Ecosystem Metabolisms and Functions
Created to support eco-literacy for designers and other stakeholders
engaged in the design and development of systems of production.
Read the academic paper here, or continue and read the overview below.
“Living matter is the most powerful geological force.”
Vladimir Vernadsky (Russian mineralogist and geochemist)
A short video introducing ecosystem metabolisms and functions. Password: ecosystems101
The word ecology is derived from the Greek oikos, meaning ‘house’ or ‘place to live’. Therefore, in a literal sense, ecology is a study of organisms ‘at home.’ Ecology is a distinct branch of biology that studies the interactions among organisms and their biophysical home.
Living organisms and their non-living environment are inseparably interrelated and interact upon each other. Any entity or ‘natural unit’ made up of a biotic community (e.g. an assembly of plants, animals and microbes) and abiotic (e.g. air, water and wind) factors, components or substances, that interact to produce a dynamically stable system, where materials are exchanged between the two factors, is known as an ecosystem.
Ecosystems have no particular size and can be as large as a desert or as small as a tree; they include agroecosystems, aquatic and marine ecosystems, coral reefs, forests, savannah and tundra and even our bodies. To follow the previous analogy, a habitat can be said to be an organism’s, or an entire community’s, ‘address.’
There can be ecosystems within ecosystems and overlaps of ecosystems. Within ecosystems there are individual species, populations of the same species, communities of different species, aggregates of narrow community groups, guilds of different species that exploit the same resource, and niches that describe the match of a species to a specific environmental condition.
Functional ecology is a sub-field of ecology that pays attention to the unique roles—or functions, that individual species, populations or communities provide within an ecosystem. For example, plants are a form of primary producer—they are able to produce their own food through the process of photosynthesis. Not all plants do this in the same way (and not all primary producers are plants), and so different plants can provide marginally different functions, and/or more than one function.
Within ecological communities, there are certain organisms that can have greater influence on the environment, and therefore also on other community members, than others. Examples include keystone species—which are considered fundamental in maintaining biodiversity within an ecosystem (such as star fish within certain marine ecosystems); foundation species—which are considered the ‘base’ or ‘bedrock’ of a ecosystem’s overall structure (such as coral within a coral reef); invasive species—which are foreign species that enter into an ecosystem and can become dominant; and ecosystem engineers—which are certain species that transform their environment, over varying space and time (such as beavers and their dams).
The role of species, individually or collectively, within an ecosystem has become an important way to understand the functions and services of ecosystems as integrative wholes. The maintenance of all life on Earth hinges on effective functioning ecosystems. Ecosystem functions denote those processes that regulate the flux of energy and matter through them; whilst ecosystem services are the benefits provided to humanity by functioning ecosystems—such as, clean water and air, and the provision of food and natural materials such as wood.
Energy mostly enters terrestrial ecosystems via plants. Therefore, they are the ‘energy gatekeepers’ for life. Through the process of photosynthesis, plants convert some of the solar energy that reaches Earth’s surface into chemical energy.
All life involves chemical reactions. So much so, that some say that life (biology) is in fact chemistry that can replicate itself. All chemical reactions change some materials—reactants, into new material(s)—products. Two examples of this from living systems, are respiration and photosynthesis.
Respiration is the decomposition of glucose with oxygen, into water and carbon-dioxide, releasing energy in the process. In order for this process to occur, a small amount of ‘activation energy’ is required (such as a catalyst), and as glucose and oxygen react, water and carbon dioxide are released, releasing energy that powers the production of ATP (the cellular energy storage and ‘currency’), and some heat.
Photosynthesis, is in principle the reverse process, requiring energy from the Sun to push water and carbon dioxide ‘back up the hill’ to produce glucose (one of the energy stores, sources of the cellular energy, and the base of material structures) and oxygen.
As can be seen through these brief examples, respiration releases energy, and so the final products have less available energy than the initial reactants—therefore, there is a reduction in potential to do work after the reaction; and, photosynthesis needs a continuous input of solar energy from the Sun, to ‘push the reaction back up the hill,’ creating final products that have more available chemical energy than the initial reactants.
Principally, a large amount of solar energy that hits the Earth is reflected, some is absorbed and remitted, and a relatively small amount (1-2%) is actually caught by producers, to power the process of photosynthesis. Consumers, such as 27 million grasshoppers can then eat around 1000 (US) tons of grass. Consumer predators, such as 90,000 frogs, can eat the grasshoppers, which can then be eaten by 300 trout (higher consumer predators). One human being can eat 300 trout per year.
Only around 10–20% of the energy stored as biomass in the prey (food) is transferred and stored as biomass in the preceding predator. A combination of reasons for this include the following: predators do not eat all the prey (e.g. some survive or die without being eaten), some elements are indigestible and are excreted (this can be around 20%), and a significant proportion is used to power respiration and is ‘lost’ as heat (this can be around 65%). Some of this heat keeps us warm while dissipating back into the environment. The elements that cannot be digested are excreted from the body as faeces or urine and decomposed by certain bacteria and fungi and other detritivores (‘debris eaters’). They extract the remaining energy from the system, while also respiring and dissipating heat.
Therefore, although there is some cycling or recycling of energy, the cycles are relatively short-lived, and so energy is said to flow through ecosystems, rather than cycle around it.
Gravity is also an input to the system, although in a different way from the solar energy coming from the Sun. Gravity is a phenomenon that forms all the stars and planets in the universe and maintains them in their interactive orbits, grasping all the rocks, liquids, and gases—and in the case of Earth, all life—retaining them together as a whole.
