An open system is a system that exchanges both energy and matter with its surroundings. An ecosystem fits this definition. Light may enter and heat leaves, while organisms and water move across its boundaries. Gases and nutrients are exchanged too.

Both energy and matter sustain an ecosystem’s structure and activity, though they behave differently. Energy passes through the system before eventually leaving as heat. Matter, by contrast, moves between organisms and the physical environment and can be reused. Systems diagrams, food webs and ecological pyramids let us trace these flows. Later, these models can also help predict how human disturbance may alter them.

The first law of thermodynamics states that as energy flows through ecosystems, it can be transformed from one form to another but cannot be created or destroyed. An ecosystem doesn’t manufacture energy; instead, it converts incoming energy into other forms.

Light energy can become chemical energy stored in biomass. Later, that chemical energy may be converted into heat. The total amount of energy remains constant through these changes, although its form and usefulness change.

Transformation of energy is a change from one form to another, such as light to heat. Transformation of matter happens in chemical reactions and can be summarized using word equations. A transfer is different: energy or matter moves between parts of a system, but its form stays the same.

During photosynthesis, light energy becomes chemical energy, while carbon dioxide and water become glucose and oxygen. In cellular respiration, the chemical energy in glucose changes into usable chemical energy and heat. At the same time, glucose and oxygen become carbon dioxide and water. The required word equations are:

• carbon dioxide + water $\to$ glucose + oxygen
• glucose + oxygen $\to$ carbon dioxide + water

In a systems diagram, boxes represent storages and arrows represent flows. Label each arrow with the process and make clear whether it shows a transfer or a transformation. When numerical data are supplied, larger storages need larger boxes, while larger flows need wider arrows. The diagram should also show all inputs and outputs.

Photosynthesis is the conversion of light energy to chemical energy in the form of glucose, some of which can be stored as biomass by autotrophs. Autotrophs can convert glucose into other carbon compounds in their biomass, such as structural or storage material.

For most food chains, this is the key point where energy enters. Keep your explanation at the ecosystem level; you don’t need to cover biochemical pathways or organelle detail.

A producer is an organism that makes its own carbon compounds and forms the first trophic level of a food chain. Most producers are plants, algae or photosynthetic bacteria. They use photosynthesis to make food.

Producers supply consumers with biomass and stored chemical energy. So, a conventional food chain always starts with a producer rather than the Sun. The Sun provides energy, but it isn't an organism and doesn't form a trophic level.

Cellular respiration is a cellular process that releases energy from glucose by converting it into a chemical form that can easily be used in carrying out active processes within living cells. These active processes include growth, movement, active transport and repair.

Both producers and consumers carry out respiration. You don't need to know the name of the cell's immediately usable energy carrier here. Focus instead on the change from energy stored in glucose to energy that cells can use.

Cellular respiration isn’t completely efficient. Only some of the chemical energy in substrates such as carbohydrates is converted into a usable chemical form. The rest is transformed into heat.

An organism cannot convert this dispersed heat back into the chemical energy of biomass. It leaves the body and is ultimately lost from the ecosystem. “Lost” doesn’t mean destroyed; the energy is simply no longer available to perform biological work in that food chain.

The second law of thermodynamics states that energy transformations in ecosystems are inefficient. It deals with energy quality. Each time energy changes form, some degrades into a less useful form, particularly dispersed heat.

Most energy loss in an ecosystem occurs during cellular respiration, so energy transfer can never be 100% efficient. While the first law accounts for the total amount of energy, the second explains why less useful energy is available to support organisms at each stage.

A consumer is an organism that gains chemical energy from carbon (organic) compounds obtained from other organisms. Different feeding strategies are used, sometimes by the same species.

• A herbivore is a consumer that eats producers. For example, a rabbit grazes grasses.
• A detritivore is a consumer that ingests and internally digests dead organic matter. A woodlouse eating leaf litter is a detritivore.
• A predator is a consumer that hunts, kills and eats another organism. One example is a dragonfly taking a mosquito.
• A parasite is an organism that obtains resources from a host while harming it. A tick feeding on a deer shows this relationship.
• A saprotroph is an organism that externally digests dead organic matter and absorbs the soluble products. Mould growing on dead fruit is an example.
• A scavenger is an animal that consumes organisms already killed by another cause. For instance, a condor may feed on a carcass.
• A decomposer is an organism that breaks down dead organic material through saprotrophic nutrition. This is commonly a fungus or bacterium.

