Biological processes

Energy from sunlight enters biological systems through photosynthesis, a metabolic process unique to plants. All other organisms, from humans and other mammals to insects and bacteria, rely directly or indirectly on plants for the energy they store as carbohydrates.

Plants are therefore called “producers,” and all the other organisms that consume other organisms (including plants) to get the energy they need to live are called “consumers.”

When small animals that consume plants are in turn eaten by larger animals, some of that energy is passed on to the larger animals. In this way, the energy is transferred as food energy on up the food chain. Some of the energy in the food chain is also consumed by fungi and bacteria when they break down dead plants and other organic material. For this reason, fungi and bacteria are called “decomposers.”

The rest of the energy is lost in the form of heat.

Photosynthesis

The energy in an ecosystem must be continuously renewed; it cannot be recycled. Energy enters the ecosystem only through plants, through a process called photosynthesis.

During photosynthesis, the chlorophyll in plant cells captures the Sun’s light energy and uses it to make carbohydrates from carbon dioxide and water. (Chlorophyll is the pigment that gives plants their green colour.) The oxygen that is also created during this process is released into the atmosphere. The plant uses the carbohydrates as a source of immediate and reserve energy, and as a means of building tissues (e.g., leaves, wood and fruit).

Respiration

During respiration, a plant does the reverse of what happens during photosynthesis. The plant consumes oxygen while oxidizing its store of carbohydrates. This creates carbon dioxide and water which are then released into the atmosphere.

All living organisms—plants, animals and micro-organisms—use respiration to get the energy they need from carbohydrates to perform all vital functions.

Decomposition

Decomposition of organic matter—plants, animals and micro-organisms—is the process through which basic elements are returned to the soil (e.g., nitrogen) or to the atmosphere (e.g., carbon, in the form of carbon dioxide). In this way, the elements are made available once again for use by plants and, ultimately, by all other living organisms.

Organic waste first feeds a host of small animals, including insects, earthworms and other invertebrates. These creatures break up the organic matter, digesting part of it, which facilitates the task of the fungi and bacteria that complete the decomposition process.Nearly all decomposition (99%) takes place in the organic waste on the ground—in plant litter (dead leaves and plants), animal droppings and dead animals. Barely 1% of the plant matter in a forest ecosystem is consumed by herbivores. Thus, nearly all decomposition (99%) takes place in the plant litter on the ground. Litter consists of dead leaves, plants and trees, as well as animal droppings and the bodies of dead animals.