To examine energy flow through biosphere: look at groups of organisms and environments. Multiple individuals of one species living in the same area are a population. Interacting populations of different species are communities. A community plus its associated non-living systems (soil, air, water etc.) form an ecosystem. Similar ecosystems are grouped into classes called biomes. For example, the tropical rain forest biome includes ecosystems in Africa, South America, Australia etc.

Organisms (mostly plants) that obtain energy directly from non-living sources, by photosynthesis or chemosynthesis, are called “autotrophs” (from Greek words for “self” and “feeding”). Other organisms are known as heterotrophs (“other feeding”); this indicates that they obtain organic matter by consuming the bodies of other organisms. Heterotrophs then obtain energy by reacting some of that organic carbon with oxygen. Animals are typically heterotrophs, but so are some plants and most fungi.

Primary production

The organic carbon and energy captured by autotrophs in an ecosystem represent the ecosystem’s gross primary production. Gross primary production is typically measured by the number of grams of inorganic carbon that are captured per square metre in a year. Some of this carbon is respired by the same autotrophs that captured it. Typically 25% – 75% of organic carbon is respired by autotrophs in this way (average ~50%). Te remaining stored organic carbon is net production.

Net production represents the carbon that is available from each trophic level. However, not all the net production makes it to the next trophic level because a certain amount is exploited by organisms that live on decomposing dead organic matter: decomposers.  Once this is removed, typically about 10% of net production makes it to the next level in a conventional “trophic pyramid”.

However, these numbers vary considerably by ecosystem. The graphic below shows actual numbers from a one of the first comprehensive studies of a whole ecosystem: the freshwater environment of Silver Springs in Florida, U.S.A.

Niches and competition

Each organism in an ecosystem is said to occupy a distinct ecological niche in its food web and ecosystem, defined by its habitat and position in food web. If two organisms come to occupy the same ecological niche in an ecosystem, they tend to compete: one eliminates the other. For example the introduced American Grey Squirrel is causing decline and possible extinction of the British Red Squirrel in the British Isles, because the two animals occupy the same ecological niche. The North American Gray Squirrel is known as an invasive species.

Biomes

Biomes, also known as ecosystem classes or ecozones, represent collections of ecosystems that have similar characteristics of energy flow, climate, and terrain, but are located in different places, often on different continents.

It’s possible to compare the gross and net production of different biomes to see how energy and carbon flow through the biosphere. The chart^[1] shows a simple classification of the biosphere into biomes, and has three parts. In the first column, the areas of the Earth occupied by different biomes are shown. By far the largest, of course, is the open ocean, which is followed by the continental shelves.  Land-based biomes each occupy much smaller areas, but in total coverage the land biomes are about 29% of the Earth’s surface, while marine areas cover the other 71%.

The second column shows estimates of the average net production per unit area of each biome. This is shown as the number of grams of inorganic carbon that are captured in each square metre in the course of a year.  The most productive biomes from this point of view are the tropical rain forests on land, and the shallow, mostly tropical, microbial beds and reefs in the ocean. Swamps and marshes on land (but in some cases along the coast) come third in primary productivity, although there is quite a lot of variability in this biome, which has led to some arguments about what is the best estimate of their average productivity.The third column is obtained by multiplying together the numbers in the first two.  You will see that there are two clear winners! Not surprisingly, the tropical rain forests are one of these. The combination of high temperatures, abundant light for photosynthesis, and lots of moisture (from rainfall under the intertropical convergence zone) lead to net production that is far ahead of any land biome.  Perhaps surprising, the open ocean is the other winner. This is due to its enormous area, although the same favourable environmental conditions are present at least in parts of the open ocean as in the tropical rain forests – moisture, heat, and light. The primary productivity in the open ocean is mostly due to microscopic free floating plants known as phytoplankton that trap light near the ocean surface. These plants supply the net carbon productivity that feeds almost all the heterotrophic organisms that inhabit the deeper parts of the ocean. (Why ‘almost all’? There are autotrophs that live on the ocean floor that obtain their energy by chemosynthesis, not photosynthesis, and contribute a small part to the production of the open ocean.)

Biomes and provinciality

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