The web of life is an appropriate metaphor for living systems, whether they are ecological, anthropological, sociological, or some integrated combination—as most on Earth now are. This phrase immediately conjures up the image of interactions and connectedness both proximate and distal: a complex network of interacting parts, each playing off one another, providing constraints and opportunities for future behavior, where the whole is greater than the sum of the parts. Networks: the term that has received much attention recently due to such common applications as the Internet, "Six Degrees of Separation", terrorist networks, epidemiology, even MySpace®, actually has a long research history in ecology dating to at least Darwin's entangled bank a century and a half ago, through the rise of systems ecology of the 1950s, to the biogeochemical cycling models of the 1970s, and the current focus on biodiversity, stability, and sustainability, which all use networks and network concepts to some extent. It is appropriate that interconnected systems are viewed as networks because of the powerful exploratory advantage one has when employing the tools of network analysis: graph theory, matrix algebra, and simulation modeling, to name a few.
Networks are comprised of a set of objects with direct transaction (couplings) between these objects. Although the exchange is a discrete transfer, these transactions viewed in total link direct and indirect parts together in an interconnected web, giving rise to the network structure. The structural relations that exist can outlast the individual parts that make up the web, providing a pattern for life in which history and context are important. The connectivity of nature has important impacts on both the objects within the network and our attempts to understand it. If we ignore the web and look at individual unconnected organisms, or even two populations pulled from the web, such as one-predator and one-prey, we miss the system-level effects. For example, in a holistic investigation of the Florida Everglades, Bondavalli and Ulanowicz (1999) showed that the American alligator (Alligator mississippiensis) has a mutualistic relation with several of its prey items, such that influence of the network trumps the direct, observable act of predation. The connected web of interactions makes this so because each isolated act of predation links together the entire system, such that indirect effects—those mitigated through one or many other objects in the network—can dictate overall relations. While this might seem irrelevant particularly for the individual organisms that end up in the alligator's gut, as a whole the prey population benefits from the presence of the alligator in the web since it also feeds on other organisms in the web which in turn are predators or competitors with the prey.
Such discoveries are not possible without viewing the ecosystem as a connected network. This chapter deals with that connectivity, provides an overview of systems approaches, introduces quantitative methods of ecological network analysis (ENA) to investigate this connectivity and ends with some of the general insight that has been gained from viewing ecosystems as networks. Insight, which at first glance appears surprising and unintuitive, is not that surprising under closer inspection. It only seems so from our current paradigm, which is still largely reductionistic. We hope these examples give further weight for adopting the systems perspective promoted throughout this book.
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