idea that food chains can be combined in food cycles, now called food webs. This reprint includes new introductions for the chapters, describing the influence of Elton’s work. It is a classic book in ecology and still a source of inspiration for the subject. Originally published in 1927.

• Jordano, Pedro. 1987. Patterns of mutualistic interactions in pollination and seed dispersal: Connectance, dependence asymmetries, and coevolution. American Naturalist 129:657–677.

  DOI: 10.1086/284665

  This paper explores the structure of assemblages of plants and their mutualistic partners (seed dispersers and pollinators). It uses the concepts of connectance and asymmetries to characterize the organization of mutualistic assemblages, anticipating the debate on the role of abundance and coevolution in shaping mutualistic networks.

• Margalef, Ramón. 1963. On certain unifying principles in ecology. American Naturalist 97.897: 357–374.

  DOI: 10.1086/282286

  This classic paper illustrates the long-lasting search for general principles in ecology. Margalef explores the relationships among the complexity of ecosystems, energy flow, perturbations, and the maintenance of complex organization in space and, especially, in time. Available online for purchase or by subscription.

• May, Robert M. 2001. Stability and complexity in model ecosystems. Princeton, NJ: Princeton Univ. Press.

  This book challenged the long-lasting view that stability is a natural consequence of complexity, firing up the complexity–stability debate. Originally published in 1973, this work popularized the qualitative stability analysis in the ecological literature, one of the most often used ways of exploring the stability of ecological networks.

• Odum, Howard T. 1960. Ecological potential and analogue circuits for the ecosystem. American Scientist 48.1: 1–8.

  This article is a key paper on the idea that ecosystem dynamics can be modeled using approaches derived from physics. The study is based on the analogies between mass and energy flow across the interactions connecting trophic levels with Ohm’s Law. Available online for purchase or by subscription.

• Paine, Robert T. 1966. Food web complexity and species diversity. American Naturalist 100.910: 65–75.

  DOI: 10.1086/282400

  This article describes one of the central results of empirical experiments in ecology. It illustrates how the removal of a top predator—a starfish—led to a reduction in the diversity within food webs due the releasing of top-down control imposed by starfish on their prey. Available online for purchase or by subscription.

• Pimm, Stuart L. 2002. Food webs. Chicago: Univ. of Chicago Press.

  This book is a well-organized and clear review of the main findings of empirical and theoretical studies on food webs prior to the 1980s. It is a wonderful introduction and summary for researchers interested in how the network approach would allow the characterization of interacting assemblages. Originally published in 1982 (London: Chapman and Hall).