Ecosystem

An ecosystem is a functional unit of nature in which living organisms interact among themselves and also with the surrounding physical environment. It has no fixed size. A single drop of pond water, a rotting log, a paddy field, a lake, a forest, or the entire biosphere can each be studied as an ecosystem, depending on the question being asked. The biosphere itself is treated as a global ecosystem, a sum of all local ecosystems on Earth.

Ecosystems are broadly split into terrestrial ones such as forest, grassland and desert, and aquatic ones such as pond, lake, wetland, river and estuary. Some are made and maintained by people: a crop field and an aquarium are man-made ecosystems. Because a pond is small and easy to observe from edge to edge, it is used again and again as the teaching example of a self-sustaining unit where all the four functions of an ecosystem can be seen working at once.

The structure of an ecosystem is described in two ways. The first is species composition, which is simply an account of which species are present and in what proportion. The second is stratification, which is the vertical arrangement of species into layers. In a forest, tall trees occupy the topmost layer, shrubs occupy the layer below them, and herbs and grasses occupy the bottom layer. Stratification matters because light, humidity and temperature differ from layer to layer, so different species can live in the same patch of ground without competing for exactly the same conditions.

Every ecosystem is built from abiotic components such as sunlight, temperature, rainfall, soil, and dissolved inorganic and organic substances, and from biotic components. The biotic components are grouped by how they get their food. Producers or autotrophs, mainly green plants and phytoplankton, fix solar energy into organic matter. Consumers or heterotrophs eat other organisms. Decomposers or saprotrophs, chiefly fungi and bacteria, break down dead bodies and waste and return the nutrients to the soil and water.

The function of an ecosystem is studied under four headings, and the rest of this chapter takes them one at a time: productivity, decomposition, energy flow and nutrient cycling. Keep the split clear in your mind. Structure answers what is present and where; function answers what is happening and at what rate.

All the organic matter in an ecosystem starts with photosynthesis, so an ecosystem is judged first by how much organic matter its plants can build. Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed as weight, in g m-2, or as energy, in kcal m-2. Because productivity is a rate, a time period must be attached: primary productivity is expressed as g m-2 yr-1 or kcal m-2 yr-1. Notice the difference in wording. Production is an amount; productivity is an amount per unit time.

Primary productivity is measured in two forms. Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis. A part of this is used up by the plants themselves in respiration. What is left over is net primary productivity (NPP). The relation is stated as:

NPP = GPP - R

Here NPP is net primary productivity, GPP is gross primary productivity, and R stands for the respiratory losses of the producers, that is, the organic matter the plants oxidise to run their own metabolism. Since R can never be zero in a living plant, NPP is always smaller than GPP. The importance of NPP is practical: it is the biomass that is actually available for consumption by the heterotrophs, meaning the herbivores and the decomposers. The quantity that supports the consumers of an ecosystem is therefore net primary productivity, not gross primary productivity.

Secondary productivity is defined as the rate of formation of new organic matter by consumers. Consumers do not create organic matter out of carbon dioxide and water; they only rebuild plant or animal matter that already exists into their own tissue, and they lose a great deal of it in respiration, faeces and heat while doing so. This is why secondary productivity is small compared with primary productivity.

Primary productivity varies enormously from place to place. It depends on the plant species growing in the area, on environmental factors such as light, temperature and rainfall, on the availability of nutrients, and on the photosynthetic capacity of the plants themselves. Taking the biosphere as a whole, the annual net primary productivity is roughly 170 billion tons of organic matter, measured as dry weight. Of this the oceans account for only about 55 billion tons, even though they cover about seventy per cent of the surface of the Earth. Open ocean water is nutrient-poor over most of its area, which is why so much surface yields so little.

Decomposition is the process by which decomposers break down complex organic matter into simple inorganic substances such as carbon dioxide, water and nutrients. The raw material of decomposition is detritus, which is dead plant remains such as leaves, bark and flowers together with dead animal remains, including faecal matter.