Along with solar energy, these two inputs also interact with the other non-living systems on Earth (e.g. the atmosphere, hydrosphere and lithosphere), which puts into motion the vast majority of Earth’s geochemical and biogeochemical cycles.
Life has the ability to turn non-living compounds into living compounds and back again, through a constant flow of elements. Life literally can turn rock into life, and life back into rock. The non-living (abiotic) elements in ecosystems include water, dead organic matter, crystalline inorganic minerals, and gases.
Liquid Water (H2O) is fundamental for life on Earth. Plants, for instance use water for photosynthesis, structure (turgor pressure), as a solution for drawing in dissolved nutrients, and for evapotranspiration–cooling. Water is also produced as a by-product of respiration by all aerobic organisms (breaking down—usually—simple sugars for fuel) with carbon dioxide and is also used for its ability to break apart molecules (either on its own or with enzymes)—known as hydrolysis. Through these examples, it is possible to imagine water cycling within organisms and around ecosystems, and how it is fundamental for all life.
Crystalline inorganic minerals are the rocks, pebbles, sands, silts and clays that make up the bedrock (lithosphere) of an ecosystem, and the mineral nutrients and salts that all life need to build matter. Although the type of bedrock determines which minerals maybe released into the soil and the type of ‘texture’ (e.g. clay or sand), it is their bio-availability that is more important for life. While around 90 different chemical elements can be found in soil, most rocks mainly comprise silicon (Si), aluminium (Al) and oxygen (O) – with the first two often bound to oxygen; and to a lesser extent iron, magnesium, calcium, sodium and potassium. All have a charge at the molecular level and combine to make up different minerals and salts. The polarity (negative or positive or neutral) of the charge is important, as it determines how mineral nutrients flow through soils, and if they are easily absorbed in water, or are held on surfaces. Clays, the smallest of the crystalline inorganic minerals, have vast surface areas covered with negative electrical charges, and can therefore hold a vast array of positively charged minerals.
Dead organic matter (or detritus) typically includes the dead bodies (e.g. animals, plants, fungi, bacteria and Protista), or dead parts of organisms (e.g. leaves, bark, skin, bone, seeds, pollen) and faecal materials–and can be around 60% carbon (dry weight). It is estimated that decomposers only respire around half of the carbon in organic matter back into the atmosphere; the remaining is left as (relatively) decay-resistant compounds, with only fungi able to break down the most complex components. Humus, which includes ulmic, fulvic and humic acids, is the by-product left over after this repeated decomposition. Humus can be thought of as a form of organo-mineral complex that contains carbon and nitrogen in specific ratios, plus a wide range of crystalline inorganic minerals. Like clays, humus also has a vast surface areas covered with negative electrical charges –and like clays can therefore hold vast amount of positively charged nutrients. Dead organic matter, such as masses of dead fungal hyphae, can also collectively hold several times their weight in water, and are important for water retention and its slow release.
Around 78% of Earth’s atmosphere (air) comprises nitrogen. In the atmosphere, nitrogen is mostly in the gas form of two nitrogen atoms, tightly triple-bounded together as dinitrogen (N2). Earth’s atmosphere also comprises of around 20.9% Oxygen (as an interesting note, the majority of the Earth’s oxygen is in the rocks, not in the atmosphere), also usually in pairs, and 0.9% Argon, which is an inert single element. The remaining, less than 1%, is made up of trace gases, such as hydrogen (H2), methane (CH4), and carbon dioxide (CO2). These figures describe dry air by volume, as water content can vary on a daily basis, but can be between 0.001% to 5%. The troposphere, the region of Earth’s atmosphere closest to the Earth’s surface, contains around 80% of the entire atmosphere’s mass–therefore the majority of all these different elements. The biosphere, the world’s oceans, water, rocks, winds, and incoming and outgoing solar energy, are constantly interacting with the atmosphere, exchanging elements and shifting thermal energy across space and time.
Metabolism is a process of chemical changes in living organisms, through the use of energy to build new matter (biosynthesis), repair and maintain existing elements, and collecting together and excreting metabolic wastes (gases and faecal matter). Metabolism has two parts– anabolism, the building up of substances (requiring energy), and catabolism, the breaking down of substances (which can release energy).
German biologist Wilhelm Pfeffer (1845-1920) is credited as the first to assign names to the different forms of feeding that organisms use to metabolise supplies from their environment.
In nature, there are principally three different forms of metabolisers, with a fourth, which is a hybrid of the main three. These are autotrophs (or producers), which are ‘self-feeders’; these organisms can build complex molecules from simple molecules and elements. Heterotrophs (or consumers) ‘other-feeders’, as these organisms need to eat ready-made molecules created by others. Moreover, saprotrophs (or decomposers) ‘dead-feeders’; these organisms also turn large molecules into smaller basic ones that the autotrophs can reuse. Finally, mixotrophs can bridge or switch metabolisms as required. There are finer distinctions within each category.
Producers predominantly eat via photosynthesis and need a source of light; therefore, they all exist near terrestrial or aquatic surfaces. These producers use this solar energy to eat the air – taking in CO2, and turning it into sugars, and more complex carbon (organic) structures.
