The Reality of Constraint and the Illusion of Control in Ecological Networks

Control has been a confusing and controversial concept in ecology. Authors sometimes fail to define how they use the term and rarely specify the controller, the controlee, the process, and the result. Control has often been depicted as one component impacting another, usually negatively, such as a predator consuming its prey or as top-down or bottom-up path control of a food chain (trophic cascade or trophic escalade). Some authors define external drivers, such as light and temperature, as controllers, however, control becomes conceptually impoverished when equated with mere straight-line change or impact. These examples do not constitute control in the cybernetic context of a system being guided toward achieving its purpose via feedback loops. Ecosystems are self-organized, networked systems with no central controller. Every species functions within a node of these networks and is continuously affected by all the pathways, mostly indirect, that transit that node. However, this does not mean that ecosystems have unlimited degrees of freedom in their organization. To be an organized system is a priori to have constraints. All living systems require a set of constraints that act in a network of feedback and regulation processes (Hofmyer, 2008), and these constraints are overwhelmingly beneficial in ensuring network integrity and behavior. Type II Loop Analysis, a signed digraph methodology, is applied here to both field and laboratory plankton communities to explore control and constraint concepts at four ecological levels: single-species interactions, two-species interactions, food chains, and food webs, as well as external drivers. Since Loop Analysis encompasses the full range of possible link types, it is a technique within ecological network analysis; however, we use the term ‘food web’ interchangeably with ecological network. Loop Analysis characterizes feedback directly as it arises from interacting functional groups of species that can simultaneously act as controllers and controlees. The food web operates as an independent, anticipatory, self-regulating system or an ecosystem chimera, which is much different than the sum of its parts. It is a network of interacting species that trade functions with each other, which individual species cannot provide for themselves. No solitary species or pathway exists in a networked ecosystem, let alone as an autonomous controller. Several structural and functional ecosystem-level constraints have been identified, including the level and sign of feedback and the overall feedback pattern, self-organizing central lattice, operating pathways, network motifs or subgraph structures required for ecosystem functions, stability measures, and Ecological Skeletons. To illustrate that random networks do not possess similar feedback and constraint properties as biologically reasonable networks, we also compare a set of 500 randomly generated food webs with an Ecological Skeleton consisting of 21 nodes and 69 links. The real-world food web is markedly different from the corresponding random ones in autoregulation, 3-node feedforward, and 4-node bi-parallel and bi-fan motifs. Some comparisons are made between food webs and metabolic and gene transcription networks. However, further study is required to ascribe specific functions to each type of ecosystem network motif and identify more potential profound similarities with other levels of biological organization.