By including under the name 'niches' all the beneficial and adverse fa
ctors or attributes for an ecological system that contribute or affect
the biomasses of the species in that system, the contributory functio
ns of these factors, together with their distributions in spatial regi
on, have been proposed. From the random nature of these functions, ter
med as 'random causes' for the ecosystem, the stochastic evolution equ
ations for the biodensities of the spreading species have been deduced
. These equations are found to be very general in nature and also desc
ribe the nonlocal contributions of the biodensities. They can produce
stochastic partial differential equations for the biodensities of the
species for various ecological systems. Finally, it has been discussed
how the experimental values of the population growth and spread over
a region with the niche distribution therein, can determine the ecolog
ical niche-contribution functions to the species of the system. The st
ationary or equilibrium transition probabilities for the general ecolo
gical systems are also given.