Decomposers predominantly eat via absorption by first decomposing some or all of their foods outside of their bodies prior to it entering the cell(s). They do this by expelling enzymes, and sometimes acids (by fungi), into their surrounding environment, to attach to and break down complex/large molecules into smaller/simpler ones. Once pre-digested, the simpler molecules can be absorbed into their bodies.
Consumers are primarily animals (Kingdom Animalia) and predominantly all eat via ingestion, which is the physical process of taking food from the external environment and bringing it inside their bodies—generally through a ‘mouth’. This usually starts with the mouth at one end of the digestive tract and the anus at the other (known as a complete digestive system); however, there are some animals, such as jellyfish, that have one opening for both (known as an incomplete digestive system).
Mixotrophs have the ability to do a mix of some of the other forms of metabolism. Some are able to switch, depending on the resources available, whilst others require symbiotic relationships with other organisms to perform the different metabolic processes.
Vladimir Ivanovich Vernadsky (1863–1945) is credited as the first to use the different metabolism forms, identified by Pfeffer, as a way to classify all life. By classifying life through metabolisms and by not distinguishing between non-living and living elements, Vernadsky developed a classification and mental framework that looked at different forms of living matter in relation to each other and their environment and how they were actively affected by and affected their proper (and others’) contexts. This collective view of relationships between the living and non-living, in one particular place, describes an ecosystem.
The different forms of metabolism can be thought about functionally in that, collectively, they interact with each other and are essentially interdependent, with the different functions collaboratively forming functioning ecosystem wholes. Therefore, metabolism is a collective activity taking place between all organisms within an ecosystem – and Earth as a whole.
Plants ‘eat’ the air and transform CO2 into sugars—the organic building blocks for all life’s structures. Animals are an example of consumers, eating plants or other animals (or mushrooms—decomposers) for food as their source of energy. Decomposers, such as bacteria and fungi, feed on dead organic matter, and through their external digestion process of decomposition, break down complex molecules; through various indirect interactions, these become available to plants as fundamental mineral nutrients. Mixotrophs are important in directly or indirectly making nutrients available for, or sharing nutrients and information between, the other metabolisms—particularly as conjunctive symbionts, for example, which are one (temporary or persistent) collective organism and support different metabolic functions.
When one organism eats another, such as an animal eating a plant, this is a direct trophic (food and feeding) interaction. These trophic relationships can be bi-directional; for example, plants feed decomposers sugars (and other organic compounds), predominantly within exudates excreted from their roots, in exchange for direct or indirect nutrients. Indirect relationships occur via other organisms or the central ‘Nutrient Pools.’ For example, a plant may drop its leaves in the autumn, and these fall to the ground, feeding the decomposers (e.g. fungi) indirectly via the Nutrient Pools during the winter; alternatively, decomposer predators feed on decomposers and produce vast amounts of excrements, which add to the Nutrient Pools and become the main inorganic mineral nutrients for plants. Many consumers and producers have symbiotic decomposers living inside them (and on them), and in the case of the consumers, this can form bi-directional relationships similar to that described between plants and decomposers.
Through these interactions, matter/energy—energy within the chemical bounds holding compounds and molecules together is flowing through the ecosystem; and particular elements, such as nitrogen and phosphorous can cycle around and around in short cycles within the same ecosystem, or longer-slower cycles through biogeochemical cycles at the scale of the entire planet. For example, elements such as nitrogen are ‘fixed’ – eaten from the air by diazotrophs—nitrogen fixing bacteria. Thanks to these bacteria, nitrogen, which is the base of amino-acids, proteins and enzymes (for example) is made available for the rest of life, and as an element within a range of different compounds, can cycle around an ecosystem a few hundred times, before it goes back into the atmosphere from whence it came.
Life can be competitive – this story has been (too) well told, and often shown to us via wild-life programs, observing male animals fighting for mating and hierarchical positions, or predators chasing their prey. However, animals are not only competitive – or dinner for another, and are far from the only form of life on Earth, although these animal examples can make for dramatic television.
Since the earliest forms of life, bacteria, as single cells, have lived and evolved in collaborative groups. Some, for example, evolved the capability to fix nitrogen and share nitrogen between the group when supplies where low. Bacteria also have the ability to share genes, or small gene packets or viruses, giving them the ability to collectively adapt and evolve.
As more complex single cell and then multi-cell organisms evolved, it was through a collaboration between different single cell bacteria coming together to form more complex cells. As a collective whole, the different elements brought together different abilities, and continued to evolve together as greater wholes. Prototista’s (e.g. algae), fungi, plants and animals all evolved from these symbiotic begins. The legacy remains in plant cells, for example, which contain two different types of organelles (small organs) that were originally bacteria. These organelles are the chloroplasts where photosynthesis takes place, and mitochondria that are the energy centres of the cell, producing ATP. All forms of life that evolved after (and with) bacteria have mitochondria. We are already all symbionts at the cellular level.
Symbiosis (from the Greek meaning ‘living together’) is any type of close and long-term biological interaction between two different organisms, whether it is mutualistic (win–win), commensalistic (win–no loss or gain) or parasitic (win–loss). Symbiotic relationships that require some form of physical attachment (which usually includes some form of physical transformation by those involved)—in which two or more organisms create a bodily union; is known as conjunctive symbiosis. When one organism lives on the surface of another, it is known as ectosymbiosis, and endosymbiosisis when they live inside.
Coevolution is where two or more species reciprocally affect each other’s evolution through the process of natural selection. A classic example is between plants and pollinators.
