Ecosystems as open systems
Living organisms cannot live alone. They depend on interactions with other organisms for supplies of energy and chemical resources. They also depend on their abiotic surroundings of air, water, soil and rock. Biologists have lake or a forest. An ecosystem is composed of all the organisms in an area together with their abiotic environment.
System are an important concept in biology. A system is a set of interacting or interdependent components. There are two main types of system:
- Open system where resources can enter or exit, including both chemical substances and energy
- Closed systems where energy can enter or exit, but chemical resources cannot be removed or replaced.
Sunlight on terrestrial surfaces
The initial source of energy for most ecosystems is sunlight. Living organisms harvest this energy by photosynthesis. Three groups of organisms carry out photosynthesis: Cyanobacteria, plants and eukaryotic algae including the seaweeds that grow on rocky shores. These organisms are known as producers. The energy fixed by producers in carbon compounds is available to other organisms that do not photosynthesize.
The amounts of energy reaching the Earth’s surface in sunlight varies around the world. Also, the percentage of this light that is harvested by producers ad therefore available to other organisms is greater in some ecosystems than others. For example, the intensity of sunlight is very high in the Sahara Desert but little energy is harvested because there are few producers. In redwood forests of Northern California, the intensity of sunlight is far lower but much more energy becomes available to the ecosystem because producers are abundant and photosynthesis rates are higher.
Sunlight on marine surfaces
In marine and freshwater ecosystems, light must pass through water to reach producers. We think of water as transparent but transmission is not 100%. Photosynthesis uses light wavelengths from 40 nm to 700 nm. Sugar wavelengths penetrate further and pure water, which is why the sea often appears blue. Water in marine and freshwater ecosystems contains living organisms and nonliving matter, which further reduce light penetration. An open ocean there is little to no light at depths greater than 200 m. Coastal waters are often turbid due to suspended clay or silt and dense populations of phytoplankton, so there is little light below 50 m. Deeper ecosystems must therefore rely on other sources of energy.
Sunlight in caves
Ecosystems have also developed in the darkness of caves. Streams entering a cave may bring dead organic matter which provides a supply of energy; for example, dead leaves contain energy produced by photosynthesis and ecosystems outside the cave. However, some cancers are isolated do not receive inputs of energy from outside ecosystems.
The producers in sealed caves are archeabacteria. They gain energy from chemical reactions that have methane, sulphide or other inorganic compounds as substrate. Energy from these reactions is used to synthesise carbon compounds in a type of metal metabolism called chemosynthesis. Microscopic vertebrae feed on biofilms of chemosynthetic archeabacteria, with other invertebrates feeding on them.
Food chains
A fortune as a sequence of organisms, each of which feeds on the previous one. There are usually between two and five organisms in a food chain. Producers are the first organisms in a food chain. The subsequent organisms are consumers.
— Most producers absorb sunlight using chlorophyll and other photosynthetic pigments. The light energy is converted to chemical energy, which is used to make carbohydrates, lipids, and all other common compounds that are required.
— consumers obtain energy from the carbon compounds in the organisms on which they feed. Primary consumers speed on producers; secondary consumers feed on primary consumers; tertiary consumers feed on secondary consumers, and so on. No consumers feed on the last organism in the food chain.
The arrows in a food chain indicate the direction of energy flow.
In an ecosystem, there are many specific food changes that provide organisms with a a supply of energy.
Apex predators are considered the top of a food chain meaning that nothing kills or eats them though parasites can obtain energy from them.
Food web
Trophic relationships within ecological communities tend to be complex and weblike. This is because many consumers feed on more than one species and are fed upon by more than one species. A food web is a model that summarises all the possible food chains in a community.
When a full weather is constructed, organisms at the same trophic level are often shown at the same level in the web. However, this is not always possible because some organisms feed on more than one trophic level.
Supply of energy to decomposers
The body of an organism that has died is available to other organisms of a source of energy. Body parts tread by plants and animals are also available— for example, the exoskeleton of an insect when it moults, or fallen leaves from a tree. Material that passes through animal guts on ingested is egested as facies. This is another form of organic matter that can provide energy. Some of these energy resources are eaten by animals such as earthworms and vultures. Large amounts are decomposed by saprotrophs.
