Need some help with this Biology ?

[Rosa M]

A particular environmental change causes the deaths of 25 individuals in a herd of 100 wild horses, and it kills 50 individuals in a herd of 200 horses. Both herds live in areas of equal size. In this case, the growth of a wild horse population is most likely limited by _____.

A) a density-dependent factor

B) food supply

C) a predator

D) an abiotic factor

E) the presence of individuals of another species that uses the same food resource

[Jesse W]

D: Abiotic Factor. In each case exactly 1/4 of the population is being destroyed. If it was an issue with food supply, because they are both in areas of equal size, then one would have expected a higher ratio of death in the larger population. If it was a density dependent factor, again, there would ahve been a larger ratio of mortality in the denser population, E is the same as B for this respect. Prey density effects rate of predation in all predator models, and as the density is not equal but the mortality is, predation is not an option either. An abiotic factor, such as a cold winter, cold enough to kill off the weakest quarter of the population is the only explanation.