What their lights revealed instead was a crowd. Around plumes of hot, mineral-rich water gushing from the sea floor stood forests of giant tube worms taller than a person, along with clams, crabs and mussels, packed densely on the rock. There was no sunlight, no plant, not a single green leaf anywhere. Yet here was a thriving community, feeding, growing and reproducing.
The animals were not living on sunlight at all. The vent water was rich in hydrogen sulphide, and bacteria there were using the chemical energy of that gas to build organic food — exactly the job green plants do on land using light. Those bacteria were the producers of this dark ecosystem, and every other animal in that community, directly or indirectly, was eating them.
The discovery made one lesson unmistakable. An ecosystem is not defined by sunlight, soil or water. It is defined by a flow of energy, moving from producers to consumers, in one direction only.
Each of these questions is answered somewhere in this chapter — watch for them as you read.
14.1 Ecosystem: Structure and Function
Before you begin, recall from earlier classes: (1) What is environmental biology? (2) What does the term habitat mean? (3) Why is the study of ecology important for human beings?
Every living thing sits inside a web of surroundings. A fish is surrounded by water, dissolved gases, sunlight, mud, food and other fish. A mango tree is surrounded by soil, minerals, rain, insects, birds and other trees. In earlier chapters you studied these surroundings in two separate lists — the biotic components (all living organisms) and the abiotic components (all non-living factors such as light, temperature, water, minerals and soil).
The two lists, however, are not really separate. The tree draws minerals from the soil; the fallen leaves of the tree put minerals back into the soil. The living and the non-living constantly act upon each other. When we stop studying them separately and instead study them as one working unit, that unit is called an ecosystem.
An ecosystem is described as self-regulatory and self-sustaining. Self-regulatory means it can hold itself in balance — if herbivores increase, the plants they eat decrease, which then brings herbivore numbers down again. Self-sustaining means that, given sunlight, it can keep running on its own, because the matter inside it is used again and again.
An ecosystem is a self-regulatory and self-sustaining structural and functional unit of nature (of the biosphere), containing both biotic and abiotic components that interact with each other and with the surrounding environment.
The word ecosystem was coined by the British botanist A. G. Tansley in 1935. Ecologists before him tended to treat a community of organisms as though it were a kind of super-organism, almost separate from its physical surroundings. Tansley argued that this was artificial: organisms and their physical environment form one system, and neither can be understood without the other. That single change in viewpoint is what allows us today to measure the energy entering a forest, follow it through its animals, and account for what is lost.
The Size and Types of Ecosystems
Ecosystems vary enormously in size. A small pond is an ecosystem; so is a large ocean. A small farmland is an ecosystem; so is an entire village. Size is not what makes something an ecosystem — the presence of interacting biotic and abiotic components, functioning as a unit, is what makes it one.
The entire biosphere can therefore be considered as one global ecosystem, made up of many smaller local ecosystems fitted together. But the Earth’s ecosystem as a whole is far too big and far too complex to be studied directly. So, for convenience of study, it is divided into two basic categories, viz. terrestrial and aquatic.
Forest, grassland and desert are the types of terrestrial ecosystems. Lakes, wetlands, rivers, seas, oceans and estuaries are the types of aquatic ecosystems. Each has its own characteristic set of organisms, because each has its own characteristic abiotic conditions of water, light and temperature.
Ecosystems can also be classified in a second, quite different way — as natural ecosystems and artificial ecosystems. This classification is based not on where they are, but on whether human beings have to keep them running.
Natural ecosystems do not require any human inputs; in other words, they are self-sustainable. A forest is not watered, weeded or fertilised by anyone, yet it persists. Artificial ecosystems, such as a farm land, a fish tank, or even a large pond used for rearing fish, require constant input in terms of energy or materials. Stop feeding a fish tank, and it collapses.
Natural vs Artificial ecosystems | Feature | Natural ecosystem | Artificial ecosystem | |—|—|—| | Origin | Formed by nature, without human design | Created and maintained by humans | | Human input | Not required | Constantly required (energy, materials) | | Self-sustaining? | Yes, self-sustainable | No, collapses without input | | Species diversity | Usually high | Usually low; often a single crop or species | | Examples | Forest, grassland, desert, ocean, natural lake | Farm land, fish tank, fish-rearing pond |
What This Chapter Sets Out to Do
In this chapter we will study and analyse the structure and the functions of the ecosystem. Think of it like the accounts of a business. There is an input — the organic matter produced by plants, measured as net productivity. There is a transfer — the movement of energy and matter through food chains, food webs and nutrient cycles. And there is an output — degradation of dead matter and the steady loss of energy.