Saprotrophs secrete digestive enzymes into the debt, organic matter and digested externally. They then absorb the products of digestion, including sugars and amino acids. In contrast, consumers in food first and digest internally. Bacteria fungi are the two main groups of saprotrophs.
Saprotrophs are also known as decomposers, because they breakdown complex insoluble carbon compounds into simpler soluble ones. By doing this, they caused the gradual breakdown with solid structures. For example, a tree trunk on the fourth floor will gradually soften then crumbled away and fallen leaves disappear. Without the action of decomposers, that organic matter would build up year by year in the chemical elements in it would not be recycled. Decomposers are the waste disposers and recyclers of ecosystems.
Autotrophs as organisms that synthesise their own energy
All organisms have nutritional requirements. They need a variety of carbon compounds:
- Amino acids for protein synthesis
-Sugars for energy supply and synthesis of polysaccharides
- Fatty acids for energy supply and four constructing membranes
- Organic bases four synthesis nucleic acids during DNA replication and transcription
- Steroids and many other groups of carbon compounds
Some organisms can make all these common compounds themselves, using carbon dioxide or hydrogen carbonate as a carbon source and nitrate, phosphate and other simple inorganic substances as sources of other elements. Organisms that do this are called autotrophs, meaning self feeding. To carry out the anabolic reactions that build carbon compounds from simple organic substances, autotroph need an external energy source. There are two possible sources of external energy: light and chemical reactions.
Photoautotrophs
Autotrophs need an external energy source to create carbon compounds. This is because carbon dioxide or hydrogen carbonate ions are in an oxidise state and the initial reduction reactions of the Calvin cycle are endothermic. Simple carbon compounds are then linked together into larger more complex compounds by condensation reactions which are also endothermic.
Autotrophs are subdivided according to the external energy source they use :
— photoautotrophs use light
— chemoautotrophs use exothermic inorganic chemical reaction reactions
So is used as an energy source by organisms that perform photosynthesis. Fusion reactions in the Sun generate vast amounts of energy in the form of electromagnetic radiation. A very small proportion of energy emitted by the Sun reaches the Earth, and a small proportion of that is absorbed by photosynthesis. Even though, suddenly is the main energy supply for most ecosystems. Plants, eukaryotic algae and cyanobacteria are photoautotrophs.
Chemoautotrophs
Autotrophs are subdivided according to the external energy source they use :
— photoautotrophs use light
— chemoautotrophs use exothermic inorganic chemical reaction reactions
Inorganic chemical reactions are used as an energy source in a variety of prokaryotes, both bacteria bacteria. A substrate is in a reduced state— such as sulphur, hydrogen sulphide, iron, hydrogen, or ammonia— is oxidised, releasing energy which is used to synthesise carbon compounds. Such organisms are autotrophic because they make their own sugars, amino acids and other common compounds.
IRON/OXIDIZING BACTERIA
Iron oxidising bacteria or chemo autotroph. Iron sulphide commonly occurs in sedimentary rocks formed in low oxygen environments. When these rocks are exposed to air, due to mining activities or natural erosion processes, the iron sulphide reacts to produce +2 iron ions, sulphate ions and sulphuric acid
FeS3 + 3(1/2)O2 + H2O —> Fe(+2) + SO4(-2) + H4SO4
Iron oxidising bacteria then remove electrons from the +2 iron ions converting them to +3 iron ions. The electrons carry energy released by the oxidation of iron. They are accepted by electron carriers in the plasma membrane of the iron oxidising bacteria. Some of the electrons are used to reduce NAD.
2Fe(+2) + NAD + 2H+ —> 2Fe(+3) + reduced NAD
Some electrons are used to provide energy for proton pumping, to increase the proton gradient across the plasma membrane formed when acid is produced from iron sulphide. This proton gradient allows ATP production chemiosmosis. these electrons are accepted by oxygen, together with hydrogen ions to produce water.
