A, only when λ > 1, the population number increases, λ remains unchanged and grows in a "J" shape, and A is correct;

B, when λ= 1, the birth rate is equal to the death rate, and the population ratio changes, and B is wrong;

C, when 0 < λ < 1, the population density decreases continuously, and C is correct;

D. Under the ideal conditions of no natural enemies, sufficient food and space conditions and suitable climate, the population grows in a "J" shape, and D is correct.

Therefore, choose: B.