Researchers study populations' geographic range, density and distribution, growth rate, and age structure when they study populations. Factors that affect population size are the birthrate, death rate, immigration, and emigration. Immigration is when organisms enter another geographic range. Emigration is when organisms leave a geographic range.
In exponential growth, population will grow exponentially, with unlimited resources. However, environments don't have unlimited resources. There is a carrying capacity. After a period of exponential growth, the population hits a carrying capacity. This is logistic growth. There is not enough resources so eventually populations will have to reach a carrying capacity. Carrying capacity is the max number of individuals of a particular species that a particular environment can support.
Limits to Growth
As stated above, carrying capacity limits the population growth of organisms. Limiting factors determine the carrying capacity of an environment for a species. Limiting factors could be competition for food, predation, parasitism and disease, unusual weather, or a natural disaster. Density-dependent limiting factors include competition, predation, herbivory, parasitism, disease, and stress from overcrowding. Unusual weather such as hurricanes, droughts, or floods, and natural disasters such as wildfires, can act as density-independent limiting factors (doesn't depend how dense the population is).