The ecosystem
Ecology is the study of the totality of relationships between living organisms and between these organisms and the environment. One of the most important concepts within this discipline is that of ecosystem. The concept was introduced in 1935 by the British ecologist Arthur Tansley and considers living organisms and the physical, chemical and geological variables of their environment as a single interacting system.
The concept of ecosystem has ushered in a new way of approaching the study of nature.
A simply classificatory vision of species is replaced by a more functional approach to understanding natural processes, in view of the diversity of life inside a given ecosystem. An ecologist therefore takes a broad and systematic approach when studying ecosystems, blurring the line between energy and matter, while focussing attention more on the totality of relationships and interactions.
Energy flows
The ecosystem approach is based on the interactions between living organisms and the non-living elements (including water, air, soil, sediments, as well as organic and inorganic matter, known as ‘abiotic components’). Size does not really matter here. Ecosystems can be very small, for example ponds or forest clearings, or large, such as boreal forests or savannahs. The important thing is how energy enters the ecosystem and is used by living organisms and how the abiotic components enter into the complex biogeochemical cycle set in motion by this energy, which involves both living and non-living components.
In the great majority of terrestrial ecosystems the energy source is electromagnetic radiation from the sun. In the early 1940s, US ecologist Raymond Lindeman suggested to classify organisms inside a given ecosystem according to their function. Organisms that capture and use solar radiation are called ‘primary producers’ and are characterised by their ability to perform photosynthesis, transforming simple molecules (carbon dioxide and water) into more complex substances, such as sugars.
Diagram comparing autotrophic organisms, or producers (e.g. plants) capable of producing nutrients (sugars) through photosynthesis for their energy needs, and heterotrophs, or consumers (e.g. animals) which instead obtain energy by feeding on other organisms.
Primary producers include species that are very different in both anatomic and metabolic terms. They range from organisms such as cyanobacteria (mainly marine prokaryotes) or unicellular algae to more complex life forms found on dry land such as Gymnosperms (conifers), Angiosperms (flowering plants), ferns or mosses. Species in this functional groups are known as ‘autotrophic’, or able to produce their own food, due to the presence of molecules capable of capturing and using solar radiation.
Autotrophs introduce energy-rich molecules into the ecosystem, which are then also available to ‘primary consumers’, such as herbivores, feeding on plants, or marine zooplankton preying on phytoplankton. Organisms at this level are called “heterotrophs” because they use organic matter produced by autotrophs.
Primary consumers are in turn food for other ‘secondary consumer’. These also range from animals with a relatively simple structure such as cnidarians (jellyfish and corals) to mammals, birds, reptiles and fish, in a range of complex and interconnected relationships that make up an ecosystem’s ‘trophic network’.
But this network could not survive without organisms able to use dead organic matter or organic matter produced by the metabolism of other organisms. The heterotrophic component of the ecosystem can therefore be divided into the consumer and decomposer subsystems. Consumers feed mainly on living tissue, while decomposers break down dead matter into inorganic substances and return them to the environment. The dead organic matter that decomposers work on is therefore essential for the ecosystem’s nutrient cycle.
Diagram illustrating the relationships between the elements of the trophic chain in an ecosystem: the energy that comes from the sun is used by producers through photosynthesis, together with water and mineral nutrients present in the soil. The produced organic matter is consumed by herbivores heterotrophs, in turn nourishment for the carnivores, and both, once dead, nourishment for the decomposing organisms. The latter release nutrients into the soil making them available again to the producers so that the cycle can start again.
The percentage of energy passing from the autotrophic level to the heterotrophic level is never 100%. This is because energy is in part dispersed in the form of heat and in part used by the organisms themselves to maintain their body’s structure and metabolism. Homeothermic animals (mammals and birds) ‘spend’ a lot of energy to maintain an even body temperature. The percentage of energy passing from one level to another therefore varies greatly: anything from 2% to 24%, depending on the species involved.
Pyramid diagram which indicates how starting from the lowest level of the producers (or autotrophs), the transfer of energy gradually decreases in the upper levels occupied by the consumers. At each level the dispersion of energy released in the form of heat or retained to maintain a constant body temperature as in homeothermic animals (Birds and Mammals).
Biogeochemical cycles
Mineral components are constantly recycled in the functioning of ecosystems, passing continuously from the abiotic to the biotic components. The activities of organisms thus profoundly influence the flow of chemicals through an ecosystem and facilitate use and reuse of the same molecules and elements throughout the trophic network.
Diagram representing the biogeochemical cycles of elements such as nitrogen, phosphorus and carbon which, through the flows between the biotic and abiotic elements of the ecosystem, become available again in the environment (air, water and soil) so that the cycles are continuous.
Certain chemical elements – including nitrogen, phosphorus and carbon among many others – are essential for ecosystems. Carbon is a key component of all biomass, while nitrogen and phosphorus availability is crucial for regulating the productivity of ecosystems over much of the Earth. The bio-availability of these elements depends on their cycles through and inside ecosystems.
