Geoscience Reference
In-Depth Information
local interactions of system components, and less with the measures or variables
that defi ne systems. Aggregate complexity places particular emphasis on the role of
individual entities and the relationships among them in defi ning system structure
and behaviour within its larger environment. The role of adaptation, learning, and
change in both system components and the system as a whole is critically important
to research in aggregate complexity and is especially important to research related
to coupled human-environment systems.
Systems like an ecosystem or an economy are driven to a great extent by indi-
vidual components and their relationships. Within ecology, for example, biotic
entities such as plants and animals have relationships defi ned by exchanges of
matter, energy, and information with other entities in larger ecological systems.
Importantly, most entities in the system have multiple relationships and play multi-
ple roles. A tree, for example, cannot survive without relationships with entities and
systems ranging from bacteria to other trees to weather systems. Of course, some
relationships are more important than others to any given component. Especially
tight links between entities will join them into larger collective groups that act as
entities in and of themselves (Allen and Holling, 2002). For example, the odds of
a single tree thriving are very dependent on whether a suffi cient number of other
trees exist to form a stand to protect individual trees from wind damage, while the
existence of many stands in close proximity is important to defi ning a larger forest
that in turn creates its own self-sustaining microclimate and habitat to which arbo-
real vegetation is better adapted than competing grassland species.
One particularly important kind of interaction is self-organisation, in which enti-
ties within a system change their relationships in a manner that enables the system
as a whole to adapt its structure and behaviour to better suit its environment (East-
erling and Kok, 2002). Sometimes these changes to internal relationships are slow
and gradual. At other times, outside forces or internal perturbations may encourage
the system to make sudden, large changes similar to the bifurcations of deterministic
complexity. Even small disturbances such as fi res have the capacity to reorder enti-
ties and relationships throughout an ecosystem, causing it to move through cycles
of destruction and rejuvenation (Holling, 2001). Self-organisation can lead to self-
organised criticality, where the system hovers on the edge of collapse and, as a
result, can quickly shift resources and internal relationships to respond to internal
or external changes (Bak, 1996). The evolution of a forested landscape in the face
of both environmental and human perturbations, for example, can be understood
as a system governed in part by self-organised criticality in which there is a balance
between disturbances (human ones such as building roads and environmental ones
such as fi res) and orderly succession of land use and cover (Bolliger et al., 2003;
Crawford, 2005).
Self-organisation is closely tied to the concept of emergence. Systems that are
treated as 'complex' by aggregate complexity can possess emergent qualities that do
not result from superposition (i.e., additive effects of system components), but
instead from synergistic interactions among components. In other words, the behav-
iour of a system can be greater than the sum of the behaviour of system's constituent
parts. Individual cells within an organ such as the brain or liver, for example, band
together to allow it to act in ways not easily surmised from examining the charac-
teristics and behaviour of individual cells. Some authors go so far as to claim that
a system is only complex if it displays emergent properties that cannot be fully
explained by analysing its components in isolation (Holland, 1998). Emergence can
