What limits population size is one of the big questions in ecology (the science of how living things interact with each other and their environment). The answer depends on the species, but the same species can be limited by different things at different times. For example, food may be limiting one year, then disease can be limiting the next.Ecologists often talk about population density, meaning the number of individuals in a given amount of space. More individuals in a place mean a higher density.

Space can be limiting in different ways. It can limit the number of shelters from weather and predators, it can limit resources, it can even limit whether new individuals will fit. For example, barnacles need a hard surface to glue themselves to. When a rock is full of barnacles, the population of that rock can't get any bigger. A population of birds may be limited by good places to put nests. Parasites are limited by the size of the hosts that they grow in or on.

Plants need sunlight, water, and nutrients to grow, so only a certain number of plants can be supported by a certain amount of space. When plant density is too high, plants don't get enough resources. Some plants may die; others just may be smaller or may not be able to reproduce. New plants may not be able to grow in a high density population.

Ground squirrels eat plants. A ground squirrel population may be limited by space because their food takes up a certain amount of space. So while you could put more squirrels in that area, they'd starve because there's not enough space to grow enough food to support them.

Population density can affect disease and predation. The closer together individuals are, the more likely it is that they will pass parasites and diseases to each other. They will also be easier for a predator to find.

Sometimes having too much space can be a problem. Can you think of ways that a low population density could keep a population from growing?

If questions like this interest you, think about a career in ecology or environmental science.

Well, living things take up space. Your body takes up a finite volume, and in order to live, you need a certain area. If there are more people in an area than that area can support, then some of them die. It's easier to think of with plants: each plant needs a certain area to collect sunlight and water to survive, and if it doesn't have that area, it will die. This is why there can be only so many trees in, say, a square mile of forest.