Five steps are recognised. Fragmentation comes first: detritivores such as earthworms break the detritus into smaller particles, which greatly increases the surface area available to microbes. Leaching is the process in which water-soluble inorganic nutrients seep down through the soil layers and get precipitated as unavailable salts; note that leaching removes nutrients from circulation rather than releasing them, so it is not the same as mineralisation. Catabolism is the step in which bacterial and fungal enzymes degrade detritus into simpler inorganic substances. These three steps do not happen one after another in a queue; fragmentation, leaching and catabolism operate simultaneously on the detritus.

The last two steps take place in the soil. Humification leads to the accumulation of a dark coloured amorphous substance called humus. Humus is highly resistant to microbial action and therefore decomposes at an extremely slow rate. It is colloidal in nature, and it acts as a reservoir of nutrients, releasing them slowly. Mineralisation is the final step, in which some microbes degrade the humus further and release inorganic nutrients into the soil. When the five steps have to be listed in order, the sequence runs: the detritus is broken up, washed, chemically digested, turned into humus, and finally stripped of its last minerals. Treat that as a listing order rather than a strict timetable, because the first three overlap in time and only humification and mineralisation follow one another in the soil.

The rate of decomposition is controlled by two things. The first is the chemical composition of the detritus. Decomposition is slower if the detritus is rich in lignin and chitin, both of which resist enzyme attack; it is faster if the detritus is rich in nitrogen and in water-soluble substances such as sugars. The second is the climatic factors of the site. A warm and moist environment favours decomposition, while low temperature and anaerobiosis, meaning a lack of oxygen, inhibit it and cause organic material to pile up. This is why litter rots away quickly on a warm humid forest floor but accumulates as thick peat in cold waterlogged ground.

One more point deserves emphasis. Decomposition is largely an oxygen-requiring process. Because oxygen is needed, drainage and aeration of the soil change the rate of nutrient release as much as temperature does.

The sun is the only source of energy for practically all ecosystems on Earth. Of the solar radiation falling on the Earth, less than fifty per cent is photosynthetically active radiation (PAR), the wavelengths plants can actually use. Out of this PAR, plants capture only about two to ten per cent. That small captured fraction has to support every animal in the ecosystem, which is the physical reason food chains are short.

Two rules govern the movement of this energy. Energy flow is unidirectional: it enters as sunlight, passes from producers to consumers, and leaves as heat. It never cycles back to the sun and it never flows from consumer back to producer. Nutrients, in contrast, are cycled. The second rule is that at every transfer a large part of the energy is lost as heat in respiration, so the amount of energy available goes down sharply at each successive step.

Organisms are placed in trophic levels according to their source of food. Producers form the first trophic level, herbivores or primary consumers the second, primary carnivores or secondary consumers the third, and secondary carnivores or tertiary consumers the fourth. The animals at the last level are the top carnivores. A given species can sit at more than one trophic level depending on what it is eating at the time, which is exactly why a real feeding network is a food web, a set of interconnected food chains, rather than a single straight line.

The amount of living material present at a trophic level at a given moment is the standing crop. It is measured either as the number of organisms in a unit area or as their biomass. Biomass may be given as fresh weight or dry weight, and dry weight is the more accurate measure because water content varies. Keep standing crop separate from productivity: standing crop is a quantity present now, while productivity is a rate of production.

The ten per cent law, put forward by Lindeman, states that only about ten per cent of the energy stored at one trophic level is transferred to the next trophic level. The remaining ninety per cent is lost mainly as heat in respiration, and some is never eaten or never digested. Since each step keeps only a tenth, the energy left after a few transfers is too small to support another level, so food chains in nature usually have only three to five trophic levels.

