A Among the three kinds of creatures: grass, mouse and snake, the mouse eats grass seeds or grass roots, and the snake eats mice. According to the writing of food chain, the food chain is: grass → mouse → snake, but the option is written backwards, which does not meet the meaning of the question.

B, the ecosystem has a certain self-regulation ability. If people prey on snakes excessively, the natural enemies of mice will be greatly reduced, and if there is enough food, they will multiply in large numbers, resulting in an increase in the number of mice in a short time.

C. As can be seen from the figure, the biomass in the ecosystem will change within a certain range and remain relatively stable, rather than having a certain stability, so it does not meet the question.

D, the ecosystem has self-regulation ability, which is limited. It is related to the species and quantity of organisms in the ecosystem. The more species and quantity, the stronger the self-regulation ability of the ecosystem. Because the food chain is simple, there are few species, and the self-regulation ability of the ecosystem is poor, it conforms to the meaning of the question.

Therefore, choose: d.