The carrying capacity is different for all populations. It can vary from species to species, habitat to habitats and even within the same habitat at different times. ... So depending on the demands of the species the environment can be capable of sustaining different population sizes of different species............... ᐛ
It is given that the allele B for the black coat is dominant over allele b for a brown coat.
A dominant allele always leads to the expression of the dominant trait in the heterozygous condition.
When a black and brown horse is crossed as given in the question and the result is a brown foal, then the genotype of the brown foal must be bb as the brown coat is a recessive trait.
Since each parent contributes one allele each for a given gene, the black horse must have b allele, and hence must be heterozygous Bb.