Lower bound technique appears to be a very useful tool in the theory of Markov operators. In particular A. Lasota and J. Yorke used it to prove the existence of an absolutely continuous measure for the Frobenius--Peron operators corresponding to piecewise monotonic transformations. Recently this technique has been extended to general Markov operators including those describing the evolution of measures for iterated function systems. Quite recently it has been shown its utility in the theory of SPDEs. In this talk, making use of lower bound technique, we are going to formulate criteria for ergodicity of an invariant measure on general complete separable metric spaces.