Final answer:
Negative natural increase rates result in a decrease in the population size, as these rates reflect a situation where the number of deaths exceeds the number of births.
Step-by-step explanation:
The potential effect of negative natural increase rates on the size of a population is that it leads to a decrease in the population size. This happens because the term 'natural increase rate' refers to the rate at which a population grows or declines based on the number of births and deaths, excluding migration. Therefore, if the death rate is larger than the birth rate, the population experiences a negative natural increase, which in turn results in a reduction of the population size over time. Other factors, such as immigration and emigration, can impact the overall population size but are not considered in the natural increase rate.