Populations

Populations

By Alejandra Rivera

What Is a Population, and What Factors Affect Population Sizes?

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Have you seen leopards in the zoo or on television? These big cats live in many different environments. Like most cats, they need certain things, such as an energy source and water, to survive.

The world’s rarest cat lives in the far east of Russia and a neighboring part of China.

It is called the Amur leopard. In 2015, there were fewer than 70 Amur leopards living in the wild. That was an increase from the 30 wild leopards living in 2007.

What factors in the Amur leopard’s environment affected its population?

Winter makes hunting hard for the Amur leopard. There is no cover and little prey. Hunting and loss of habitat have made survival difficult for this big cat.

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All of the Amur leopards in eastern Russia are one population. Biologists use population numbers to determine if a specific type of living thing is endangered. That means it is at a high risk of extinction in the wild. In 2016, the World Wildlife Fund listed 33 endangered species and 17 as critically endangered, including the mountain gorilla and the Amur leopard.

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So what factors, including those in their environment, caused these animals to become endangered? How do scientists keep track of population sizes? What would happen if the populations decreased or increased? In this concept, you will learn the answer to those questions and more as you explore the science of populations.​

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Open Ended

What is necessary for survival and what is not?

Necessary: _______

Not necessary: __________

How Do Populations Interact within Communities and Ecosystems?

A population is a group of organisms of the same species that live in the same area. This could be a population of mallard ducks. It could be a population of swallowtail butterflies. Or it could be a population of humans. Organisms of the same species can breed with each other.

An ecosystem will have many different populations. Each population will be made up of members of a particular species. A forest might have a population of blue jays. It may also have a population of gray squirrels and a population of oak trees. All of the populations of different organisms that live in an area make up a community. An ecosystem is made up of a community of interacting populations as well as nonliving factors. These factors include rocks, soil, water, and weather patterns.​

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A population does not have to consist of only a single species of vertebrate organisms. All species have populations. Examples include species of plants, fungi, algae, and bacteria. Within a community, populations influence each other in many ways. For example, a population of wolves may prey on a population of deer. Similarly, the population of deer eats acorns from a population of oak. Many species depend on each other for food, shelter, and other resources within an ecosystem. The size of one population depends a lot on the size of many other populations. If one population grows or shrinks, other populations may change as a result. This can affect populations that depend on them. This is a type of ripple effect. It spreads throughout the ecosystem, and can have a big impact on a system in balance.​

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Within an ecosystem, abiotic factors also affect populations. Abiotic factors are nonliving parts of the environment. These factors include the climate or habitat. Imagine that a grassland has a drought. The population of a grass species may decrease. This could lead to a decrease in food for a species of small insect. Many small bird species feed on small insects. So the populations of small bird species may also fall. This will decrease a population of a snake species that preys on their nestlings. If the rains return, all these species may again increase in numbers. The connections among populations form a food web. This web includes the populations that feed, and are fed upon by, other populations in a community, habitat, or ecosystem.

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How Do Various Factors in a Habitat Limit Population Size?​

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There are many factors that influence the size of a population. Populations do not continually increase over time. This is because other populations that may prey on them limit them. The resources available in the environment also limit populations. These resources include food, water, sunlight, and shelter. The carrying capacity of an ecosystem refers to the size of a population that is supported by the resources in a given location.

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A population of wild horses is affected by the availability of water and food, such as grasses. Predators, such as mountain lions, also affect them. A population of grasses is affected by the amount of rainfall and the growing season. A population of mountain lions is affected by the availability of prey organisms. All of the populations in a given location are influenced by both abiotic (nonliving) and biotic (living) factors.​

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Natural causes and human activities can change biotic and abiotic factors in an environment. These changes affect the environment’s carrying capacity. Carrying capacity is the greatest number of individuals in a population that an ecosystem can support. The carrying capacity is limited by factors. An example is the amount of food. Natural enemies also limit the carrying capacity. These could include parasites and predators. Carrying capacity is most affected by the availability of energy. Organisms must be able to get enough energy. They need energy to have sufficient resources to reproduce. In some cases, the environment changes so much that some organisms can no longer survive and reproduce. When the needs of a species are no longer met, it can no longer survive to reproduce. Then its population will decrease. Eventually the species might become extinctglossary term (opens in a new window).​

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What is a population?

A population consists of all of the organisms of a single species living in an area.

 

What factors in a habitat can limit population size?

Population size is affected by changes in the surrounding habitat. Populations are limited by the availability of food and water, access to sunlight, and the availability of shelter. The carrying capacity of a habitat refers to the number of organisms the habitat can support based on the availability of these limiting factors.​