Population Ecology: Environmental Effects on Population Size

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Populations can range from the very small to the very large, but the size and prosperity of all populations are influenced by a few basic things. In this lesson we'll discuss the important factors that cause a population to grow or decline in size.

What is a Population?

Have you ever wondered what factors affect where people and animals chose to make their homes, and why more live in some areas than others? The measurement of the number of individuals in a certain area is called a population, which can come in all shapes and sizes. They represent groups of individuals that may be very large or small in size, may be spread out or tightly packed together, and may contain a diverse variety of individuals or individuals that are very similar.

For example, we can refer to the human population in the city of Chicago, but we could also refer to the human population of the entire planet. These are two very different groups but both are populations! We call the area over which a population occurs its distribution and the number of individuals in that area its abundance.

But no matter their shape, size, or variety all populations are subject to different environmental effects and factors that influence how they grow and sustain themselves.


Food is by far one of the most important environmental resources that affect population size. All organisms need to take in nutrients in one way or another, if they don't they will starve and die. As individuals in a population die out the size of the population decreases, especially if new individuals aren't being born into the population fast enough to replace them.

All organisms need food to survive. The amount of food available is a major influence on the size of a population.
sheep grazing in a field

However, this can be a bit of a double-edged sword. If the population grows large enough there may not be enough resources to sustain it. Think about it this way: if you buy only enough food at the grocery store for your 4-person family but three of your friends come over for the weekend there won't be enough food for everyone. You can go back to the grocery store to get more food, but this isn't an option for other types of populations. Instead, as the population grows it either has to spread out or move to a new area to find more food.

Disease & Predation

Another factor that can greatly affect the size of a population is disease. When disease enters a population it may affect only a few individuals or it may affect a large percentage and endanger the survival of the population. For example, if the disease spreads easily from one individual to another, more individuals will be affected quickly which will have a greater influence on the size of the population.

Predation removes individuals from the population, affecting its size.
lions hunting

Disease may also influence which animals are more susceptible to becoming someone else's dinner. Large predators like lions will identify the sick or weak members of a herd and go after them as opposed to chasing the healthiest and fastest individuals.

Predators themselves feel the effects of fluctuations in the size of other populations. If there are more predators than prey, the predators may exhaust the prey population and end up starving themselves! This is why prey organisms tend to be more abundant than predatory organisms; if predators eat all of the prey they too will have a difficult time surviving for very long.

Water & Shelter

Like food, water is also a limiting factor of population size, meaning that it influences population growth or decline. All organisms need water to survive but what varies is how much water the population uses compared to how much is available. For example, desert animals fare far better without water than do animals adapted to the tropical rain forest.

Water also serves other purposes aside from consumption and bathing. For example, fish and many other specialized organisms can only live in water, so a healthy water ecosystem provides essential habitat for these populations. Amphibians lay their eggs in water and without this resource their populations would surely decline rapidly.

Shelter on land is also important to many organisms like mammals, reptiles, and birds. Shelter provides protection from weather and also from predators..

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