Within both symbiotic and coevolution relationships, there is a form of division of labour, specialisation, and a form of exchange. Some relationships include a purposeful exchange, whereby one organism makes a food for another (such as nectar for pollinators), in return for a different type of food or a service (such as the act of pollinating). In other cases, it can be said to less purposeful, where a waste becomes a food, such as a respired gas or faecal matter. Further relationships can include sharks swimming into areas where cleaner fish can eat any parasites from their skin—the parasite in this case is not produced by the shark but forms a beneficial symbiotic relationship.
By coming together as symbionts, organisms are able to ‘straddle the line’ between the different forms of metabolism—therefore, many are collectively mixotrophs; and by forming coevolution relationships, organisms are able to meet their own needs, whilst supporting, and being supported, by another. Conceivably, it is through these perhaps most compromised forms of collaboration that they are often the foundational organisms in an ecosystem, helping to colonise new environments and put into action the primary stages of pedogenesis—the building of new soils and soil crusts, for instance. Relatedly, these collaborative organisms can create a niche in early ecological succession ecosystems, where nutrient cycles can be poor, the environment can be highly demanding—and neither may be able to survive alone.
A cascade describes a process occurring in a series or in a succession of stages, in which each stage derives from or acts upon the product(s) of the preceding stage. Cascades can describe a ‘falling’ – such as water cascading over rocks, or the initial activation of a sequence of events, or interactions between different elements within the same cascade, but are not necessarily directly interacting. Some examples include biochemical cascades, living cascades, trophic cascades and the related grazing optimisation hypothesis.
Biochemical cascades, are a series of chemical reactions that occur when initiated by stimulus. For example, a cut in the skin sets off the coagulation cascade, which leads to the formation of fibrin-a fibrous protein that clots the blood (along with platelets), forming a homeostatic plug over the wounded area.
Living cascades, can be a way to think about the constant turnover of matter in living systems, at three levels. Within our cells, proteins are constantly being deconstructed and made anew. This demands a constant input of energy, and new materials, as some are recycled, but others are disposed of waste. Entire cells are also turning-over, although on a slower cycle to proteins, they are also continuously being deconstructed and made anew. Depending on the organ in our bodies, the turnover of cells can be relatively fast (e.g. cells lining our acidic stomachs) or slow – from days (e.g. skin cells) to months to a year or so (e.g. bones). This turnover of our cells, means that materially we not the same person from one year to the next. At a third level, and a longer time frame, we also turning over as living organisms. We are born, we grow and we die. The cascade of the population can remain, but the individuals are different.
Trophic cascades describe indirect reciprocal interactions between different metabolic groups. Examples are generally discussed, which are said to be ‘top-down’ or ‘bottom up’ (or subsidy, but this will not be discussed here). Top down give example from ecosystems, where consumer predators (the so called ‘top’ of food pyramids) can have positive (indirect) affects on producers, by controlling primary consumers population, and therefore the consumption of the producers. Through these dynamics, producers can thrive, limited populations of herbivores can thrive (more food to go around), and predators can thrive (healthy producers, support healthy primary consumers, which then supports healthy predators). When consumer predators are removed from an ecosystem, then this dynamic balance can fail, leading to over feeding by primary consumers and unchecked growth of their populations. Examples include, sea otters (consumer predators), which help maintain sea urchin populations (primary consumers), in kelp forests (producers).
Variations of these processes exist, with two further classic examples. The first example is that of whales. Whales (consumer predators) eat krill (primary consumers). Krill eat plankton (photosynthetic plankton are producers). Whales also eat squid, which they find by diving down into the depth of the ocean. However, at these great depths the stress on their bodies (and the need to breath air) means, that they need to return to the surface to defecate. In this way, they are transporting nutrients, which can cascade to lower levels in the ocean, and bring it back up to higher levels, near the ocean surface. The photosynthetic plankton needs to be near the surface to have access to light, but also needs nutrients. The whale therefore is acting as a nutrient pump, supporting the production of producers, which also supports the food resource for krill, and therefore, the food source for the whales.
With the reintroduction of gray wolves into the Yellowstone National Park, the wolves changed the behaviour of the elk (primary consumers), and reduced their consumption of seedling riparian trees. More riparian trees were able to grow, which helped stabilise riverbanks, and reduce erosion. Therefore, these cascades can also have positive affects on non-living systems.
So far these have been top-down cascades, and usually bottom-up cascades are said to start with plants. However, within this framework, bottom-up includes what is happening below the soil, with the decomposers. An important bottom-up cascade in healthy soils, are the earthworms, nematodes and protozoa (decomposer predators), that feed on the bacteria and fungi (primary decomposers), which feed on the dead plant-biomass, or the plant exudates, or can be plant pathogens. The predators maintain populations, and this has similar benefits to consumer predators; however, they also defecate large amounts of minerals, as the bacteria and fungi are so nutrient dense. These are some of the key available nutrients for plants.
Grazing optimisation hypothesis, states that herbivore consumers, can have indirect benefits for their producer food source, which can overcompensate the damage, so that producer fitness can be increased.
