Ecosystem structure and function

The first principle

An ecosystem is a place where living things and their physical surroundings exchange energy and matter, and two physical laws decide how it works. Energy cannot be created or destroyed, but at every transfer much of it turns into heat that no organism can use again, so energy flows one way, from the sun to heat. Matter, by contrast, is never used up: carbon, nitrogen and phosphorus pass from soil and air into bodies and back again. Almost everything in this note, from the shape of a pyramid to the danger of a pesticide in a food chain, follows from this contrast between energy that flows and matter that cycles.

The core

Structure: what an ecosystem is made of

• The idea and its components. The British ecologist A G Tansley coined the word ecosystem in 1935 to treat organisms and their environment as one working system. Its abiotic part is light, temperature, water, air, soil and the nutrients in them, while its biotic part is the producers, consumers and decomposers that live on those resources.
• Producers, consumers and decomposers. Green plants, algae and some bacteria are producers, because they turn light into chemical energy by photosynthesis. Herbivores are primary consumers, carnivores that eat them are secondary consumers, and decomposers such as bacteria and fungi break down dead matter, while detritivores such as earthworms first break it into small pieces.
• Limiting factors. Liebig's law of the minimum says that growth is set by the scarcest essential resource, not by the total of resources, and Shelford's law of tolerance adds that too much of a factor limits a species as surely as too little. This is why adding phosphorus to a lake, where it is usually scarce, can transform it.

Function: how energy moves

• Capturing sunlight. About half of the sunlight that reaches the ground is photosynthetically active radiation, the wavelengths plants can use, and plants capture only about 2 to 10 per cent of it. That small fraction sustains nearly every living thing on Earth.
• Productivity. Gross primary productivity (GPP) is the rate at which producers fix energy, and net primary productivity (NPP) is what remains after their own respiration, so NPP equals GPP minus respiration. NPP is the food available to every consumer, while secondary productivity is the rate at which consumers form new tissue.
• Food chains and webs. In a grazing food chain energy passes from living plants to herbivores and carnivores, while in a detritus food chain it passes from dead matter to decomposers. In water the grazing chain carries most of the energy, but on land a far larger share flows through the detritus chain, and real chains interlock into food webs, which give an animal alternative food when one prey becomes scarce.
• The ten per cent law. Raymond Lindeman showed in 1942 that only about a tenth of the energy at one trophic level, or feeding level, reaches the next, because the rest is spent on respiration, lost in waste or never eaten. This is why food chains rarely run beyond four or five levels and why a tiger needs a large area of prey.
• Decomposition. Dead matter passes through fragmentation, leaching of soluble nutrients, catabolism by enzymes, humification into dark humus and finally mineralisation, which releases inorganic nutrients back to plants. It is fastest where it is warm and moist and slow where it is cold, dry or waterlogged, which is why peat builds up in bogs.

Ecological pyramids

• Three measures, three shapes. A pyramid stacks trophic levels, with producers at the base, and can measure numbers, biomass or energy. The pyramid of energy is always upright, because of the ten per cent law, but a pyramid of numbers is inverted on a single tree carrying thousands of insects, and a pyramid of biomass is inverted in the open sea, where fast dividing phytoplankton weigh less at any moment than the zooplankton they feed.
• Standing crop and limits. The living mass at a level at a given time is its standing crop, and a pyramid shows that crop, not the rate of production. Pyramids also ignore detritus feeders and species that feed at more than one level, which is why they simplify rather than describe a real web.

Dimensions

• Ecological. Productivity is uneven across the planet. Coral reefs, estuaries and tropical rainforests produce the most per square metre, deserts and the open ocean the least, yet the ocean covers about 70 per cent of the Earth and so adds a large share of the global total, which textbooks estimate at about 170 billion tonnes of dry matter a year.
• Economic. The Millennium Ecosystem Assessment of 2005 grouped ecosystem services into provisioning (food, water, timber), regulating (climate, floods, pollination), cultural (recreation, spiritual value) and supporting (soil formation, nutrient cycling). Robert Costanza and colleagues estimated in 1997 that these services were worth on average about 33 trillion US dollars a year, more than world gross national product at the time.
• Health. Persistent chemicals that organisms cannot break down build up in body tissue, which is bioaccumulation, and rise in concentration at each trophic level, which is biomagnification, so top predators and humans carry the highest loads.
• Social and legal. Article 48A of the Constitution directs the State to protect and improve the environment, and Article 51A(g) makes it a fundamental duty of every citizen; both were added by the 42nd Amendment in 1976.