There are two kinds of food chain. The grazing food chain (GFC) begins with living producers being eaten by herbivores. The detritus food chain (DFC) begins with dead organic matter and is made up of decomposers, chiefly fungi and bacteria, which secrete digestive enzymes onto the dead matter and absorb the products. In an aquatic ecosystem the grazing food chain is the major route of energy flow. In a terrestrial ecosystem a much larger fraction of the energy flows through the detritus food chain instead. The two are not sealed off from each other; organisms of the detritus food chain are eaten by animals of the grazing food chain, so the two are linked at several points.

An ecological pyramid is a diagram that compares the trophic levels of an ecosystem by stacking them, with producers forming the base and the top carnivores forming the apex. The bar for each level is drawn in proportion to some measured value, and there are three such values, giving three kinds of pyramid: the pyramid of number, the pyramid of biomass and the pyramid of energy. Two words are used to describe the result. A pyramid is upright when the measured value falls steadily from the producers up to the top carnivores. It is inverted when the value at some higher trophic level is greater than the value at the level immediately below it.

The pyramid of number counts individuals at each level. In a grassland, many grass plants support fewer grasshoppers, which support still fewer frogs and snakes, so the count falls at each step and the pyramid is upright. In a forest built around large trees the picture reverses. A single big tree supports a very large number of insect herbivores feeding on it, and those insects support a smaller number of birds. Because one producer supports thousands of primary consumers, the count at the second level is far larger than at the first, and the pyramid of number becomes inverted.

The pyramid of biomass uses the mass of living material at each level. On land it is normally upright, because the total mass of vegetation greatly exceeds the mass of the animals living on it. In the sea it is generally inverted. The phytoplankton that act as producers are tiny and short-lived, but they multiply and are eaten so fast that a small standing mass of them can feed a much larger standing mass of fish and other consumers. So the measured biomass of the consumers at any instant exceeds that of the producers, even though the producers still supply all the energy over the year. The lesson to carry away is that biomass is a snapshot of what is present, not a record of what was produced.

The pyramid of energy compares the energy that flows through each trophic level over a period of time. Because every transfer loses a large part of the energy as heat, each level can only pass on a fraction of what it received. For this reason the pyramid of energy is always upright and can never be inverted.

Pyramids carry three limitations worth knowing. They do not accommodate a food web, since they assume a simple linear food chain. They do not handle a species that belongs to two or more trophic levels at once. And saprophytes, that is the decomposers, are not given a place in an ecological pyramid even though they play a vital role in the ecosystem.

The communities living in an area do not stay the same for ever. Ecological succession is the gradual and fairly predictable change in the species composition of a given area, in which one community is replaced by another until a stable community is reached. The whole sequence of communities that appear one after another in an area is called a sere, and each of the transitional communities within it is a seral stage or seral community. The species change, and so does the total number and kinds of species present, along with an increase in total biomass. The final community, which is in near equilibrium with the environment of the place, is the climax community.

Succession is of two types depending on what was there to begin with. Primary succession starts in an area where no living organisms ever existed. Newly cooled lava, a bare rock face, a newly created pond and a newly made reservoir are the standard examples. Here soil has to be formed before higher plants can establish, so primary succession is very slow; it may take thousands of years for a climax community to develop. The first organisms to colonise such a site are the pioneer species.

Secondary succession begins in an area that lost all its living organisms but still has its soil. Abandoned farmland, a burned or cut forest and land that has been flooded are examples. Because the soil, with its seed bank and nutrients, is already present, secondary succession is much faster than primary succession. Speed, and the presence or absence of soil, are the two features to quote when the two types are compared.

Succession is also classified by the water condition of the site. Hydrarch succession takes place in wetter areas, such as a pond, and moves from hydric conditions towards mesic conditions. Xerarch succession takes place in very dry areas, such as bare rock, and moves from xeric conditions towards mesic conditions. Note the common endpoint carefully: both hydrarch and xerarch succession converge on mesic conditions, that is, moderate moisture, neither too wet nor too dry.