Diagram of photosynthesis which illustrates the two phases that take place inside the chloroplasts in the plant cell: the light-dependent phase, in which solar energy together with water is transformed into chemical energy in the form of ATP and NADPH molecules with the release of oxygen in the atmosphere, and light-independent or“dark” phase where ATP and NADPH with carbon coming from carbon dioxide captured from the atmosphere will be used to produce organic molecules (sugars) used by plants for their growth and survival.
The first step in carbon’s biological cycle is utilisation of gaseous carbon dioxide by autotrophic organisms to synthesise organic compounds. Photosynthesis is a two-phase process. In the first ‘light’ phase, reactions transform light energy into a temporary form of chemical energy, e. g. molecules such as ATP. Oxygen becomes a waste product that exits the leaves in this aerobic photosynthesis stage.
In the ‘dark’ phase, a plant’s cells use the products of previous reactions to synthesise sugars. These molecules are a more permanent form of chemical energy that can be stored, transferred elsewhere or metabolised. They can then be used by plants, algae and cyanobacteria for the synthesis of other organic compounds essential for their life.
Where light is present, both sets of reactions occur simultaneously in chloroplasts, organelles present in all organisms capable of photosynthesis. Sugars can be converted into the materials used to ‘build’ the plant body, such as cellulose, or stored up as more complex molecules, such as starch.
Terrestrial plants have developed three different types of photosynthesis that depend on environmental conditions, CO2 concentrations in the atmosphere and water availability. They use different metabolic pathways and different enzymes, but above all they have different levels of efficiency.
In the presence of high humidity, plants use ‘C3 photosynthesis’. More efficient ‘C4 photosynthesis’, on the other hand, is used in hot and dry climates with low CO2 concentrations. CAM photosynthesis is used by succulent plants growing in arid and semi-arid environments, as well as by epiphytes in tropical forests.
Other elements such as nitrogen and phosphorus circulate inside and among ecosystems, following pathways that are often elaborate and difficult to retrace. Nitrogen is a very important factor for determining the productivity of many terrestrial and marine ecosystems and most agricultural and forest ecosystems.
Unlike carbon, almost all nitrogen relevant to biogeochemistry is contained in a single reservoir (the atmosphere), with relatively small amounts found in oceans, rocks and sediments.
Like nitrogen, phosphorus is an essential but often scarce nutrient, limiting, for example, the productivity of high-altitude lake waters. Most of the phosphorus on the Earth’s surface is found in marine and freshwater sediments and terrestrial soils. Most organic phosphorous is found in plant or microbial biomass. Most of the phosphorous available to organisms comes from recycling such organic matter when it dies.
How organisms adapt to the environment
There are many different ways for organic matter and elements to pass from the first to the following steps, in a constant struggle to eat – by overcoming the defences of organisms we consider food – or be eaten – if we fail to defend ourselves from those who consider us food.
The main classes of organisms fall into three broad categories: herbivores (organisms that eat plants), carnivores (organisms that feed on animals) and detritivores (organisms that feed on non-living organic matter, normally the remains of plants and animals).
These categories may not capture the full trophic diversity of nature, but they are not arbitrary. Herbivores, carnivores and detritivores evolved to live off essentially different energy and food sources. Despite the great availability of plant sources of food in many land environments, herbivores still have to overcome the physical and chemical defences of plants. Certain physical defences are obvious, like thorns that discourage some herbivores while slowing down the feeding of others. Plants also use a range of more subtle defences. Grasses absorb large amounts of abrasive silica into their tissue to render it hard to eat.
Many other plants harden their tissue with large quantities of cellulose and lignin to produce fibrous leaves that are difficult to chew. Besides, most animals can digest neither cellulose nor lignin. Those that can are helped by bacteria, fungi or protists that live in their digestive systems. This suggests that cellulose and lignin in plants might be a first line of chemical defence against herbivores, which most herbivores overcome with the aid of other organisms. The desired result can therefore be obtained not just by competition and predation, but also by cooperation between different species.
Carnivores must overcome other obstacles, because their prey are often equipped with features and behaviours that make them hard to see, or else prevent or slow down their capture. Spines, armour, poisons, together with a range of body alterations, from crypsis to aposematism to various types of mimicry (Batesian, Müllerian and so on), either stop or hamper predators.
Detritivores and decomposers, for their part, feed on material of plant and animal origin that is released into the environment. In some environments this is a very abundant resource that nonetheless lacks important nutrients such as phosphorus and nitrogen. Detritivores recycle dead organic matter, making the nutrients in the detritus available to the ecosystem.
This results in ‘detritus chains or networks’, as opposed to the ‘grazing chains’ arising from matter and energy passing through living organisms. Taken as a whole, these chains interlink and complete to form the great network of interactions between organisms and the environment that defines the ecosystem.
