This chapter is a pre-review version.

Introduction

In this section we examine how reduced carbon, carrying energy, is cycled through the different organisms that make up the biosphere. In doing so, we will look at the different ways organisms obtain energy, whether by trapping it themselves or by feeding on other organisms. These relationships are studied by the part of biological science known as ecology.

As awareness of human impacts has increased, the term “ecology” has been increasingly used to designate awareness of these impacts and efforts to combat them. However, in its original, scientific sense, “ecology” refers to any set of relationships between organisms living in the same area.

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.

Ecosystems

Autotrophs and heterotrophs

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.

Trophic levels, food chains and food webs

Because all the energy enters a community via autotrophs, heterotrophs must survive by eating either autotrophs or other heterotrophs. Therefore, it’s possible to arrange species in a community on food chains in trophic levels. The trophic level of an organism indicates how many steps it is along its food chain from the autotrophic primary producers, which always belong to trophic level 1.

If we look at a whole community, these food chains combine to form a food web. Arrows represent flow of organic matter and energy. In complex communities, it may be harder to assign a unique trophic level to some organisms that have varied diets.

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; 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. ^[2] 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 surprisingly, 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 autotrophs known as phytoplankton that trap light near the ocean surface. These phytoplankton 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 around hot springs, not photosynthesis, and contribute a small part to the production of the open ocean.)

Biomes and provinciality

Barriers to migration have resulted in regions with distinct faunas and floras. These are known as biogeographic provinces, or sometimes as ecozones.

A single biome can exist in multiple provinces but is populated by different species. For example, mammals specialized for eating ants, commonly known as anteaters, exist in the rainforest biome on several continents. These animals are quite distantly related to one another in terms of their evolution – all they have in common is that they are mammals – but they have evolved some similar features, the most notable of which is a long snout.

Anteaters occupy equivalent niches in different biogeographical provinces

Echidna (Australia)	Tamandua (Central America)
Pangolin (Malasia)	Giant Anteater (Venezuela)

Provinciality in Earth History

We know enough about the ancient Biosphere, from fossil collections, to be able to identify biogeographic provinces that existed in the past. For example, during the Paleozoic Era of Earth history (around 540 – 250 Ma) fossils called trilobites were common. They are arthropods (animals with a hard jointed exoskeleton), now extinct, but probably relatives of modern king crabs and crustaceans. Many species have been described, and it’s clear that they show different faunal provinces.

Remarkably, the same species are found in western Newfoundland and in northwest Scotland. Eastern Newfoundland shows quite different species, but these are similar to those found in southern Britain:- England and Wales. Both lived in the continental shelf biome but in different provinces. This was one of the first lines of evidence for an ancient ocean, now known as the Iapetus Ocean, that had closed up as the supercontinent Pangea formed. West Newfoundland and NW Scotland lay on one side of that ocean while east Newfoundland and southern Britain lay on the other side of the ocean in a different faunal province. This was one of the earliest lines of evidence for the theory of plate tectonics, developed during the 1960s, and was figured out by Canadian geophysicist J. Tuzo Wilson in a paper entitled “Did the Atlantic close and then re-open”, published in 1966.

^[3][4]