  (Abridged) We study numerically the applicability of the effective-viscosity
approach for simulating the effect of gravitational instability (GI) in disks
of young stellar objects with different disk-to-star mass ratios \xi. We adopt
two \alpha$-parameterizations for the effective viscosity based on Lin &
Pringle (1990) and Kratter et al (2008) and compare the resultant disk
structure, disk and stellar masses, and mass accretion rates with those
obtained directly from numerical simulations of self-gravitating disks around
low-mass (M_\ast ~ 1.0 M_sun) protostars. We find that the effective viscosity
can, in principle, simulate the effect of GI in stellar systems with \xi <=
0.2-0.3, thus corroborating a similar conclusion by Lodato & Rice (2004) that
was based on a different \alpha-parameterization. In particular, the Kratter et
al's \alpha-parameterization has proven superior to that of Lin & Pringle's,
because the success of the latter depends crucially on the proper choice of the
\alpha-parameter. However, the \alpha-parameterization generally fails in
stellar systems with \xi >= 0.3, particularly in the Class 0 and Class I phases
of stellar evolution, yielding too small stellar masses and too large
disk-to-star mass ratios. The failure of the \alpha-parameterization in the
case of large \xi is caused by a growing strength of low-order spiral modes in
massive disks.
