Basic Needs

  • All living organisms have basic needs. Organisms must be able to acquire the things that they need from their habitats in order to ensure their survival.

Basic Needs of Animals

The basic needs of animals include shelter, food, water, air, space, waste removal, and a particular range of temperatures.
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  • Shelter—Animals need shelter because it protects them from the elements and from predators. A bird's nest is an example of shelter.
  • Food—Animals need food because it provides energy and nutrients necessary to the body to perform life functions. Some animals eat plants. Some animals eat other animals.
  • Water—Animals need water because it transports nutrients and wastes in the body, and it helps keep body temperatures constant.
  • Air—Animals need air because oxygen is needed for the chemical process that releases energy from food.
  • Space—Animals need space because overcrowded areas lead to starvation and disease.
  • Waste removal—Animals need waste removal because waste is produced as animals use food, water, and air for life processes. Animals must be able to get rid of this waste to survive.
  • Temperature range—Animals are best suited to survive in certain temperature ranges.
Animals meet their needs in the habitat, or environment, in which they live. The environment is everything that surrounds and affects an animal. This includes the plants and other animals in the area, rocks, soil, air and water. Both living and non-living things are part of an environment.

Basic Needs of Plants

The basic needs of plants include sunlight, water, air, minerals, and space.
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  • Sunlight—Plants get energy from the Sun. Chlorophyll found in plant leaves uses sunlight to change water, minerals, and carbon dioxide into food. This process is called photosynthesis.
  • Water—Plants must have water because it helps carry nutrients from the soil to the plant’s roots. Water is a part of plant cells and is a main ingredient for photosynthesis.
  • Minerals—Plants need soil because they use it to get water, nutrients, and minerals.
  • Air—Plants also get nutrients from the air. During photosynthesis, plants take in carbon dioxide from the air and release oxygen.
  • Space—Plants also need space to grow. If there are too many plants for the amount of sunlight, soil, minerals, or space, some of the plants will not survive.

A Walk in the Forest

Carrying Capacity & Population Dynamics

  • The growth of a population in an ecosystem is limited by the availability of resources. Populations can only grow to a certain point before there are not enough resources available for all of the organisms to survive.

Population Growth

Left to themselves (and assuming infinite resources), populations of organisms will experience exponential growth, in which the rate at which they grow is proportional to how many organisms there are. The image below shows an example of exponential growth.
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Populations do not experience exponential growth forever because they will eventually outgrow the resources available to them. The resource that limits the growth of a certain population is known as that population's limiting factor.

Limiting Factors

Limiting factors are things that limit a population's growth. These factors can be resources that organisms need in order to live and that are present in limited quantities. Or, they can be things that limit the growth of the population in other ways, such as a population's rate of reproduction or the presence of a disease. Limiting factors can be biotic or abiotic.
For a plant population, a limiting factor might be the availability of light, or it might be the availability of fertile soil. For a hyena population, a limiting factor might be competition for food with other predators, or it might be a low reproductive rate. The number of plants or hyenas in a specific ecosystem will be limited by the resource that is the population's limiting factor. Different ecosystems can have different limiting factors, even for similar populations.
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The limiting factor for a population of hyenas could be many different things, including competition with other predators or a low reproductive rate.
Limiting factors can be created by human activities. If there is a chemical spill that kills plant and animal life in an area, this could limit how successful the surviving populations can be. Humans also destroy large amounts of plant and animal habitats. So space is often a limiting factor for many wild plant and animal populations. All stable populations are subject to at least one limiting factor.
The limiting factor of a population determines the population's carrying capacity, or the maximum number of this kind of organism that a specific ecosystem can support over a long period of time. The carrying capacity is limited by the available energy, water, air, space, food, and minerals. It is also limited by the ability of the ecosystem to recycle dead organisms through the activities of decomposers, such as bacteria and fungi.
When a population is less than the carrying capacity, it tends to grow. When a population is greater than the carrying capacity, it tends to shrink.

Patterns of Population Growth

The pattern of population growth will vary as changes in environmental conditions arise. For example, variations might result from:
  • a change in the food supply
  • a change in the water supply
  • a disease epidemic
  • the creation of new vaccines
  • a change in temperature
  • storms, earthquakes, and other natural disasters
  • crowding and competition
  • predator-prey relationships


Since there are limited amounts of resources in an ecosystem, if one organism gets a particular resource, another does not. This leads to competition as two organisms try to access the same resources. Food, water, sunlight, and space are examples of resources that organisms compete for.
Plants and animals of the same species may compete for resources such as food, water, shelter, and space. Populations of different species will also compete with one other if their needs are the same as the needs of another population in that ecosystem.
For example, trees in a forest compete for sunlight. As one tree grows taller, the shorter trees are shaded by it, and they receive less sunlight. The shorter trees may die as a result.
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The tall tree and the shorter trees in this picture are competing for sunlight.

Population Dynamics

Populations in an ecosystem affect one another. A change in the number of one type of organism will cause a change in the number of other types of organisms. These changes can be studied by looking at the population dynamics of an ecosystem.
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The food web above represents four populations within a savanna ecosystem. Antelopes depend on leaves from trees as their major food source. African wild dogs and lions compete for access to antelopes as a food source.
The number of organisms within these populations will always be changing as some organisms are born and other organisms die. The population size of one species will also change in response to a change in the population size of another species.
For example, if a drought in the ecosystem caused many of the trees to die, the antelope population would decrease because there would not be enough food for all of the antelope to survive. If the antelope population decreased, the number of African wild dogs and lions would also likely decrease because they would be losing a major food source.
Another example would be if humans began hunting and killing many of the African wild dogs and lions. This would cause an increase in the antelope population because they would not have as many predators, which would lead them to eat more leaves from trees, and decrease the amount of this resource in the ecosystem.
All populations within an ecosystem are related in some way or another. This means that a change in even one population in an ecosystem can have drastic effects on the ecosystem as a whole.