Along the way we will also look at the relationships, chains and webs that are created precisely because energy flows through the system. Nothing in an ecosystem eats, competes or decays in isolation; every link exists because energy is moving from somewhere to somewhere else.
Species Composition and Physical Structure
We already know that biotic and abiotic components influence each other. Let us now look at them with an integrated approach and see how the flow of energy takes place in an ecosystem.
The interaction of biotic and abiotic components results in a physical structure that is characteristic for each type of ecosystem. A forest looks like a forest — layered, shaded, tall — because of the way its plants and its climate have acted on each other. A grassland has an entirely different physical structure for the same reason.
Identification and enumeration of the plant and animal species of an ecosystem gives its species composition. In simple words, species composition answers the question “which species live here, and how many of each?”, while physical structure answers “how are they arranged in space?” Together, these two make up the structural aspect of an ecosystem.
Spatial Patterns: Stratification and Zonation
Biotic and abiotic components are not spread evenly. They differ as locations vary in space and in time. The variation that arises due to space results in a spatial pattern. There are two types of spatial patterns, viz. stratification and zonation.
The vertical distribution of different species of plants and animals occupying different levels is known as stratification. In a forest, trees occupy the top vertical strata or layer, shrubs the second, and herbs and grasses occupy the bottom layer, as shown in Fig. 14.1.
Why does this layering arise at all? Because light is the resource being competed for, and it arrives from above. The tallest plants intercept it first, so only the species that can survive in dim, filtered light succeed underneath. The vertical ladder of a forest is really a vertical ladder of decreasing light.
Stratification is not limited to land. It is also observed in the open seas, as the epipelagic, meso-pelagic, bathy-pelagic and benthic zones — again arranged from the sunlit surface downward into darkness.
Stratification is the vertical distribution of different species of plants and animals occupying different levels of an ecosystem.
Find out how animals are stratified in the Amazon rain forest — which animals live in the canopy, which in the understorey, and which on the forest floor.
The horizontal distribution of plants and animals on land or in water is called zonation. Zonation is observed in aquatic (wetland) as well as terrestrial ecosystems, but it is most easily seen at the junction of the two — where land meets water. The edges of a large lake or a beach show pronounced zonation in the form of inter-tidal, littoral and sub-littoral zones. Fig. 14.2 shows this banding in a wetland.
Zonation is the horizontal distribution of plants and animals on land or in water, giving rise to distinct zones or bands.
Stratification vs Zonation Both are spatial patterns, but stratification is vertical (layer above layer, like the storeys of a building), while zonation is horizontal (band beside band, like strips drawn on a map).
Think about it — why is zonation more pronounced at the edges of a habitat than in its middle?
The Four Functional Processes of an Ecosystem
Structure alone does not make an ecosystem. Its biotic and abiotic components are all linked together to function as a single ‘ecosystem unit’ through various processes — productivity, decomposition, nutrient cycling and energy flow. These four are the functional aspects of an ecosystem.
Any ecosystem must perform all four of these processes for its sustenance, that is, to be self-sustaining. This holds true whichever ecosystem is under study — it may be as small as a pond or as large as the entire biosphere. The four processes are summarised in FC 14.1.
Productivity is the process by which inorganic chemicals are converted into organic material with the help of the radiant energy of the sun by the autotrophs, and the autotrophs are then consumed by heterotrophs. It is the doorway through which both energy and matter enter the living part of the ecosystem.
Decomposition is the breakdown of dead organic material and the mineralization of dead matter. Without it, nutrients would remain locked inside corpses, faeces and fallen leaves, and the ecosystem would eventually starve amid its own waste.
Nutrient cycling is the storage and transport of nutrients. The minerals released during decomposition are used again by autotrophs, so the same atom of nitrogen or phosphorus can pass through many organisms over time.
Energy flow is the unidirectional flow of energy from producers to consumers, with final dissipation and loss as heat. Notice the crucial contrast with nutrient cycling: nutrients go round in a circle, but energy travels in a straight line and leaves the system for good.