As herbivores, such as bison, feast on grassland plants, they obviously reduce plant’s total biomass – and may kill some plants altogether. However, as herbivores eat they also excrete, urinate, and push mulch into the soil. This speeds up the cycling of inorganic nutrients, and increases the availability of nutrients to the decomposers through ‘pre-digestion’ via the consumers’ stomach. Some additional indirect mutualistic effects include herbivores eating or pushing down dead or dry biomass, creating space and light for new growth, and through eating the plants, the plants are provoked to mobilise resources (e.g. sugars) stored in the roots, which are released to the decomposers, also boosting nutrient cycling. Perhaps the greatest, and the least (or non-) contestable, indirect mutualistic interaction by herbivores feeding on plants is that they reduce the amount of total dry plant biomass (e.g. by eating the drier grasses or keeping the grasses pruned, and therefore, not allowing grasses to elongate and age so much); thus, they convert a significant proportion of the total ecosystem plant biomass into their own meat biomass (their bodies). This reduction in burnable biomass reduces the total biomass lost during annual fires, and particularly, it reduces the loss of important inorganic nutrients, such as nitrogen, that are stored in grass biomass and released into the atmosphere (such that they are lost from the soils) during fires. Therefore, although herbivore consumers generally have a negative direct effect on plants through their consumption, they compensate for this with a potentially greater amount of positive indirect effects.
Organisms interact with their environment, not just through metabolic (trophic) interactions directly, but also through a rich array of indirect trophic and direct non-trophic interactions, whereby they modify, maintain or create new habitats purposefully (such as building nests) or as a consequence of being alive. By changing thestructure of their own environment, they can also consequently modify/maintain/create an entire ecosystem, and therefore, the environment for other organisms. By affecting structure, life also affects the functionality of an ecosystem, in terms of its ability to decompose organic materials, nutrient cycling, energy flows and primary production for instance.
Autogenic ecosystem engineers are organisms that change their environment with their own physical structures (e.g. their bodies or biofilms). Allogenic ecosystem engineers change their environment by changing the physical environment around them (e.g. woodpeckers and beavers).
Interlinked with these two principle categories, there are a range of different ways that organisms can be ecosystem engineers. For example, structural ecosystem engineers can change the structure of their environment; bioturbator ecosystem engineers burrow, excavate, disturb and mix materials in their environment; chemical ecosystem engineers can modify the chemistry of water, air or soil for instance; another type are light ecosystem engineers that can affect the light quantity or quality in an area.
Some organisms can be more than one type. Producers, consumers and decomposers can be any type, however, arguably, producers are more often light ecosystem engineers, consumers are more often bioturbators, and decomposers are generally more often chemical ecosystem engineers. All are structural engineers of some form.
Physical and chemical factors, such as the Sun and water interact constantly with non-living and living-matter, through processes such as weathering, erosion and deposition, and phenomena such as isostasy. Life also interacts with non-living matter, through processes such as bioweathering, bioerosion, and migration. These processes occur within ecosystems, and also link different ecosystems, via the activation of flows and cycles between them.
From bacteria to fungal spores and plant pollen and seeds, to insects, birds, and herbivore animals, the migration of life across ecosystems, shifts and releases (mineralises) or immobilises (stores) nutrients within ecosystems and between ecosystems. Whether moving nutrients within their bodies as biomass, or through their excrements or exudates, life can bring-in (subsidise) from, or take out important nutrients to, other ecosystems. The production of, particularly organic matter, but also water and mineral nutrients, being brought into an ecosystem from the outside is known as allochthonous production (whilst local production is autochthonous).
Weathering is the breaking down of matter (organic matter or mineral) within the same place. Erosion is the process by which matter, produced by weathering, is carried away from one place to another. Isostasy, which is not a force or a process, describes the uplift of the floating continental crust: when top soil is eroded, the lower rock floats upwards to compensate for the loss— much like icebergs do, as snow melts on the top. It is estimated that land surfaces drop only around 5 centimetres for every 30 cm of rock taken off the top of the land. Deposition is the process, in which sediments, soils, and rocks add to another landmass, fed by erosion.
Weathering, whether by non-living processes or living processes are important in releasing nutrients, and decomposing nutrients within ecosystems. Bioweathering of rocks by fungi and lichen, and lithotrophic bacteria for instance, release and bring-in minerals into the food web; as well as decomposing organic matter. Erosion is an important part of life. At very slow natural rates—equal to, or below, weathering rates— erosion can refresh soils, by allowing isostasy to move new rock from below into the area, and for new weathering to take place. This is particularly important in landscapes where, due to high levels of rainfall, clays are highly weathered, and collapsed and/or lack nutrients. The rate of erosion to weathering, which is affected by factors such as temperature, rainfall, winds, slope, soil texture, parent bedrock, and the forms of life, will generally evolve to a dynamic equilibrium, with soil profiles around 30 to 100 cm deep. Once soils form, bio-weathering is the predominant process, breaking down bedrock faster than abiotic processes do.
Anadromy, is the migration of fish, from salt water to fresh water as adults. Salmon migrate from the ocean, and swim to the upper reaches of rivers for spawning in the gravel beds. After spawning, many Atlantic salmon die, however many are also eaten on-route by consumer predators, such as bears, otters, and bald eagles. The predators proceed to excrete nutrient-rich urine and faeces (containing elements, such nitrogen, sulphur, carbon and phosphorus), and leave partially eaten carcasses (which can be up to half of the salmon); bringing an estimated 4 tonnes per hectare – providing as much as 24% of the total nitrogen into these riparian woodland ecosystems. Catadromy is the opposite to anadromous migration. A well-known example is the migration of all ells, from fresh water rivers into the sea to spawn.