In xerarch succession on bare rock, lichens are usually the pioneer species. They secrete acids that dissolve the rock and help form a thin layer of soil, which allows bryophytes such as mosses to establish. Larger herbs follow, then shrubs, and finally trees, so the climax is a forest. In hydrarch succession in a water body, the pioneers are small phytoplankton. These are replaced with time by rooted submerged plants, then rooted floating plants, then reed swamp and marsh vegetation, then scrub, and finally trees. As sediment and dead plant material fill the basin, the water body is slowly converted into land, and the climax is once again a forest.

Two general points follow. Succession in an area disturbed by natural events or by human activity starts again, so the seral stages can be seen repeating in the same area over time. And whatever the starting point, wet or dry, the direction of change is towards the mesic climax community that the climate of the region can support.

Energy flows through an ecosystem and is lost, but nutrients are used again and again. The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling, and because these cycles involve both living organisms and the geological environment, they are also called biogeochemical cycles. Alongside standing crop, an ecosystem has a standing state, which is the amount of nutrients such as carbon, nitrogen, phosphorus and calcium present in the soil at any given time. The standing state varies greatly from ecosystem to ecosystem and also with season.

Nutrient cycles are of two kinds. In a gaseous cycle the reservoir of the element is the atmosphere, as in the carbon and nitrogen cycles. In a sedimentary cycle the reservoir is the Earth's crust, as in the sulphur and phosphorus cycles. Environmental factors such as soil moisture, temperature and pH regulate how fast nutrients are released from the reservoir into circulation.

In the carbon cycle, carbon makes up about forty-nine per cent of the dry weight of organisms and is therefore the backbone of all organic matter. About seventy-one per cent of global carbon is found dissolved in the oceans, and this oceanic store regulates the amount of carbon dioxide in the atmosphere. The atmosphere itself holds only about one per cent of total global carbon. Carbon enters the living world when photosynthesis fixes atmospheric carbon dioxide, and roughly four times ten to the power thirteen kilograms of carbon are fixed in the biosphere through photosynthesis every year. Carbon returns to the atmosphere as carbon dioxide through the respiration of producers and consumers, through the activity of decomposers on dead organic matter, through burning of wood and forest fires, through combustion of fossil fuel, and through volcanic activity. Human activities, chiefly deforestation, the burning of fossil fuel and forest fires, add extra carbon dioxide to the atmosphere.

The phosphorus cycle works differently. Phosphorus is a major constituent of biological membranes, of nucleic acids and of cellular energy transfer systems, and animals also need large amounts of it for shells, bones and teeth. The natural reservoir is rock, which contains phosphorus in the form of phosphates. When rocks are weathered, small amounts of these phosphates dissolve in soil solution and are absorbed by the roots of plants. Herbivores and other animals obtain this element from plants. Decomposers, including phosphate-solubilising bacteria, release phosphorus from the waste products and dead bodies of these organisms and return it to the soil.

Two differences between the carbon and phosphorus cycles must be stated exactly. First, atmospheric inputs of phosphorus through rainfall are much smaller than the carbon inputs. Second, gaseous exchanges of phosphorus between the organism and the environment are negligible, whereas carbon moves freely between organisms and the atmosphere as carbon dioxide. That is the reason carbon has a gaseous cycle and phosphorus has a sedimentary one.

Finally, the products of ecosystem processes that people benefit from are called ecosystem services. Healthy forests and other natural ecosystems purify air and water, mitigate droughts and floods, cycle nutrients, generate fertile soil, provide habitat for wildlife, maintain biodiversity, pollinate crops, store carbon and provide aesthetic, cultural and spiritual value. Economists have attempted to put a money value on these services. In one such estimate, Robert Constanza and his co-workers valued the natural services of the biosphere at an average of about thirty-three trillion US dollars a year, close to twice the value of global gross national product at the time. In that estimate soil formation alone accounted for about fifty per cent of the total, while climate regulation and provision of habitat for wildlife came to about six per cent each, and services such as recreation and nutrient cycling came to less than ten per cent each.