  Many systems in nature, from ferromagnets to flocks of birds, exhibit
ordering phenomena on the large scale. In physical systems order is
statistically robust for large enough dimensions, with relative fluctuations
due to noise vanishing with system size. Several biological systems, however,
are less stable than their physical analogues and spontaneously change their
global state on relatively short timescales. In this paper we show that there
are two crucial ingredients in these systems that enhance the effect of noise,
leading to collective changes of state: the non-symmetric nature of
interactions between individuals, and the presence of local heterogeneities in
the topology of the network. The consequences of these features can be larger
the larger the system size leading to a localization of the fluctuation modes
and a relaxation time that remains finite in the thermodynamic limit. The
system keeps changing its global state in time, being constantly driven out of
equilibrium by spontaneous fluctuations. Our results explain what is observed
in several living and social systems and are consistent with recent
experimental data on bird flocks and other animal groups.
