In ecology we need a body of theory to address relationships that are consequences of changing levels of analysis, which call for altered definitions. For instance, the contemporary fracas over definitions of plant competition could benefit from recognizing differences in the scope and type of the framework used by the respective partisans. With distinctions between levels of analysis made clear, each school may test their respective hypotheses in peace aware of which theories actually compete and when they merely address some other level of discourse. The contentious literature surrounding overcompensation of plants in response to losses to grazing was significantly a matter of pulse versus press consumption in relation to different timescales for assessing recovery. The Clements/Gleason debate over the proper definition of plant community might not have lasted the body of the twentieth century had hierarchy theory been available at the onset of hostilities when Nichols attacked Gleason's paper at the 1926 International Congress of Plant Sciences (see Community). Hierarchy theory's focus on level of analysis offers such clarification. It lays subtle distinctions bare, so that definitions work for the ecologist instead of ecologists working for their definitions.
If big, slow things were always on top, such that hierarchical levels were only a matter of scale, the problem would reduce to a straightforward technical scaling issue. Not to underestimate the challenges of scaling in engineering, but that technical setting does not need something as grand as a theory to deal with hierarchies. But in ecology, scaling is complicated by higher ecological levels giving lower levels meaning. In ecology, the move upscale to be more inclusive often changes significance more than it invokes a change of size, and so we do indeed need a special body of theory to deal with difference of quality, not just quantity. Differences in scale quickly become large enough to cause qualitative change in perception, which forces a change in the level of analysis. Thus scale is soon embroiled in values, judgment, and arbitrary choices, not just as an inconvenience, but as a necessity for proper understanding. While scaling as an engineering technicality actively ignores such messy issues, hierarchy theory explicitly includes value-based decisions of the observer in creating hierarchies.
To control for observer values, technical measurement and analysis in science keeps its criteria constant across the local discourse. But large discoveries precisely amount to the recognition of a change in value. New scientific ideas indicate a specific change in the preanalytic stage, before deciding what might be relevant data. In the terms of Russell and Whitehead (made accessible by Gregory Bateson), new scientific ideas amount to the definition of new logical types. Hierarchy theory's central activity is recognizing logical type. Logical types are tied to some new level of inclusivity, a new hierarchical level with its own meaning. Notice how left and right sides are possessed by organisms at their own level of existence. Meanwhile, the notions of up and down refer to a larger discourse that includes an environment, which is shared by many organisms. As a result, a mirror switches the image left and right, but with no switch in up versus down. The larger scope invoked by the idea of up introduces a new logical type, even though left and right may often be simply at right angles to up and down. If left and right contrasted with up and down can be problematic, ecosystems are a nightmare. While exquisitely holding criteria constant in formal scientific calibration will help, it is insufficient for large discoveries, which turn on recognizing when a new type is necessary to solve some puzzle.
Ecology in particular invites many logical types because its hierarchies are so rich. A new type invokes new aggregation criteria, which come explicitly from observer decisions. Consider the difference between a community conception of vegetation as opposed to the process-functional conception that prevails in ecosystem modeling. A forest can be considered as a collection of trees on a tract of land. Alternatively those same tree trunks may be aggregated as a separate class from the leaves (Figure 1). If leaves in a forest are the production system independent of species, then the boles are part of the carbon storage function. This assignment has the peculiar effect of
unifying the tree trunks with soil carbon in a single carbon storage compartment. A community focus aggregates trees set in an environment of soil and atmosphere. Meanwhile a flux-process conception splits the trees into at least two parts, one of which aggregates with the soil. But the soil was part of the environment in the community conception. Thus the same pieces of soil and plant biomass are aggregated into different higher units, depending on the type of system that is recognized as being in the foreground by the observer. Note how forests under either conception may be called forest ecosystems, suggesting that one use of hierarchy theory is to untangle alternative meanings in commonplace ecological terminology. The difference between a process-focused ecosystem and a community is a change in logical type.
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