2Fe(+2) + 1/2O2 + 2H+ —> 2Fe(+3) + H2O
Reduced NAD and ATP are used to fix carbon dioxide and produce carbon compounds by means of the Calvin cycle. Acidithiobacillus ferrooxidans is an example of an iron oxidising bacterium. It is adapted to highly acidic environments because the initial reaction produces sulphuric acid.
Heterotrophs as organisms that obtain energy from other organisms
Many organisms to fill their nutritional requirements by obtaining carbon compounds from other organisms. These organisms are heterotrophic, which means they feed on others. They digest carbon compounds that were part of another organism, then use the products of digestion to build the large complex carbon compounds they need. The process of absorbing carbon compounds and making them part of the body is called assimilation
Assimilation requires absorption of carbon compounds into cells so the molecules must be small and soluble enough to pass across the cell membranes. Proteins, polysaccharide, nucleic acids and other large compounds must be digested before they can be absorbed. Heterotrophs are subdivided according to whether they digest food internally or externally.
- Saprotrophs grow into or across the surface of food and secrete hydrolytic enzymes to digest the food externally
- Consumers ingest their food
- Multicellular consumer such as Cunico take food into their gut by swallowing it. Then they mix the food with enzymes from digestive glands. This is regarded as internal digestion although the food has not yet entered any cells
- Unicellular consumers such as Paramecium take the food into their cells by endocytosis, then digested inside phagocytic vacuoles. They absorb products by digestion from the vacuoles in their cytoplasm.
Release of energy by cell respiration
All organisms require supplies of energy in the form of ATP in their cells. They use this energy for vital activities, including:
- Synthesizing large molecules such as DNA, RNA and proteins
- Pumping molecules or ions are cross membranes for active transport
- Moving structures within cells, for example, chromosomes or bicycles; and muscle cells array of protein fibres are moved to causes muscle contraction
- Maintaining constant body temperature
In both autotroph’s and heterotrophs ATP is produced by cell respiration. Carbon compounds such as carbohydrates and lipids are oxidised to release energy and this energy is used to phosphorylate ADP producing ATP.
Classification of organisms into trophic levels
Ecologists classify organisms into groups according to how they obtain energy and carbon compounds and therefore where they are positioned in food chains. These groups are called trophic levels
Most consumers have a very diet and can occupy different trophic levels in different food chains.
- Producers are autotrophic they make their own carbon compounds using external energy sources so they are at the start of food chains.
- Primary consumers (herbivores) feed on producers so they are second in food chains.
- Secondary consumers eat primary consumers so they are third in food chains.
- Tertiary consumers eat secondary consumers so they are fourth in food chains and so on.
reduction of energy availability, incomplete consumption
In any ecosystem, there are largely losses between trophic levels. As a result, there is less energy available to each successive trophic level. There are three main forms of energy loss.
- Incomplete consumption.
The organisms in a trophic level are not usually entirely consumed by organisms in the next trophic level. For example, locusts sometimes consume all the plants in an area but more usually they only eat parts of some plants. Similarly, predators do not usually eat bones or hair from the body of their prey. The energy and dead organisms are dead parts of organisms, passes to saprotrophs such as fungi or detritus feeders such as earthworms rather than passing two organisms in the next trophic global. This energy is lost from the food chain.
reduction of energy availability, incomplete digestion
In any ecosystem, there are largely losses between trophic levels. As a result, there is less energy available to each successive trophic level. There are three main forms of energy loss.
- Incomplete digestion.
Not all parts of food ingested by organisms are digested and absorbed. For example, some animals do not digest through fibrous plant matter containing cellulose. In indigestible material is egested in faeces. Energy in faeces does not pass on along the food chain. Instead it passes to saprotrophs or detritus feeders.
reduction of energy availability, cell respiration
In any ecosystem, there are largely losses between trophic levels. As a result, there is less energy available to each successive trophic level. There are three main forms of energy loss.
- Cell respiration
Carbohydrates, proteins and other energy containing substances are used as substrate in cell respiration. These substrate are oxidised to come dioxide and water, releasing energy. The carbon dioxide and water are waste products that cannot supply energy to the next trophic level. Only carbon compounds that are not oxidise and respiration can be passed to organisms in the next trophic level.