Here’s the key distinction: detritivores ingest the material, whereas saprotrophs digest it outside their bodies before absorbing it.

A food chain is a linear model showing how carbon compounds, biomass and energy pass through feeding relationships. The chain starts with producers, which make carbon compounds through photosynthesis. Primary consumers eat producers, secondary consumers eat primary consumers, and further consumers follow.

To construct a food chain from given data, find the producer first. Then arrange the organisms according to who eats whom. Each arrow must point from the food to the feeder, showing the direction of energy flow and biomass transfer.

A trophic level is a stage in a food chain. During feeding, carbon compounds and their chemical energy pass from one organism to the next. Producers occupy the first trophic level, followed by primary consumers at the second, secondary consumers at the third, and so on.

Simple food chains traditionally leave out decomposers because they receive carbon compounds from dead organisms and waste from many trophic levels. They play a major role in food webs, transforming chemical energy through respiration and returning matter to the abiotic environment.

At each transfer, only some of the available organic matter becomes new biomass at the next trophic level. Follow the sequence: some available food isn’t harvested, some harvested material isn’t consumed, some consumed material isn’t absorbed, and some absorbed material isn’t stored.

Material may remain uneaten or be indigestible and pass out as faeces. It may also be excreted or used in cellular respiration, which transforms part of its chemical energy into heat. Organic-matter transfer is therefore never 100% efficient.

Gross productivity (GP) is the total gain in biomass by an organism. Net productivity (NP) is the amount remaining after losses due to cellular respiration. So:

Consumers commonly lose more biomass through respiration than producers. Activities such as movement and hunting use substantial amounts of energy. Net productivity is the biomass potentially available to the next trophic level. It is also the maximum sustainable yield that could be removed without reducing future availability.

Energy released during respiration is dispersed as heat, so it’s unavailable to higher trophic levels. Uneaten parts, unabsorbed material and other losses reduce the transfer even more. Commonly, 10% or less of the energy flowing into one trophic level reaches the next. Eventually, too little energy remains to support another viable level.

This limits food-chain length. It does not mean that an individual top predator must eat a greater mass of food than every organism below it. The argument concerns the total energy available at each trophic level.

To calculate transfer efficiency, divide the energy reaching the higher trophic level by the energy available at the preceding level, then multiply by 100. Make sure the units and time periods match.

A food web is a model of interconnected food chains within a community. It gives a more realistic picture of feeding than a single food chain, since organisms often have several food sources and predators.

Arrows run from the food to the consumer. They show the direction of energy flow and biomass transfer. One species may occupy different trophic levels in different pathways. For example, an omnivore acts as a primary consumer when it eats seeds, but as a secondary consumer when it eats herbivorous insects.

When building a food web from data, put the producers near the base and then add the consumers. Draw every stated feeding link in the correct direction, including decomposer links where the information supports them.

Biomass is the mass of organic matter in organisms or a trophic level. Since water accounts for most of the inorganic matter in living samples, dry mass provides an approximation of biomass.

To measure plant biomass, collect representative samples, often using randomly located quadrats. Record the mass of the container first, then weigh the fresh sample. Dry it in an oven at about $80^\circ\text{C}$, allow it to cool and weigh it again. Repeat this process until two consecutive masses agree. Subtract the container mass, then express the dry biomass per unit area. Using replicates improves reliability, while extrapolation allows sample values to estimate biomass across the whole site.

To estimate energy content, burn a known dry mass in a calorimeter and measure the heat transferred to a known mass of water. The result can then be extrapolated to the total biomass. Use plant material only and follow fire and heat precautions. Because some heat escapes to the surroundings, a simple calorimeter underestimates energy content.

An ecological pyramid is a diagram representing the relative numbers, biomass or energy of successive trophic levels. Its bars are centred on a vertical axis and have equal heights. Each one is labelled with its trophic level, while its width is proportional to the value supplied.