Many birds migrate, and also through nesting activities on land and by feeding at sea, they are also shifting important nutrients between different ecosystems. For example, if an island has woodlands, or holes in rocks, typically, birds come and bring oceanic nutrients onto the land (such as guano) fertilising the land with their excrement nutrients. If trees are cut down, birds stop coming, and a key input can be lost to an ecosystem. These nutrients can also cascade down into coral reef ecosystems that may be surrounding the island.
Insects such as butterflies, and locusts also migrate, as do large groups of herbivore animals, such as the wildebeest across the Serengeti.
Ecosystems, and all living systems are in dynamic equilibrium, far from static equilibrium.
Part of this dynamism of life is evolution by natural selection. The basic three principles are that individuals vary; these variations/differences can have consequences on their abilities to survive and reproduce; and descendants resemble their parents. Those traits that support survival and reproduction will become more widespread, and over time, individual organisms will become increasingly adapted with their environment.
Increasingly it is becoming accepted within the scientific community, that this process is also happening at different ‘levels’ —within groups, populations, communities and ecosystems, known as Multilevel Selection Theory (MLS).
MLS describes a hierarchy of evolutionary processes which are organised within nested wholes. At the centre, is a single organism, where there is selection between genes within an individual; at the next level there is selection that acts upon the relative fitness of the individual to other individuals; and at a higher level, there is selection that acts upon groups within a population, and so on. It is important to underline that between-group selection is occasionally a weak evolutionary force – and sometimes a very strong evolutionary force – depending on the case being looked at; and that adaptation at any level tends to be destabilised by selection at a lower level.
MLS can be best explained through a chicken and egg laboratory experiment. Here hens were kept in groups within cages, and two kinds of selection for egg productivity was made. In the first experiment, the individualwith the highest production of eggs, within each group (cage) was selected to breed the next generation. In this case, the most productive hen achieved her productivity essentially by bullying the other hens. After repeating the selection process over six generations, a hyper-aggressive strain had emerged, which resulted in a nosedive in total productivity. In the second experiment, the group with the highest collective production of eggs were all selected to breed the next generation of chickens. This was done repeatedly over six generations. The result was a docile strain of chickens with group egg productivity increase of 160% over the six generations. This is an example of ‘between-group selection.’
A third, and unintentional version of this experiment seems to show that ‘ecosystem level selection’ is also possible. This is where a selection of strains of yeasts and bacteria that are used to produce kefir (a yogurt-like drink) is selected for its taste and health benefits over others. Not only does the process select a multi-species microbial community, but the community has evolved to aggregate into clusters, all held together by a sugary matrix – which makes it possible to move it across batches as one single ‘unit.’
Producer, consumer and decomposer communities can show an orderly change in species composition and community structure overtime, as they colonise newly available ecosystems (e.g. land raised above sea-level after tectonic activities) or after a major disturbance (e.g. fire) in an existing ecosystem. This remarkably predictable process—over the long run—is known as ecological succession. For example, ‘higher’ plants can gradually replace other plants within ecological succession until the previous plants are no longer present (or are present in very reduced numbers); this means that it also includes dynamic competitive interactions, as well as mutualist and parasitic ones. As the plants change, so do the insects and animal communities above ground (consumers), and the bacteria and fungi and their predators below ground (decomposers).
One of the most prominent features of ecological succession, is the displacement of species with those of greater size, lifespan, colonisation abilities and growth rates (increasing biomass, particularly in plants, and increasing production, particularly in primary production). All these attributes contribute to decreasing biomass turnover rates and increasing resource-use intensity.
Ecological resilience is the ability of an ecosystem to maintain its normal pattern of community, nutrient cycling, and energy flow following an ecological disturbance. This is important in relation to ecological succession, as this can determine how ecosystems proceed through this process–or not. For example, some ecosystems can become ‘locked’ within an undesirable state (from a human perspective); such as a eutrophic lake, where an overabundance of nutrients has created a hypoxic (depleted oxygen level) system, leading to the death of many fish species followed by the spread of undesirable pests. Furthermore, some ecosystems can be said to be more ‘brittle’ than others, meaning that disturbances—such as farming techniques originally developed in less brittle ecosystems, such as western Europe, which are then transferred to more brittle ecosystems, such as areas of Africa, do not cope with the same amount and type of disturbances, and can lead to ecosystem collapse.
For an ecosystem to function over the long term, there needs to be some form of buffer, so that an ecosystem can reorganise itself and recover after disturbances. This is ecological resilience. After a forest fire, there are remaining structures, such as tree stumps, and remaining seeds, and many organisms that may survive by burying themselves underground. This becomes, what is known as ecological memory, which, supported by nearby areas that remain untouched by the fire (becoming external memory), support regeneration. If, ecological memory is degraded, then this affects resilience.
This section looks at the model as a whole, selecting three different ecosystems and one human made ecosystem, and looking at them through the integrative lens.
The seagrass ecosystems can be large and dense enough that they can be seen from space. They are amongst the most productive ecosystems in the world, providing food and shelter (structural components), for an incredibly diverse community of producers, consumers and decomposers. These ecosystems can be found along most coastlines of the worlds continents.
Seagrass is different to both seaweed and terrestrial plants, with the ability to be totally submerged (unlike plants), has roots and an internal vascular system (like plants), and have flowers and seeds (like plants). Known as the ‘lungs of the sea,’ one square meter of seagrass can generate 10 litres of oxygen every day via photosynthesis.
