A, in the ideal environment without natural enemies, the population is J-shaped, and A is correct;

B, with the increase of population, the number of animals (natural enemies) that feed on this population will also increase, and B is wrong;

C, when the population density increases, intraspecific competition will intensify, and C is wrong;

D, because natural resources and space are always limited, the population is S-shaped and D-shaped.

So choose: a.