Smaller amounts of energy flow to each successive trophic level because there are smaller amounts of carbohydrates, proteins and other energy containing substances. It is not because the substances contain less energy program in higher trophic level that they have the same energy content per trophic level. If anything, higher trophic levels contain more energy per unit of biomass, not less.
It is often said that 90% of energy is lost between trophic level with only 10% passed on however there is much variation between food chains.
Heat loss due to conversion of chemical energy to heat cell respiration
Energy can be converted from one form to another. Energy enters ecosystems in the form of sunlight. It is transformed into chemical energy by photosynthesis. Chemical energy flows along food chains and decomposers. Ultimately, all of this energy is transformed to heat.
All organisms — both autotroph and heterotrophs— convert some of their chemical energy into heat. Person mammals sometimes increase their rate of heat generation to maintain a constant body temperature. In other organisms, heat is generated as an inevitable side-effect of activity. Cell respiration is a major source of heat in living organisms. The second law thermodynamic states that energy transformations are never 100% efficient. This means that not all of the energy from the oxidation of carbon compounds in cell respiration is transferred to ATP. The remainder is converted to heat. Chemical energy is also converted to heat when ATP is used in cell activities.
Body warmth helps organisms to remain active. However, this heat is ultimately lost due to the abiotic environment according to the laws of thermodynamics: heat passes from hotter to cooler bodies. This energy cannot be converted back into chemical energy are used by other trophic levels or any other organisms. For this reason, energy flows through ecosystems and cannot be recycled. The heat may remain in an ecosystem for a while, but it will eventually radiate out through the atmosphere and on to space.
Restriction on the amount of trophic levels
The number of stages in a food chain varies. We might expect food chains to be limitless with one species being eaten by another and so on forever. However, this does not happen. This is because so much energy is lost at each step in a food chain. There is less energy available to each successive trophic level so after only a few stages, there is not enough energy to support another trophic level. For this reason the number of trophic levels in food chains is restricted.
It is important to understand the animals and trophic levels do not have to eat more food to gain enough energy. They pre-contains a large amount of energy per unit of mass— there is not much prayer available. A peregrine falcon for example is mainly a tertiary consumer and may need a territory of more than 100 km² to find enough to eat.
Primary production
Production and ecosystems is the accumulation of carbon compounds in biomass. Biomass accumulates when living organisms grow. Reproduction can increase the numbers of growing organisms and thus contribute to production.
Both autotrophs and heterotrophs produced bio mass by growth and reproduction. Plants and other autotroph‘s are primary producers because they synthesise common compounds from carbon dioxide and other simple substances.
— gross primary production is the total biomass of carbon compounds made in plants by photosynthesis
— net primary production is gross primary production minus the biomass lost due to respiration of the plant. This is the amount of biomass available to consumers.
Both GPP and NPP are generally measured over long time intervals like a year at the ecosystem level. The units of measurement are usually grams of carbon accumulated per square meter of an ecosystem per year. There are other measures.
Biomes in their capacity to accumulate biomass, depending mainly on rates of photosynthesis. Satellite imaging can be used to monitor primary production.
Secondary production
Secondary production is the accumulation of common compounds in biomass by animals and other heterotrophs. Carbon compounds such as sugars and amino acids are ingested from food and then built up into proteins and other macro molecules.
Carbon compounds are used as respiratory substrates by all organisms at every trophic level. Cell respiration results in a loss of carbon compounds and therefore a loss of biomass in every trophic level. For this reason that production is always lower than gross production and secondary production is lower per unit area than primary production in an ecosystem. Secondary production declines with each successive trophic level from primary consumers onwards.
Differences in Production are reflected in farm yields. Production of crops per unit area is always much higher than production of meat and other animal products. This means that more humans can be fed per hectare of farm land if they eat plant products then meat. This is one of the reasons for increased interest in plant based diet among environmentalists.
Carbon cycle
Ecologist used the terms “pool” and “flux” when describing the carbon cycle and recycling of other elements.
- A pool is a reserve of the element. And can be organic or inorganic. For example, carbon dioxide in the atmosphere is an inorganic pool of carbon. The biomass of producers in an ecosystem is an organic.
