Final answer:
Population size can be determined by its growth rate, which is affected by birth, death, immigration, and emigration rates. Exponential growth can lead to rapid increases in population size, while variables such as average age and local economy also influence growth.
Step-by-step explanation:
The population size can be determined based on the change in population growth rate, which is influenced by the rates of birth, death, immigration, and emigration. The formula to calculate the population growth rate is r = (b + i) - (d + e), where b stands for birth rate, i stands for immigration, d stands for death rate, and e stands for emigration.
For example, if a population experiences exponential growth, such as the hypothetical town that sees its population size double every 10 years, this would correspond to a 7% annual growth rate. In contrast, a more modest growth rate of 1% would lead to the population size rising from 7 billion to 8 billion in less than 14 years.
Factors like the average age of the population, local economy, and environmental conditions, such as available food and moisture, also affect a population's growth. It's important to note that population growth rates may fluctuate over time due to these factors.