Limiting factors and carrying capacity
A limiting factor is any resource or condition that caps how large a population can grow in a given place.
A worked example
A pond that supports at most 300 trout is at its carrying capacity, so adding 80 more only raises deaths from competition and disease.
The mistake people actually make
Carrying capacity is the ceiling a population levels off at, not the number present today. Removing habitat, as when a wetland is drained, lowers that ceiling rather than moving the species somewhere else.