Geoscience Reference
In-Depth Information
Figure 7.2
Relationships between levels in a system, as conceived in hierarchy theory.
Processes at the focal level are constrained by the level above. They are driven by
interactions among components at the level below. Figure by Darin Jensen.
and Wright, 2002). The prevailing approach in biophysical geography, as in ecology,
is hierarchy theory (Allen and Starr, 1982), in which phenomena are classifi ed based
on functional relations or operational scales. Wu and Loucks (1995, p. 451) argue
that ecological studies should examine (at least) three levels: the level of the process
at issue, plus the levels above and below it (fi gure 7.2). 'The higher level provides
a context and imposes top-down constraints on the focal level, and the lower level
provides mechanisms and imposes bottom-up constraints'. Note that causality here
is not unidirectional (
contra
Leitner and Miller, 2007): The outcome at a given level
is determined both at that level and by the interaction of processes that link it
'upwards' and 'downwards' to adjacent levels.
Most biophysical systems are theorised as constitutive hierarchies. This means
that relations are not simply bureaucratic, in which 'higher' levels dictate what
happens at 'lower' ones (known to political scientists as an exclusive hierarchy).
Nor are they inclusive, as in taxonomy, in which each level simply encompasses
those below it. In a constitutive hierarchy, units at one level, when combined at the
next level up, may display patterns of self-organisation and 'emergent properties'
that cannot be discerned in, or deduced from, their behaviour at the focal level
(Gibson et al., 2000). The idea is often expressed as 'the whole is greater than the
sum of its parts'. Landscape ecology descends in part from this insight, sometimes
glossed as 'holism' or the study of 'holons' (Naveh and Lieberman, 1984). Similarly,
'complex adaptive systems' are defi ned by heterogeneity and unpredictability as
'pattern emerges from the interplay between processes that generate novelty and
those that winnow that novelty' (Chave and Levin, 2004, p. 31). Chaos theory and
panarchy (Gunderson and Holling, 2002) are other recent attempts to make sense
of such phenomena, which Church (1996, p. 167) locates 'in the zone between
mechanistic and contingent explanation'.
Scale as Relation
It is here that scale as relation emerges. Not only is there no single 'correct' scale
for understanding social or ecological systems, but neither can one assume linearity