A pyramid of numbers displays organism counts. Sometimes it’s inverted because a few large producers can support many small consumers. A pyramid of biomass shows standing crop: the biomass present per unit area at one particular time, commonly measured in $\text{kg m}^{-2}$. This pyramid may also be inverted. That happens when producers have a small biomass but turn over rapidly, as can occur in aquatic systems.

A pyramid of energy shows the energy flowing to each trophic level per unit area per unit time, often reported as $\text{kJ m}^{-2}\text{ year}^{-1}$. It always narrows towards the top because energy transfer is inefficient, so it cannot be inverted. Rather than giving a standing-crop snapshot, it accounts for change over time and seasonal effects.

To create a pyramid from data, first decide whether the values represent counts, standing biomass or energy flow. Use a consistent numerical scale, then plot the bars symmetrically around the central axis.

A non-biodegradable pollutant is a pollutant that natural biological processes do not readily break down. Polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT) and mercury are examples.

Bioaccumulation refers to the increasing concentration of non-biodegradable pollutants in organisms or trophic levels over time (as more are absorbed). The key here is change over time within an organism or trophic level.

Biomagnification refers to the increasing concentration of non-biodegradable pollutants along a food chain (due to the loss of biodegradable biomass through, for example, cellular respiration). A predator takes in pollutants from many prey, but ordinary biomass is respired or lost in other ways. As a result, concentrations tend to be highest in long-lived top consumers. Toxic effects may then reduce survival or reproduction and disrupt the wider food web.

Microplastics are plastic particles smaller than 5 mm. Their surfaces can absorb non-biodegradable pollutants from the surrounding water. When organisms ingest these particles, the plastic and the pollutants attached to it can move more readily into food chains.

In the North Pacific subtropical gyre, plankton may consume plastic fragments carrying pollutants. Small fish then eat the plankton, before being eaten by larger fish. Microplastics and their associated pollutants pass through the food web in this way, potentially reducing feeding, growth or reproduction. Citizen-science programmes can monitor beach or water samples to reveal particle abundance, though they need contamination controls and consistent size categories.

Human activities alter ecosystem storages and change the rates of flows between them.

• Burning fossil fuels moves carbon that has been stored for long periods into the atmosphere as $CO_2$. Where $CO_2$ is a limiting factor, the extra supply may increase photosynthesis. However, warming and changes in water availability, along with co-released pollutants, generally lower primary productivity and disrupt food webs.
• Deforestation removes stored biomass and cuts both photosynthetic capacity and carbon uptake. The remaining residues release $CO_2$ through respiration and decomposition, while erosion carries soil matter into waterways.
• Urbanization replaces vegetation with roads and buildings, so biomass and primary production fall. Habitats become fragmented and food webs are disrupted. At the same time, external energy use increases, as do imports of materials and the production of wastes.
• Agriculture replaces diverse ecosystems with simpler food webs. It changes biomass storage and removes matter through harvested crops or livestock. Agricultural inputs and disturbance may cause further changes to productivity and transfers.

Deforestation, urbanization and agriculture show a common pattern: ecosystem biomass is lost, food webs are disrupted and the capacity for photosynthesis declines. Systems diagrams reveal these connections by showing how storages and flows differ before and after disturbance.

An autotroph is an organism that synthesizes carbon compounds from inorganic sources of carbon and other elements. A heterotroph is an organism that obtains carbon compounds from other organisms.

Every living organism belongs to one of these two nutritional groups. Autotrophs use an external energy source to build biomass. Heterotrophs rely on that biomass, either directly or indirectly.

A photoautotroph is an autotroph that uses light as an external energy source in photosynthesis. This category includes plants, algae and many bacteria.

A chemoautotroph is an autotroph that uses exothermic inorganic chemical reactions as an external energy source in chemosynthesis. These organisms live in many environments, but they are especially important where little or no light is available. Around deep-sea hydrothermal vents, chemoautotrophic microorganisms support food webs as their energetic foundation, without sunlight.

Primary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements. It applies to autotrophs and can be measured as either gross or net productivity.