Nutrient cycling vs Energy flow Nutrients are recycled — the same phosphorus atom can be reused endlessly. Energy is not recycled — it enters as sunlight, moves in one direction, and is lost as heat. This is why every ecosystem needs a constant fresh supply of solar energy, but does not need a constant fresh supply of matter.
the ecosystem Zoom in: an ecosystem is made of populations of many species, each population made of individual organisms, each organism of organ systems, organs, tissues and cells. Zoom out: many local ecosystems together make up a biome, and all the biomes together make up the biosphere — the single global ecosystem.
A Pond as a Working Model
Think of a small pond ecosystem. It is fairly a self-sustainable unit, and it explains the complex interactions which exist in any aquatic ecosystem. A pond is a shallow water body in which all four of the above basic processes of an ecosystem can be observed at once, which is why ecologists use it so often as a teaching model.
The abiotic component is the water, with all the dissolved inorganic and organic substances in it, together with the rich soil deposit at the bottom of the pond. The solar input, the cycle of temperature, the day-length and other climatic conditions regulate the rate of function of the entire pond.
The biotic components fall into the familiar three groups. The producers include the phytoplankton, algae and other aquatic plants. The consumers are represented by the zooplankton, aquatic insects and fish. The decomposers are the fungi and bacteria located at the bottom of the pond.
Productivity
A constant input of solar energy is the basic requirement for any ecosystem to function and sustain itself. That input is captured by plants, and the rate at which it is turned into living matter is what ecologists call productivity.
Productivity refers to the rate of generation of biomass in an ecosystem. Because it is a rate, it must be expressed per unit surface (or per unit volume) per unit time — for instance grams per square metre per day, written \(g/m^2/day\). Here the mass unit may relate to dry matter, or to the mass of carbon generated.
Productivity is the rate of generation of biomass in an ecosystem, expressed as mass per unit surface (or volume) per unit time, e.g. \(g/m^2/day\).
Productivity can be divided into gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis — the total organic matter made, before anything is spent.
But plants are living things, and living things respire. Plants themselves use a considerable proportion of this GPP for their own respiration. Hence, gross primary productivity minus respiratory losses (R) constitutes the net primary productivity (NPP):
\[GPP - R = NPP\]
Here GPP is the gross primary productivity (rate of organic matter produced in photosynthesis), R is the respiratory loss (organic matter consumed by the plant’s own respiration) and NPP is the net primary productivity — all three expressed in the same units, e.g. \(g/m^2/day\).
Gross primary productivity (GPP) is the rate of production of organic matter by producers during photosynthesis.
Net primary productivity (NPP) is the gross primary productivity minus respiratory losses (R); it is the biomass actually available for consumption by heterotrophs — herbivores, carnivores and decomposers.
Net primary productivity is the number that really matters to the rest of the ecosystem, because it is the available biomass for consumption by heterotrophs (herbivores, carnivores and decomposers). GPP is what the plant earns; R is what the plant spends on itself; NPP is what is left over for everybody else.
GPP vs NPP | Feature | Gross primary productivity (GPP) | Net primary productivity (NPP) | |—|—|—| | Meaning | Total rate of organic matter produced in photosynthesis | GPP remaining after the plant’s own respiration | | Relation | — | \(NPP = GPP - R\) | | Value | Always the larger of the two | Always smaller than GPP | | Significance | Total energy captured by producers | Energy actually available to heterotrophs |
In a grassland plot, the gross primary productivity is measured as \(20\ g/m^2/day\) (dry weight), and the plants lose \(7\ g/m^2/day\) through their own respiration. Calculate the net primary productivity, and state how much dry matter becomes available to herbivores from an area of \(100\ m^2\) in one day.
The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, the productivity of the oceans is only 55 billion tons. The rest, of course, is from land-based ecosystems.
NPP of the biosphere — land versus ocean The oceans cover roughly seven-tenths of the Earth’s surface, yet they contribute only 55 of the 170 billion tons of annual NPP — well under a third. Read the two numbers together and the conclusion is unavoidable: per unit area, the open ocean is far less productive than land. The vastness of the sea hides how thin its production actually is.
Use your brain power — what could be the reason for the low productivity of the ocean?
Primary productivity (GPP) depends on the plant species inhabiting a particular area, since different species photosynthesise at different rates. It also depends on a variety of environmental factors, on the availability of nutrients, and on the photosynthetic capacity of the plants. Therefore, it varies widely in different types of ecosystems.