- Flux is the transfer of the elements from one pool to another
There are three main types of carbon flux due to living organisms and ecosystems:
- Photosynthesis— absorption of carbon dioxide from air or water and its conversion to carbon compounds
- Feeding— gaining common compounds from other organisms
- Respiration— released to the atmosphere of carbon dioxide produced by respiring cells.
Carbon dioxide is abundant in the atmosphere it comes from cell respiration and producers cell respiration consumers, combustion of fossil fuels and cell respiration in saprotrophs and detritivores. It is used for photosynthesis in producers which are then eaten by consumers. Carbon dioxide is also produced in the egestion of consumers and in their death either through carbon and dead organic matter which is consumed by saprotrophs and detritivores or through incomplete decomposition and facilitation of organic matter which turns to coal, oil and gas.
Ecosystems as carbon sinks and carbon sources
Ecosystems are open systems because both matter and energy can enter or exit. Carbon enters and exits mostly in the form of carbon dioxide, throughout the processes of photosynthesis and respiration. The rate of these two processes for an ecosystem as a whole are not necessarily equal.
- If photosynthesis exceeds respiration, there is an Uptake. The ecosystem is acting as a carbon sink.
- If respiration exceeds photosynthesis, there is a net release. The ecosystem is acting as a carbon source.
In most ecosystems, saprotrophs digested or organic matter and release the carbon from it as carbon dioxide due to respiration. Conditions in some ecosystems inhibit decomposition. For example, acidic and anaerobic conditions in water habitats like bugs or swamps, prevent decomposition, so Peat accumulates. During some periods in the past, this pear has turned to call. The carbon coal is removed from the carbon cycle for millions or even hundreds of millions of years. Removal of carbon from the cycle is known as sequestration.
Periodic fires occur naturally in some ecosystems and species and these communities are adapted to it. During a fire in a forest or other ecosystem, carbon dioxide is produced by combustion of carbon compounds in living organisms and dead organic matter. The ecosystem therefore becomes a carbon source.
Release of carbon dioxide into the atmosphere
Biomass, peat, coal, oil, and natural gas are carbon sinks, because the carbon in them can remain sequestered for indefinite lengths of time. They were formed at different times in the past by a variety of processes:
- Natural gas and oil formed over the past 550, million years or more, from the pre-Cambrian geological period and onwards. Deep burial of partially decomposed organic matter under sediments, where high temperatures caused chemical changes and produced oil and natural gas that were trapped in porous rocks.
- Coal was mostly formed 325 to 250,000,000 years ago, during the Pennsylvanian and Permian geological periods. Accumulation of wood and other plant matter and swamps, where it was braided under other sediments caused the formation of coal.
- Peat was mostly formed over the past 10,000 years in the period since the last glaciation of earth. Incomplete decomposition of dead plant matter due to acidic and anaerobic conditions in water love bugs and swamps caused the formation of peat.
- Biomass is formed from plant biomass derived from photosynthesis, with transfer along food chains to animal biomass. Wooden trees has accumulated over the past few thousands of years when organic matter is more recent.
When any of these materials burn in air, carbon dioxide is produced and released into the atmosphere. The scientific term for burning air is combustion. Combustion only begins if a specific ignition temperature is — This is called the flashpoint. For coal, crude oil and natural gas. This is over 500°C. For normal dry wood it is over 400°C. These temperatures are never reached during normal weather conditions on earth. However, a volcanic activity and lightning strikes can cause fires, especially when ecosystems have been desiccated by hot weather and dropped.
Most coal is deeply buried so it cannot burn in air. At the end of the Permian period, 250 million years ago, volcanic activity in Siberia may have led to combustion of coal deposits on a huge scale. This triggered the release of carbon dioxide, leading to extremely high temperatures on earth due to the greenhouse effect. This caused the greatest mass extinction event of all time.
In recent years summer wildfires have become increasingly frequent in areas of tundra located inside the Arctic cycle. Combustion of peat that accumulated over thousands of years has released large quantities of income dioxide. It is estimated that in 2020, nearly 250 mega tons of income dioxide was admitted as a result of these fires— but this is far less than the 340 megatons released by humans burning fossil fuels in that year.
