  We introduce a multi-species network model which describes the resistance
fluctuations of a resistor in a non-equilibrium stationary state. More
precisely, a thin resistor characterized by a 1/f^alpha resistance noise is
described as a two-dimensional network made by different species of elementary
resistors. The resistor species are distinguished by their resistances and by
their energies associated with thermally activated processes of breaking and
recovery. Depending on the external conditions, stationary states of the
network can arise as a result of the competition between these processes. The
properties of the network are studied as a function of the temperature by Monte
Carlo simulations carried out in the temperature range 300 \div 800 K. At low
temperatures, the resistance fluctuations display long-term correlations
expressed by a power-law behavior of the auto-correlation function and by a
value approx 1 of the alpha-exponent of the spectral density. On the contrary,
at high temperatures the resistance fluctuations exhibit a finite and
progressively smaller correlation time associated with a non-exponential decay
of correlations and with a value of the alpha-exponent smaller than one. This
temperature dependence of the alpha coefficient reproduces qualitatively well
the experimental findings.