Producers are not the only ones adding organic matter. Secondary productivity is defined as the rate of formation of new organic matter by consumers. Alternatively, it is the rate of assimilation of food energy at the level of consumers. It is the amount of energy available to a consumer for transfer to the next trophic level.
Secondary productivity is the rate of formation of new organic matter by consumers, i.e. the rate of assimilation of food energy at the consumer level.
Primary vs Secondary productivity Primary productivity is the work of producers (autotrophs) building organic matter from inorganic chemicals using sunlight. Secondary productivity is the work of consumers (heterotrophs), who do not create organic matter from inorganic material — they merely convert food that already exists into their own body matter.
Decomposition
Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients, and the process is called decomposition. If productivity is the ecosystem’s way of building, decomposition is its way of dismantling — and both are equally essential.
The raw material for decomposition is detritus: the dead remains of plants and animals, including faecal matter. Every fallen leaf, every dead insect, every bit of dung is detritus waiting to be processed.
Decomposition is the breakdown of complex organic matter (detritus) into inorganic substances such as carbon dioxide, water and nutrients, by decomposers.
Detritus is the dead remains of plants and animals, including faecal matter; it is the raw material for decomposition.
The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralization. The sequence, and the way it feeds back into the soil, is shown in Fig. 14.3 and set out step by step in FC 14.2.
Detritivores (e.g. the earthworm) break down detritus into smaller fragments or particles. This process is called fragmentation. Its purpose is simple but vital: breaking a leaf into fragments increases the surface area exposed to microbes, so everything that follows can happen faster.
By the process of leaching, water-soluble inorganic nutrients go down (percolate) into the soil horizon and get precipitated there as unavailable salts. Note the word unavailable — leaching does not release nutrients to plants; it locks a portion of them away.
Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called catabolism. Here the decomposers are working chemically, not mechanically, cutting large organic molecules into small ones.
It is important to note that all the above steps in decomposition operate simultaneously on the detritus. They are listed one after another only for convenience of description; on a single decaying leaf, fragmentation, leaching and catabolism are all happening at the same time.
Fragmentation vs Catabolism Fragmentation is physical breakdown into smaller pieces, done mainly by detritivores such as earthworms. Catabolism is chemical breakdown into simpler inorganic substances, done by bacterial and fungal enzymes.
Humification and mineralization occur during decomposition in the soil. Humification leads to the accumulation of a partially decomposed, dark coloured, amorphous, colloidal organic substance called humus, that is resistant to microbial action and undergoes decomposition at an extremely slow rate.
Humus formation changes soil texture and increases the water holding capacity of the soil. Being colloidal in nature, humus serves as a reservoir of nutrients — a slow-release store that holds minerals in the soil instead of letting them wash away.
humus Dark, amorphous, colloidal and resistant to microbes → decomposes extremely slowly and binds soil particles and water → acts as a long-term reservoir of nutrients, improves soil texture and raises the water holding capacity of soil.
The humus is further degraded by some microbes, and the release of inorganic nutrients from it occurs by the process known as mineralisation. This is the final step, and the most important one for plants, because only in inorganic (mineral) form can nutrients be absorbed again by roots. Mineralisation is the point at which decomposition hands the nutrients back to nutrient cycling.
Mineralisation is the degradation of humus by microbes, releasing inorganic nutrients into the soil.
a fallen leaf in a forest 1. The leaf drops from the tree and joins the litter on the forest floor — it is now detritus. 2. Earthworms and other detritivores chew and shred it into fine fragments (fragmentation), greatly increasing its surface area. 3. Rain water dissolves some inorganic nutrients from the fragments and carries them down into the soil horizon, where they precipitate as unavailable salts (leaching). 4. Bacteria and fungi secrete enzymes onto the fragments and degrade the complex organic molecules into simpler inorganic substances (catabolism). 5. Part of the material resists complete breakdown and accumulates as dark, colloidal humus (humification), which improves the soil’s texture and water holding capacity. 6. Microbes slowly degrade the humus, releasing inorganic nutrients into the soil (mineralisation). 7. The roots of the same tree absorb those nutrients — and the atoms of the old leaf are built into a new one.
Decomposition as a process requires oxygen — it is essentially an oxidative breakdown carried out by aerobic microbes. Temperature and soil moisture are the most important factors that regulate decomposition indirectly, by helping or hindering the soil microbes that do the actual work.