Seagrass meadows accumulate vast amounts of organic matter (sediments), representing an important nutrient reserve for the plants. This helps capture nutrients lost from the land from erosion, and also act as buffers for coastlines from storms. However, over time, this build-up of nutrient-rich sediments can eventually cause toxicity, which is detrimental to the seagrass and other organisms.
Within these waters, many organisms, such as bacteria, and microalgae (such as diatoms) grow directly on the seagrass leaves, much like lichens or moss grow on terrestrial trees. Some grazing invertebrates, such as small crustacea and snails feed on these small organisms, helping to keep the seagrass clean (and helping them maintain their photosynthetic functionality). These small invertebrates then feed larger crustaceans, fish and birds.
Within the seagrass ecosystem are also bivalve clams, which are primary consumers, feeding on small organisms, such as plankton and micro-algae, and organic matter suspended in the water. This cleaning function is not performed by the clams alone, however, as within the clam’s gills, live sulphide-oxidising bacteria which have the ability to decompose the organic matter and detoxify the water for the seagrass and the clams (and the greater community). Dead seagrass leaves also feed a diverse community of decomposers, also including nitrogen-fixing bacteria, which support the nitrogen input into the ecosystem, as they are eaten, and the nitrogen cycles through the food web. This is a clear, producer, consumer and decomposer integrative system.
A large variety of larger herbivore consumers feed on seagrasses, such as adult green sea turtles, manatees and dugongs (marine mammals), and geese. Prior to the over-hunting of sea turtles, it was thought that the number of sea turtles in the Americas, outnumbered the large hooved primary consumers in the Serengeti Desert today, by 15-20 times. A large adult sea turtle can feed on an estimated 2kg of seagrass per day.
In certain seagrass ecosystems, sharks can be the primary predatory of the larger herbivores, such as adult (and late juvenile) green turtles. And here, the sharks can play an important top-down cascade role, by maintaining turtle populations, so that they do not overgraze the seagrass system.
Seagrass ecosystems are also rich and important nurseries, that form refuge, shelter and food for a vast array of organisms.
Pedogenesis (from the Greek pedo-, or pedon, meaning ‘soil, earth,’ and genesis, meaning ‘origin, birth’) is the process of soil formation, in a place, overtime. The starting-point is the initial weathering of the parent rock. The physio-chemical condition of the clays for example are fundamentally important. If they are collapsed, or washed-out, they will be both poor in nutrients, and unable to hold and exchange nutrients; whilst flocculatedclays, which are structured and with spaces, can contain and hold nutrients.
Saprotrophic bacteria in soils and on surfaces within aquatic environments, just like producers and consumers, build vast structures; however, unlike many producers and consumers, which primarily build their physical bodies, single-cell organisms produce vast structures outside their bodies, known as biofilms. Biofilms are a sticky matrix made up of simple sugars, proteins and DNA. Some bacteria use these biofilms as a means of transport, saving them from drying out (desiccation), and they can be a defence against antibiotics produced by other organisms (up to 1000 times more resistance). In soils, biofilms also bind micro-aggregates of soil together, representing the smallest building blocks of soil structure.
Unlike bacteria, saprotrophic fungi predominantly build autogenic structures using their bodies (hyphae) to hold micro-aggregates together, building macro-aggregates—the larger structures in soils. AM mycorrhizal fungi also produce significant amounts of glues, known as glomalin. As a gluey substance, glomalin helps build soil macro-aggregates, and with its wax-like properties, it is not very soluble; therefore, it helps maintain the stability of soil aggregates during rain events. Humus can also hold significant amounts of water, which is a by-product of decomposition, particularly by fungi; some mycorrhizal fungi create water storage structures around plant roots, helping to keep plants hydrated during periods of water scarcity. Collectively, this makes saprophytic and mycorrhizal fungi critical autogenic structural engineers and water engineers—helping bacteria and their biofilms build the ‘carbon soil sponge.’
Earthworms, which are decomposer higher predators, are also important ecosystem engineers. As engineers, they are burrowers (like ‘micro-ploughs’), as they pull organic matter down from the soil surface underground—making it bioavailable for many other organisms, fungi and bacteria, and speeding up cycling of dead leaves, for example, from 1–2 years to around 3 months. Earthworms increase soils’ porosity through tunnelling, and therefore, support water and air flow through the soil. Collectively, this means that earthworms, and arthropods, such as beetles, are highly important micro-bioturbators (and burrowers), particle matter processors and water engineers.
The other decomposer predators, such as nematodes and protozoa, support tropic-cascades, by controlling bacteria and fungal populations, and by producing micro-faecal matter, which is the fundamental nutrients for plants.
Powering this is underground system are the exudates from the plants, which turn carbon dioxide (CO2) and water (H2O) into food and structure, and above ground, create food and shelter for decomposers and consumers alike.
Plants—particularly trees—create important autogenic structures that are used by microbes, insects, birds, small mammals and many other plants, from mosses to vines and lichens. Plants also act as ‘litter trappers’, which are able to catch falling leaf litter and other organic debris; on a larger scale, riparian woodlands can hold riverbanks together. In this process, they concentrate sediments and organic matter and create new autogenic structures and ecosystems.
Plants, through the loss of dead leaves, branches (and roots), generate litter under and around them. This keep soils cool from the hot Sun; reduces water loss through evaporation; protects soil from direct rainfall (which compacts bare soils); reduces germination of young plants, which need plenty of light; creates habitat for soil macro fauna; and becomes an unfrozen water zone in winter, where soil fungi can eat and thrive (while others lie dormant). In these different ways, plant litter also makes plants light, water and autogenic structuralengineers.
