“…There are also experimental evidences (Carpenter et al, 2011) that environmental noises can play a key role in ecological systems. Given this, stochastic dynamical modelling, which is one of the powerful tools to investigate the effect of noise and toxicant on the population in a polluted environment, has attracted many researchers interests (Hallam et al, 1983;Hallam and Ma, 1986;Duan et al, 2004;Yang et al, 2007;Liu and Wang, 2009). On the other hand, due to the nature of Gaussian white noise, we need to consider another noise to reflect phenomena in ecosystems suffering switch abruptly to a contrasting alternative stable state (Scheffer et al, 2001).…”