A warm and moist environment favours decomposition, because microbial enzymes work fastest there. Low temperature and anaerobic conditions inhibit decomposition, because the microbes slow down or cannot function at all.
These same two factors explain why we refrigerate food. Cooling it does not kill the microbes; it merely slows their enzymes, so decomposition (which we call spoiling) is delayed. The same principle operates on a much bigger scale in nature: in cold northern coniferous forests, decomposition is so slow that a thick carpet of undecomposed litter builds up on the forest floor, while in a warm, moist tropical forest the litter disappears within months. Waterlogged, oxygen-poor peat bogs preserve plant remains for thousands of years for exactly the same reason — no oxygen, no decomposition. And in a garden compost pit, gardeners deliberately turn the heap to let air in and sprinkle water on it: they are supplying the oxygen, warmth and moisture that decomposers need.
14.2 Energy Flow
Before you begin, recall from earlier classes: (1) What is a food chain? (2) What are trophic levels in a food chain?
Where the Energy Comes From
The Sun is the only source of energy for all ecosystems on the earth, except for the deep-sea ecosystems. Everything green that grows, every animal that runs, and every microbe that decays a leaf is ultimately spending sunlight that arrived some time ago.
But only a fraction of that sunlight is usable. Of the total incident solar radiation, less than 50 % of it is photosynthetically active radiation (PAR) — that is, radiation of wavelengths that chlorophyll can actually absorb and use. The rest lies outside the useful range and merely heats the earth and the air.
Plants and photosynthetic bacteria (autotrophs) fix this energy to prepare food from simple inorganic materials. Even of the PAR that reaches them, plants capture only 2–10 %. Take a moment to appreciate how small that is — and yet this small amount of energy sustains the entire living world.
Q1. Deep-sea vent animals live in total darkness. If sunlight is the energy source for almost every ecosystem on Earth, where does the energy in that community come from?
A. From chemicals instead of light. The deep-sea ecosystem is the one exception to the rule that the Sun is the source of energy for all ecosystems. At hydrothermal vents, the producers are bacteria that use the chemical energy of hydrogen sulphide rising from the sea floor to build organic food — a process called chemosynthesis. These bacteria occupy the first trophic level exactly as green plants do elsewhere, and every consumer in that dark community feeds on them, directly or indirectly. The energy source changes; the pattern of energy flow does not.
Find out — (1) Is there any presence of living organisms in the perpetual darkness of deep oceanic trenches? (2) In the absence of solar radiation, what is their source of energy? (3) Which organisms serve as the producers in the food chains of the deep oceans?
Energy Flows in One Direction Only
Since almost all life runs on captured sunlight, it becomes very important to know how the solar energy captured by plants flows through the different organisms of an ecosystem. Directly or indirectly, all organisms are dependent for their food on producers.
Hence there is a unidirectional flow of energy from the sun to the producers, and then from the producers to the consumers. The direction cannot be reversed — a deer cannot hand energy back to the grass, and the grass cannot return it to the Sun. Energy can be used only once in the ecosystem.
Energy flow in an ecosystem is unidirectional: energy passes from the sun to the producers and then to the consumers, is never reversed, can be used only once, and is finally lost from the ecosystem as heat.
This one fact explains why every ecosystem needs a fresh delivery of sunlight every single day, yet does not need a fresh delivery of nitrogen or phosphorus — those it can recycle. Cut off the sunlight and the ecosystem runs down: the matter remains, but the energy to move it does not.
Producers and Consumers
The autotrophs need a constant supply of energy to synthesize the molecules they require. Because they are the point at which energy enters the living world, the autotrophs are called producers.
In a terrestrial ecosystem, the major producers are herbaceous and woody plants. Likewise, the producers in an aquatic ecosystem are phytoplankton and algae — individually invisible to the eye, but collectively feeding the entire ocean.
All animals directly or indirectly depend on plants for their food. They are hence called consumers (heterotrophs). If they feed directly on the plants, they are called primary consumers; and if the animals eat other animals which eat plants, they are called secondary consumers. Likewise, you could have tertiary consumers too.
The primary consumers are also known as herbivores. Some common herbivores are insects (grasshopper, aphids), birds (parrot) and some mammals (sheep, cattle, goat, donkey) in the terrestrial ecosystem, and molluscs in the aquatic ecosystem.