Trees (and other plants) can create wind and sunshade for animals and other plants. (Some plants profit, while others can suffer). The further interactions of consumers and their predators on the system is described in the section on cascades.
Although not strictly classed as an ecosystem, this example, extends the vision to a plant cell. A cell is the most elementary autopoietic unit. A plant cell is also made up of some of the same key foundational producer, consumer and decomposer functions within its organelles (small organs), which have already been described at the ecosystem level. In fact, this is a result of symbiogenesis – individual bacteria with these different abilities, coming together as a functioning ‘ecosystem’ at the cellular level, through the process of evolution by natural selection.
In green, are the main processes occurring within the chloroplast—where photosynthesis takes place— the ‘producers’ equivalent. The blue circles represent the mitochondria, where the cell generates chemical energy in the form of ATP through aerobic respiration—the ‘consumers’ equivalent. At the top, in dark blue, is the nucleus, and the other apparatus where proteins are coded, synthesised (built) and ‘shipped’ out to the rest of the cell—the equivalent in is potentially spread across the network. The orange circle represents the leucoplasts, where excess glucose from photosynthesis can be stored in the form of starch — the equivalent to the ‘nutrient pools’ equivalent— and the cytosol also performs a nutrient pools function. and wastes are recycled, digested and excreted by the lysosomes and peroxisomes, shown in purple (see ‘decomposers’ equivalent).
At their most basic, fractals can be described as objects that look the same at all scales or at any magnification level. In nature, this can be seen in the evolution of hierarchical branching network systems, such as the mammalian circulatory system. In effect, plants have evolved the ability to perform the elementary metabolic functions at the cellular level, which shows the sophistication of plants and that this mirroring—or ‘fractal’—at the scale of an ecosystem, also suggests that these three functional groups/metabolisms (producers, consumers and decomposers) are indeed elemental for complex life to exist. Many single-cell organisms (such as bacteria) are able to do parts of this process but need others to perform the corresponding missing processes to survive. As documented in the previous text, there do not seem to be any plants that can live without decomposers (they need inorganic elements)—and many benefit from or vitally need certain consumers. Therefore, for plants, life is fractal, as the pattern needs to exist at the two scales, and this may be why plants are the ‘keystone kingdom’ for the rest of terrestrial life.
‘Mesocosms’ (also known as ‘eco-machines’) are, usually aquatic-based, systems that are often designed to carry out ‘work’ for society, such as cleaning water, growing food or fuels, or remediating polluted landscapes, based on ecological design.
If one wants to design a wastewater system, for instance, the model can be used to think about the different forms and elements within the system. For example, the system will include synergistic living elements, such as phytoremediation plants, different types of aerobic or anaerobic fungi or bacteria, potentially protozoa (e.g. algae) and perhaps some fish (producers, consumers and decomposers). The system may also require a physical structure, which could be designed following the circularity principles, and it will probably require some form of aeration or pumping system or vortex to support the functioning and provide energy (if direct sunlight and the producers are not enough). We can then consider potential cascading systems that develop food-web style cascades to support the enhanced cleaning of the water and even produce some high-value by-products (collecting a ‘yield’), which may be able to finance the initial investment and/or its running costs. Examples can include wicker from coppiced willow, which may be one of the functional plants in the system; this can also be harvested annually to make baskets that can then be sold. It can also be possible to think about the system as a combination of different ecosystems, linking up, for example a natural river or bog system, to clean/filter different elements out of the water.
Metabolism is collective: Through direct and indirect, trophic and non-trophic interactions, metabolism is a dynamic and collective activity between living and non-living systems.
Metabolism is integrative: Highly functioning, diverse and modern ecosystems, dynamically integrate all forms of metabolism.
Energy and matter move in different ways: Within this integrative metabolic system, matter and energy moves as cycles, and via collaborative synergies, and through cascades. Energy and matter also flow into and out of ecosystems.
Qualities are Dynamic: Living and non-living systems are continuously interacting, and transforming the qualities of stocks and flows of matter and energy, through an aggregation and disaggregation of structure and functionality. This affects energy and matter availability, efficiency, and ability to do work for individuals and collective groups, and creates potentials for new forms of integrations and collective benefits to take place.
Ecosystems are Dynamic: Life, and therefore ecosystems, are far from equilibrium, and have different trajectories and resilience to change, dependent on their historical biotic and abiotic factors.
The proposed value adding of the ecosystem metabolisms and functions model for designers:
- Metabolism helps explain how different organisms use energy to find and transform matter and what types of metabolic wastes they produce.
- Understanding the different forms of metabolisms and their wastes explains how different organisms relateto each other, in and to a place (an ecosystem).
- It is through these relationships that the different forms of metabolism express their function within ecosystems (e.g. producers producethe building blocks of life, consumers consume them and decomposers decompose them and make them available for producers); this helps explain interdependency – and how life creates and recreates the conditions for more life.
- Functions can also be non-trophic, and making this explicit supports understanding of how organisms affect other organisms and their environment in ways that are not necessarily expressed, for example, in food webs.
- By understanding functions, designers can select different organisms, depending on the functions they provide—and the functions that are needed and for the specific place in which they will function—to build ‘mesocosms’ (see work by J. Todd), compost (see work by E. Ingham), holistic grazing plans (see work by A. Savory), a forest garden (see work by M. Crawford) or integrated elements, for example, within passive buildings.