Final answer:
The population pyramid described is one with rapid growth, common in many developing countries with high birth rates and declining death rates. This pyramid is characterized by a wide base and a tapering shape, indicating increasing population size with a high proportion of younger individuals.
Step-by-step explanation:
The type of population pyramid described by characteristics such as a very high birth rate, a high but declining death rate, a short life expectancy, and an increasing population size is known as the pyramid of rapid growth. This pyramid shape is commonly seen in developing countries where access to birth control is limited, leading to a large number of births, and where improvements in health care and living conditions have started to lower death rates. As a consequence, more individuals survive to reproductive age, resulting in a population increase. Nigeria's pyramid is an example of this type as it appears with a wide base representing a high number of young children and tapers off with fewer older adults, exemplifying the high birth and death rates.
Mexico's population pyramid from 1998 is illustrative of a Stage 3 population, indicating that the birth rate has begun to fall, reflecting a transition phase towards more stabilized growth. Populations in countries like Sweden represent a Stage 4 pyramid, with slow or no growth due to sustained low birth rates. Finally, countries like Italy that have reached the hypothetical Stage 5 of demographic transition depict a top-heavy pyramid due to birth rates falling below death rates, resulting in negative population growth.