  Statistical mechanics of relative species abundance (RSA) patterns in
biological networks is presented. The theory is based on multispecies
replicator dynamics equivalent to the Lotka-Volterra equation, with diverse
interspecies interactions. Various RSA patterns observed in nature are derived
from a single parameter related to productivity or maturity of a community. The
abundance distribution is formed like a widely observed left-skewed lognormal
distribution. It is also found that the ``canonical hypothesis'' is supported
in some parameter region where the typical RSA patterns are observed. As the
model has a general form, the result can be applied to similar patterns in
other complex biological networks, e.g. gene expression.
