  In this paper, we study a single-species population model with pulse toxicant
input to a small polluted environment. The intrinsic rate of population is
affected by environment and toxin in organisms. The toxin in organisms is
influenced by toxin in environment and the food chain. A new mathematical model
is derived. By the Pulse Compare Theorem, we find the surviving threshold of
the population and obtain the sufficient conditions of persistence and
extinction of the population.