Gross primary productivity (GPP) is the total rate at which producers produce biomass. Net primary productivity (NPP) is the rate remaining after producers' respiratory losses. The relationship is:

The guide usually reports productivity in the more practical unit $\text{kg C m}^{-2}\text{ year}^{-1}$.

In the laboratory, estimates may be made by comparing photosynthesizing samples under controlled light conditions. Field measurements use initial and final dry biomass from replicated sample areas over a known interval. An open plot shows the net change. Comparing it with an otherwise similar opaque-covered plot can provide an estimate of respiratory loss and, from this, gross production. Control area, duration, plant type, temperature and water availability.

Secondary productivity is the gain in biomass by consumers using carbon compounds absorbed and assimilated from ingested food. Faecal matter doesn’t count because it was never digested or absorbed.

Gross secondary productivity is represented by:

$GSP = I - F$

Here, $GSP$ is gross secondary productivity ($\text{kg C m}^{-2}\text{ s}^{-1}$), $I$ is the rate of food ingestion ($\text{kg C m}^{-2}\text{ s}^{-1}$), and $F$ is the rate at which unabsorbed material leaves as faeces ($\text{kg C m}^{-2}\text{ s}^{-1}$). The reporting units used for primary productivity also apply here.

To estimate this in the field, combine population-size estimates with the mean dry mass per individual at the beginning and end of a known period. Calculate the change in total dry biomass, then divide it by the area and the elapsed time. Sampling must represent the population, while capture or marking methods must minimize harm.

Net primary productivity is the amount of new producer carbon compounds that can be supplied sustainably to primary consumers. Gross production also counts biomass later used in producer respiration. NPP, rather than GPP, therefore provides the renewable basis of food chains.

In natural ecosystems, NPP is the plant growth available to herbivores. In farming and silviculture, it is the growth that farmers and foresters could potentially harvest. For consumers:

$NSP = GSP - R_c$

where $NSP$ is net secondary productivity ($\text{kg C m}^{-2}\text{ s}^{-1}$), $GSP$ is gross secondary productivity already defined, and $R_c$ is consumer respiration expressed as a carbon-equivalent rate ($\text{kg C m}^{-2}\text{ s}^{-1}$).

When using data, first make sure all values have matching units. Subtract respiratory loss from gross productivity, and don’t confuse faecal loss, which happens before assimilation, with respiratory loss after assimilation.

Maximum sustainable yields (MSYs) are the net primary or net secondary productivity of a system. They represent the fastest rate at which new biomass is replaced and, therefore, the theoretical upper harvest rate that won’t reduce future availability.

For a natural fish population, MSY corresponds to net secondary productivity. In a crop or plantation system, it corresponds to net primary productivity. If harvesting exceeds MSY, biomass is removed faster than growth and recruitment can replace it, so natural capital declines. In practice, managers usually set a precautionary harvest below the theoretical maximum because productivity varies and measurements are uncertain.

Each trophic transfer loses energy. As a result, lower trophic levels have more productivity per unit area than higher ones. When humans eat producers directly, one or more inefficient transfers are avoided, so the same primary production can provide more food energy.

Plant-based foods are therefore easier to produce at sustainable yields than foods from high trophic levels. They generally use less land, which reduces pressure for habitat conversion and livestock pollution. Sustainability still depends on the farming method and local conditions, as well as waste, transport and nutrition. Even so, eating lower in the food chain retains its thermodynamic advantage.

Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next level. Calculate it using:

Both energy values must be measured over the same area and interval.

Efficiency differs between ecosystems, species and trophic transfers. Consumption and digestion vary, as do activity and respiration. The familiar 10% figure isn't fixed or a true average, so calculate the actual percentage whenever data are provided.

Entropy refers to the amount of disorder within a system. As biomass and energy move through ecosystems, each transformation disperses energy. Under the second law, this produces an overall increase in entropy.

Living organisms keep their structures highly organized, with locally low entropy, by taking in concentrated energy and matter. This comes at the cost of a larger increase in the entropy of their surroundings, largely because cellular respiration releases dispersed heat and waste. Local biological organization therefore doesn't contradict the second law; total entropy still increases.