The most frequently cited definition of the ecosystem concept owes its origin to Sir Arthur G. Tansley in 1935 (Pickett and Grove 2009). It has proven to be a very flexible concept, and can be applied to any scale that includes aggregations of physical environment and organisms, plus the interactions among all physical and biological components. The interactions are just as important as the material, energy, and organisms that exist within ecosystems. Nevertheless, ecosystems are usually specified as locations or places, that is volumes of some part of the Earth, whether watery, dry land, or wetland (Figure 1).
Ecosystem as Interaction
Yet, there is a view of ecosystems that places the interactions themselves as the focus. This usage is most often heard in the vernacular or in discussions about corporate products and the services they deliver. Hence, one hears about such things as the "Apple ecosystem," or the "Google ecosystem," or the "health care ecosystem." Clearly such usages place the connections and flows of information, products, or outcomes at the center of concern. A bounded and contiguous place is not the point in this use of ecosystem as a network.
Networks are a primary concern of many sciences. Information sciences, neurosciences, industrial ecology, and studies of governance (Figure 2) often emphasize connections and flows of information, power, influence, materials, and energy. Traffic engineers and planners envision their topic in terms of networks, although those networks can be seen to serve both close and distant territories. The network rather than the territory or place is the point.
So it may seem that the network perspective and the place-delimited view of ecosystem are contradictory and divergent approaches to science. But this is not the case. In fact, ecology is a science of interaction. A traditional definition of ecology states that it is the study of the interaction between organisms and their environments. However, this definition can be expanded to acknowledge the variety of units or entities other than just organisms that are involved in those interactions. For example, in the 1950s ecology expanded to consider ecosystems as entities in which biogeochemical processes were the concern. In the 1980s in the United States, landscape ecology was imported and adapted to emphasize the interactions that involved spatially heterogeneous mosaics at various scales. Even populations and communities came to be seen as comprising subsets that interacted across space, as in metapopulation or metacommunity theory. The explicitly interaction-focused definitions of ecology (Box 1) work equally well with the place-based or the network-based approachs to systems.
Box 1: The Cary Institute Definition of Ecology. The scientific study of the processes influencing the distribution and abundance of organisms, the interactions among organisms, and the interactions between organisms and the transformation and flux of energy and matter.
A network is also a system, but specifically a system of interaction pathways, mechanisms, or flows. In other words, a network can be considered to be an infrastructure or a process that takes place over that infrastructure. Networks as infrastructure may involve materially connected continuous pathways, like roads or wires. But infrastructural networks can also involve fields, like broadcast via electromagnetic radiation (McGrath and Shane 2012). Certainly, the underlying fields are continuous, but the function of networks that rely on electromagnetic media seem more discontinuous, since the information can travel great distances with no tangible expression of the network until the message is delivered and decoded.
Joining Networks and Places
Some theories in urban ecology attempt to deal simultaneously with the place-based and network-based conceptions of systems. For example, the continuum of urbanity recognizes that local to global connections are key to understanding urban systems now (Seto et al. 2012, Boone et al. 2014). The fine scale connections can be via road, rail, pipes, fiber, or copper. Even at regional scales, these physical networks remain important. Shipping lanes and airline routes are a different kind of infrastructure, but they support regional to global connections in which urban areas partake. Satellite communications, radio, and wireless act at various scales and can be seen as virtual networks. The regional and global networks have become a part of daily urban system functioning.
These networks connect places, and those places embody lifestyles, livelihoods, and environmental conditions that are key drivers of urban ecosystem structure, function, and change. The continuum of urbanity expresses the seminal role of connections when it acknowledges that rural, urban, and wild places -- ecosystems in the sense of Tansley -- contain and are parts of extensive continuous and virtual networks. So ecosystems are both place and network.
Boone, C. G., C. L. Redman, H. Blanco, D. Haase, J. Koch, S. Lwasa, H. Nagendra, S. Pauleit, S. T. A. Pickett, K. C. Seto, and M. Yokohari. 2014. Reconceptualizing land for sustainable urbanity. Pages 313–330in K. C. Seto and A. Reenberg, editors.Rethinking urban land use in a global era. MIT Press, Cambridge.
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Pickett, S. T. A., and J. M. Grove. 2009. Urban ecosystems: what would Tansley do? Urban Ecosystems 12:1–8.
Michele Romolini, J. Morgan Grove. 2013. Assessing and comparing relationships between urban environmental stewardship networks and land cover in Baltimore and Seattle. Landscape and Urban Planning 120:190–207.
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