How Ecological Factors Limit Population Growth

Instructor: Jayne Yenko

Jayne has taught health/nutrition and education at the college level and has a master's degree in education.

In this lesson, we are going to discover why populations do not continue to grow at unlimited rates, and what ecological factors limit population growth. We'll also discover how humans factor into population limits.

What is Population Density?

Let's imagine you're getting ready to set up an aquarium. You have to decide on freshwater or marine, size of tank, species of fish, filter, food, and where to put the tank in your home. While this is an artificial environment, you are playing Mother Nature making decisions that will determine how many fish your tank is able to sustain. In nature, similar factors determine how large a specific population will grow, and even whether that population will become extinct.

What is the population?
Population sign

We can divide these factors into two extremes:

  • Density-dependent, which impact populations only when they reach a certain size, such as food supply. These are also called biological factors.
  • Density-independent, which affect populations regardless of how large or small they are, such as earthquakes or floods. These are also called physical factors.

Density refers to the number of organisms within a population. It can be large or small, or somewhere in between. In some environments, one or the other factor may dominate, while others may have a combination. The environment itself is the ultimate cause of population stabilization. If you put two guppies in your aquarium, you'll probably have an explosion in the population. They have an ideal environment, which means lots and lots of guppies will be born.

However, once the population reaches a certain size, it should level off due to competition, predation, disease, or even natural disasters. That leveling off is called the carrying capacity.

In your goldfish bowl filled with guppies, unless you intervene, the population will, at some point, level off. The overcrowding, limited access to space, oxygen, and other resources (not counting food), plus the lack of ability to recycle waste will be limiting factors. Only one factor may severely limit population size. This is called the law of the minimum, which tells us that population growth is limited by the resource in shortest supply.

Let's look more closely at these extremes.

Density-Dependent Factors

Density-dependent factors include:

  • Competition for resources such as food, oxygen, water, etc. occurs when the population of a species reaches a large enough level where there aren't enough resources to go around. For example, some aquatic plants spread rapidly in ponds and use most of the oxygen needed by other plants and fish. If you have one guppy in your aquarium, food isn't going to be a problem, unless you forget to feed it. But if you have 50 guppies in a goldfish bowl, there is probably going to be competition for food, space, oxygen, etc. In nature, shortages of food will cause animals to die, or move to areas with more food and/or less competition. Unfortunately for your guppies, they are stuck in their goldfish bowl.
  • Predation - if the jackrabbit population in the high desert of Nevada migrates to Arizona in search of food, the foxes left behind will have less food. The foxes will be unable to maintain their population density, and some will die. On the other hand, a surplus of predators, such as wolves, can cause a density limit on the elk herd in Yellowstone National Park. They're eating all the elk they want, so the elk will not be reproducing as they had been. Predation isn't an issue in the goldfish bowl, unless you introduce an Angelfish, who will eat the guppies.

Predation affects populations that have reached a certain size.
Predation

  • Parasitism - this is where one species benefits while harming the other. Parasites work more slowly than predators. Some don't kill their hosts, but others do. Too many parasites on a single host can cause the death of the host. For example, ticks often swarm on young moose in spring, sucking their blood and introducing infections. This can cause the death of the moose.
  • Disease can wreck havoc on crowded populations. In fact, diseases need organisms to be in close proximity in order to spread from one organism to another. Think about cities like New York or Hong Kong. Millions of people live there and a new disease can spread rapidly. If you consider rural areas, such as Nevada or Wyoming, there aren't as many people living closely together, so diseases aren't able to spread as quickly, if at all.

Density-Independent Factors

Density-independent factors affect all populations the same, regardless of how large or small they may be. Examples of these types of factors include:

  • Natural disasters, such as droughts, floods, hurricanes, and fires, can all influence whatever populations are in the area at the time. Not only do these occurrences kill individuals in all populations, they also disrupt the availability of resources for survivors. For instance, your dog, Cujo, wags his tail and knocks over the goldfish bowl of guppies. Water and guppies go flying all over the room. Some guppies will be killed outright, while others may survive to be returned to their bowl.

Forest fires affect all populations, regardless of size.
Forest Fire

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