Scale problems invited hierarchy theory into the discipline. Ecologists have long been aware of scale, investigating the properties of quadrats in obtaining estimates of vegetation in the 1950s. Then change in variance across quadrat size was used to measure aggregation of plants on the ground. Hierarchy theory remains associated with scale today. The observation protocol brings attention to a universe of a certain extent, while making a second distinction, the finest grain at which observation units are distinguished from one another. Grain and extent together characterize the scalar level in question in many ecological hierarchies. Grain and extent are connected. Wider extents require coarser grains, if the mass of data are to be remembered, analyzed, and understood. Modern computational power has widened the gap between grain and extent, where remotely sensed areas are captured in billions of pixels. Even so, explicitly linking items in the grain across the extent becomes difficult, and generally impossible as the extent widens by much.
In contrast to linking across scales, it is possible to unify ecology across types of ecological system that correspond to the main subdisciplines of ecology: organism; population; community; ecosystem; landscape; biome; and biosphere. These types for ecological subdisciplines are explicitly not scale based, and so are not required to be assigned to level in the order given in the previous sentence. When scale is parsed away from type, the various approaches to ecology achieve a sharper depth of focus, offering clear relief between types of investigation. The subdisciplines of ecology are not scalar levels. If they are levels at all, they are type-based levels of organization, with the different types related to one another by asymmetric relationships made explicit in the definitions. As a separate issue, a typed level of organization itself contains scale-based hierarchies, as in fractal landscapes (see Landscape Ecology). In that scaled universe, the ecosystem modeling strategy may apply across a range of sizes, where local processes are part of more global processes. Communities too may be variously inclusive of species across narrow or wider areas. Under the organism criterion, examples are found from redwood trees to mites. An ecological hierarchy may change the scale and type at the same time, but it is fraught with conceptual danger. Indeed, hierarchy theory is often invoked to clean up the mess in the aftermath of scale and type being mixed together. There is no prohibition changing both together, but only so long as the relationships at each new level are explicit. This matters because most descriptions of ecological material precisely do change type across widening scalar levels, although most of them do not follow the textbook ordering from organism to biosphere. For instance, in a forest community, a rotting tree trunk may be considered an ecosystem, whose upper surface is landscape, on which grows a community of bryophytes.
The copious variety of materials, entities, and sizes in ecology invites hierarchy theory into ecology. Indeed, it is in ecology that hierarchy theory has been used most often to significant effect, as in the NESH studies mentioned above. Hierarchy theory can capture a rich set of scaled examples across a mixture of types. Ecology is a multiple-scaled labyrinth of types. Hierarchy theory is the ball of string that we can trail behind, so that ecological scientists do not get lost.
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