Introduction - Methods for Multilevel Analysis and Visualisation of Geographical Networks - page 5

For urban systems more generally, the process implemented at the level of actors
and organisations is primarily responsible for establishing the urban hierarchy. The
hierarchy is not unitary but is composed of multiple subsystems that are simultane-
ously interwoven at different thematic and specialised levels of geographical space
( Batty , 2006 ; Dicken , 2011 ; Rozenblat , 2007 ).

1.3 Multilevel Networks

Social networks are multilevel because different levels of aggregation allow the
emergence of collective processes. Regardless of their spatial scope, networks are
created, maintained and destroyed by individual actors or organisations that deploy
assets and relationships to consolidate or improve their position in a space that
is structurally identified and perceived at different spatiotemporal scales ( Pumain,
Paulus, Vacchiani-Marcuzzo, & Lobo , 2006 ). Due to the immediacy of access
(within timespans of an hour), these networks rely on local territories - cities -
from which they extract, exchange and expand human, material and informational
resources to varying degrees (see Fig. 1.2 ).
As Pumain et al. ( 2006 , p. 172) note:

[N]ew properties emerge and characterize the city as a collective entity. Some of these new
properties can be directly related to the intention of some institution, but most of the time
they are the unexpected (and sometimes unwanted) result of collective interaction.

The city operates at an intermediate level in the organisation of societies, which
Pumain ( 2006 ) termed the “meso level” (see Fig. 1.2 ). The intermediate level
catalyses the social system daily on a local scale in a self-organising fashion (Batty
2005 , 2006 ; Pumain , 1997 ; Pumain, Sanders, & Saint-Julien , 1989 ; Wilson , 2000 ).
These places, which exhibit “immediacies of access” in interactions at the regional,
national, continental or global level, due to actors' competitive or collaborative
global strategies, comprise the total urban system that constitutes the macro level
( Berry , 1964 ; Pred , 1977 ; Pumain et al. , 1989 ; Sanders , 1992 ). This macro level
provides feedback to the meso level that integrates the inner city's networks into
larger interurban networks. However, the initial power belongs to individual micro
level actors who create, maintain or destroy social and economic networks, although
this process operates under macro and the meso-level constraints despite bottom-up
feedback effects.
The “small-world” network approach identifies network components, termed
“clusters”, that are denser and more cohesive than the entire network ( Barabasi ,
2002 ; Barabasi & Albers , 1999 ; Newman et al. ( 2006 ); Watts , 1999 ; Watts & Stro-
gatz , 1998 ). Clusters not only exhibit a high level of dependency between individual
components but also a low level of “vulnerability” in networks that contain many
redundancies ( Burt , 2000 , 2005 ). Thus, networks organise new topological “prox-
imities” through direct links and more indirect pathways that are simultaneously
spatial, social, cultural and organisational. Geography incorporates economic and