  Percolating networks based on interparticle tunneling conduction are shown to
yield a logarithmic divergent piezoresistive response close to the critical
point as long as the electrical conductivity becomes nonuniversal. At the same
time, the piezoresistivity or, equivalently, the conductivity anisotropy
exponent $\lambda$ remains universal also when the conductive exponent is not,
suggesting a purely geometric origin of $\lambda$. We discuss our results in
relation to the nature of transport for a variety of materials such as
carbon-black--polymer composites and RuO_2-glass systems which show
nonuniversal transport properties and coexistence between tunneling and
percolating behaviors.