In practice

• The vulture collapse. India's Gyps vultures, among them the white rumped vulture Gyps bengalensis (Critically Endangered on the International Union for Conservation of Nature (IUCN) Red List, Schedule I of the Wildlife (Protection) Act 1972 as amended in 2022, and Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES)), fell by more than 95 per cent from the 1990s to the mid 2000s. The cause was diclofenac, a veterinary painkiller left in cattle carcasses, which causes kidney failure in vultures, and the government banned its veterinary use in 2006.
• Further action on the same chain. The Ministry of Environment, Forest and Climate Change (MoEFCC) adopted the Action Plan for Vulture Conservation 2020-2025, and in August 2023 the Union health ministry also banned ketoprofen and aceclofenac for animal use, since both harm vultures in the same way.
• Endosulfan. In 2011 the Supreme Court, in an interim order in Democratic Youth Federation of India v Union of India, banned the production, use and sale of the pesticide endosulfan across India, after decades of aerial spraying on cashew plantations in Kasaragod, Kerala, were linked to severe illness in nearby villages. The same year it was listed for elimination under the Stockholm Convention on Persistent Organic Pollutants.
• Counting what ecosystems give. An expert group under the economist Partha Dasgupta, set up by the Ministry of Statistics and Programme Implementation (MoSPI), proposed a framework for green national accounts in 2013. MoSPI now publishes environmental accounts, including the extent and services of ecosystems, in its annual EnviStats India reports, following the United Nations System of Environmental Economic Accounting.

Issues and way forward

• Services are invisible in the accounts. A forest is counted when it is cut and sold but not when it holds soil and water, because regulating and supporting services have no market price; mainstreaming green accounts in planning, as the Dasgupta group proposed, and paying communities for services they protect are the correction.
• Persistent chemicals keep entering food chains. New pesticides and drugs are approved on their direct toxicity, not on what happens when they climb a food chain, so testing for effects on scavengers and predators and for persistence and biomagnification before approval, together with integrated pest management, is the remedy that the vulture and endosulfan cases point to.
• Simplified food webs. Fragmentation, hunting and monoculture remove links from webs, which leaves the remaining species with fewer alternatives and makes populations swing more sharply; keeping large connected habitats and diverse farms keeps webs resilient.
• Degraded soils slow decomposition. Heavy use of chemicals, loss of organic matter and burning of crop residue weaken the soil organisms that return nutrients, so adding organic matter and protecting soil biology restore the detritus chain on which farms rely.

Common confusions

• GPP and NPP. Gross primary productivity is all the energy producers fix, while net primary productivity is what is left after their own respiration, and only NPP is available to consumers.
• Productivity and standing crop. Productivity is a rate, such as grams per square metre per year, while standing crop is the mass present at one moment; phytoplankton have a small standing crop but high productivity.
• Bioaccumulation and biomagnification. Bioaccumulation is the build up of a chemical inside one organism over its life, while biomagnification is the rise in concentration from one trophic level to the next.
• Decomposers and detritivores. Detritivores such as earthworms eat and fragment dead matter, while decomposers such as bacteria and fungi break it down chemically into simple nutrients.
• Energy flow and nutrient cycling. Energy passes once through an ecosystem and leaves as heat, while nutrients go round again and again; the cycles are in the biogeochemical cycles note.

For the exam

• Keyword bank: abiotic and biotic components, trophic level, grazing and detritus food chains, food web, ten per cent law, ecological pyramid, inverted pyramid, gross and net primary productivity, standing crop, decomposition and humification, limiting factor, biomagnification, ecosystem services, green accounting.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: Plants capture only about 2 to 10 per cent of the photosynthetically active light that reaches them, and only about a tenth of that energy passes from one trophic level to the next.
  • Introduction with a report: The Millennium Ecosystem Assessment of 2005 found that about 60 per cent of the ecosystem services it examined were being degraded or used unsustainably.
  • Conclusion: Because energy flows one way and nutrients cycle, any policy that breaks a food chain or poisons the soil weakens the base on which farming, water and health depend.
  • Conclusion: Counting ecosystem services in national accounts, as the Dasgupta framework proposed, would let India see the cost of degradation before it shows up as lost crops or failed rivers.