The principal reason for declining natural increase rates in less developed countries today is

Declining crude birth rates.

Explanation:

Birth rate may be defined as the number of births takes place per year per thousand of the population. The birth rate and death rate may change the population dynamics.

The developing countries shows the expanding population graph as they have more number of people of the reproductive age. The advancement in the medical field results in the decline birth rate that causes the decline of the nature resources.

Thus, the answer is declining crude birth rates.