  We study the time evolution of two ecosystems in the presence of external
noise and climatic periodical forcing by a generalized Lotka-Volterra (LV)
model. In the first ecosystem, composed by two competing species, we find noise
induced phenomena such as: (i) quasi deterministic oscillations, (ii)
stochastic resonance, (iii) noise delayed extinction and (iv) spatial patterns.
In the second ecosystem, composed by three interacting species (one predator
and two preys), using a discrete model of the LV equations we find that the
time evolution of the spatial patterns is strongly dependent on the initial
conditions of the three species.
