The main factor affecting the growth rate of a population is the availability of resources, such as food, water, and space. These resources determine a population's carrying capacity, influencing its reproductive and mortality rates. If resources are abundant, populations can grow rapidly; however, as resources become scarce, growth slows down or may even lead to population decline. Additionally, external factors like predation, disease, and environmental conditions also impact growth rates. Studying these factors is essential for understanding population dynamics and managing ecosystems effectively.