The term “carrying capacity” sounds like it means how many objects you could possibly carry in your arms -- which might be a fun experiment! That’s not how the term is used in biology, but the idea is actually pretty similar.

In biology, “carrying capacity” refers to how many organisms an environment can support. An environment has a certain amount of resources like space and food, which is only enough for a certain number of organisms. Each population within an environment has a carrying capacity, which is stated as an approximate number of individuals who can survive there.

For example, if a small garden had enough sunlight, water, and space to allow about 5 tomato plants to grow, you could say the tomatoes in the garden have a carrying capacity of 5. If tomato growth is only controlled by available resources, their population will approach their carrying capacity. That means if there are only 3 tomato plants in the garden, more tomato plants will grow. If there are 7 tomato plants, they will have to compete for access to space, light, or water. Because there’s not enough resources in the garden for that many plants, some won’t survive.

Again, you can also think about it just like carrying objects. Maybe you have enough strength and enough space between your arms to carry 10 baseballs. As you start to pick up baseballs, you still have extra space and strength, so you can pick up more. But, if you pick up 11, 12, or 13, you may start to drop some because they are too heavy or there just isn’t enough room for them.

In the natural world, populations rarely exist at their carrying capacity because there are a lot of other factors that control population size. Predation, disease, and weather conditions can also impact how big a population can grow. In our garden example, rabbits could get into the garden and eat some of the plants. Even though there is room and resources for 5 plants, maybe only 3 survive every year because of the rabbits. Natural disasters, environmental changes over time, and even human actions can also affect the size of populations in the wild.