12. From the individual to the ecosystem

12.1 Populations form communities

Imagine a lake that is full of perch. Each perch individual belongs to the species Perca fluviatilis. All the fish that belong to this species and live in the lake form the perch population of the lake. The individuals that belong to the same population are capable of reproducing with each other. In contrast, the perch from two different lakes belong to two different populations, as they cannot move between the two lakes.

The lake is also the home of many other species that form their own populations. All of these different populations living in the same enviroment form the lake's organism community. In communities, the different populations interact with each other. 

A community of organsims = the populations (groups of individuals) of all the species living in a single environment.


Individuals form populations. An individual perch is part of the perch population of its home lake.

12.2 From communities to ecosystems

The natural world around us is comprised of a large variety of different ecosystems. An ecosystem consists of the non-living (abiotic) environment of a certain area and the community of organisms which inhabits that environment. The organisms of an ecosystem interact both with each other and with their non-living environment.

The biosphere is a term that means the parts of our planet where life can be found.

Usually, the term "ecosystem" is used to mean a certain, well-defined part of the environment, such as a pond or a lake. The populations of an ecosystem are affected by the different environmental factors of that ecosystem. For example, the fish population of a lake is affected by such environmental factors as the water's oxygen and nutrient concentrations and acidity. 

Ecosystems are often connected to one another. For example, brooks and rivers often run into ponds or lakes. Likewise, these ponds and lakes are often connected by rivers to the nearest sea. Water ecosystems also often have connections with their surrounding land ecosystems, such as forests. 

An ecosystem = a community of organisms + their abiotic environment

Some of ecosystems are large, like the Baltic Sea. Larger ecosystems can also be divided into smaller parts: for example, the Baltic Sea can be divided into different archipelago ecosystems that are different to the ecosystems found in the open sea. The organisms of these ecosystems have adapted to their different conditions. For example, dark lakes are the home for organisms that survive with only a small amount of sunlight, whereas the organisms that inhabit sea ecosystems have adapted to the water's high salt concentration.

12.3 Ecosystems rely on energy from the Sun

Ecosystems receive their energy from the Sun. The energy from sunlight is converted into sugar by plants, algae, and photosynthetic bacteria. They use this energy to grow and to support their vital functions. The sugar produced by these organisms also forms the energy source of all the other organisms of the ecosystem.

Plants and algae are producers, as they use energy from the Sun to produce energy for their ecosystems. They do this through the process of photosynthesis.

Photosynthesis occurs in the chloroplasts  of plant cells. To photosynthesize, a producer needs light energy from the Sun, water, and carbon dioxide. Because sunlight is essential to producers, plants do not live in environments where sunlight is not available. 

To receive enough sunlight, aquatic plants often live near the surface of the water. Aquatic plants have no problems in receiving enough water to photosynthesize, but the amount of carbon dioxide in water is often quite limited. Aquatic plants receive carbon dioxide directly from water. If plants photosynthesize too efficiently, the resulting lack of carbon dioxide can become a limiting factor. Aerial shoot plants and floating-leaf plants gather carbon dioxide from the air.

Photosynthesis produces sugar and oxygen. Both of these substances are essential to the organisms of all ecosystems. The plants use most of the sugar themselves to grow and reproduce. Plants use some of the produced oxygen themselves as well, but release the majority of it into the environment, where it is used by other organisms.

The formula of photosynthesis is: carbon dioxide + water → oxygen + sugar, the reaction requires solar light energy

To grow, producers also need nutrients. They are chemical substances that plants need in order to grow. Nitrogen and phosphorous are two of the most important plant nutrients. Normally, these nutrients circulate through the ecosystem. 

12.4 Terminology

  • A population is a group of organisms from the same species inhabiting a single area. 
  • An organism community consists of all different populations living within the same area.
  • An ecosystem consists of the abiotic (non-living) environment and the organism community that exists within this environment.
  • A producer is a plant, alga, or cyanobacterium that produces sugar via photosynthesis. 
  • In photosynthesis, carbon dioxide and water are used to produce oxygen and sugar with the help of solar light energy.
  • A nutrient is a type of chemical substance that plants need to grow.