If I understand your question correctly you are probably referring to the carrying capacity of a population. The more genetically diverse a population is, the more resistant they become against factors that would reduce the population such as disease, lack of resources and other environmental factors. As genetic diversity increases, carrying capacity of the population increases. This is why catastrophic events which lead to genetic bottlenecks are so damaging to a population. If genetic diversity is removed, the population is more susceptible to one particular environmental factor.