The consumers that feed on these herbivores are carnivores (secondary consumers). Those animals that depend on the primary carnivores for food are called secondary carnivores. Notice that these names describe not the animal itself but what it eats — which is exactly what application-based questions test when they place the same animal in two different chains.
Trophic Levels
Every organism occupies a place in the ecological community according to the source and the method of obtaining its food. The specific place that an organism occupies in a food chain is its trophic level.
Producers belong to the first trophic level, herbivores (primary consumers) to the second, and carnivores (secondary consumers) to the third trophic level. The tertiary consumer, or top carnivore, occupies the fourth trophic level. Chart 14.4 sets out these levels with examples.
A trophic level is the specific place an organism occupies in a food chain, decided by the source and the method of obtaining its food.
the four trophic levels in order P–H–C–T: Producer → Herbivore → Carnivore → Top carnivore. First it is made, then mown, then hunted, then hunted again.
Food Chains and Their Types
You have already studied several food chains and food webs that exist in nature. Food chains are always straight, and they usually have four or five trophic levels.
There are three types of food chains, viz. grazing, detritus and parasitic. Starting from the plants (or producers), food chains and food webs are formed such that an animal feeds on a plant or on another animal, and in turn is itself food for another.
The energy trapped by the producer is either passed on to a consumer, or it remains trapped until the producer organism dies. The death of an organism is the beginning of the detritus food chain or web — the energy is not wasted, it simply changes route.
A simple grazing food chain (GFC) is shown in FC 14.3. It begins with a living producer.
The detritus food chain (DFC) begins with dead organic matter. It is composed of decomposers, which are heterotrophic organisms — mainly fungi and bacteria. They meet their energy and nutrient requirements by degrading the detritus, and they are known as saprotrophs (saprobes). The sequence is shown in FC 14.4.
Decomposers secrete enzymes that break down dead organic materials into simple, inorganic materials, which are then absorbed by them. Notice that they digest their food outside the body and absorb it afterwards — the reverse of the order in which an animal digests.
The detritus food chain may be connected with the grazing food chain at some levels. In a natural ecosystem, some animals like cockroaches, crows, bears and man are omnivores. Omnivores eat producers as well as consumers, and so they belong to more than one chain at the same time.
Grazing food chain vs Detritus food chain | Feature | Grazing food chain (GFC) | Detritus food chain (DFC) | |—|—|—| | Starting point | Living producers (green plants) | Dead organic matter (detritus) | | First link feeds on | Living plant tissue | Dead remains and faecal matter | | Main organisms | Herbivores and carnivores | Saprotrophs — mainly fungi and bacteria | | Mode of nutrition of first consumer | Ingestion of living tissue | Extracellular digestion, then absorption | | Link between the two | Death of any organism of the GFC starts a DFC | Nutrients released return to the producers |
These natural interconnections of food chains make a food web, shown in Fig. 14.5. A food web is what a food chain becomes once we stop pretending that an animal eats only one kind of food.
Can you tell — from the given food web diagram, at which trophic levels is the eagle present?
Food chain vs Food web A food chain is a single straight line of “who eats whom”, with one organism at each trophic level. A food web is many food chains interconnected, so one organism may occupy several trophic levels and have several sources of food. Food chains rarely exist alone in nature; the food web is the reality.
Use your brain power — (1) What could be the connecting points between the GFC and the DFC? (2) Would you classify man as a carnivore (primary or secondary) or as an omnivore, and why? (3) At how many trophic levels do human beings function in a food chain?
The 10 % Law and the Limit on Food Chains
The amount of energy available decreases at each successive trophic level. This is not a gentle decrease — it is a very steep one, and Fig. 14.6 shows just how steep.
The number of trophic levels in any food chain is restricted, because the transfer of energy follows the ‘10 % Law’ (R. Lindermann, 1942). The law states that only 10 % of the energy is transferred to each trophic level as net energy, from the previous trophic level.
10 % Law (R. Lindermann, 1942): Only 10 % of the energy present at one trophic level is transferred as net energy to the next trophic level; the remaining 90 % is lost from the chain, largely as heat.
reading the energy flow in Fig. 14.6 Start with the 1000 J of light energy falling on the grass. Only 100 J of it — one-tenth — ends up as net energy in the deer, while 900 J is lost to the environment. Of that 100 J, only 10 J reaches the lion; 90 J is lost. Of that 10 J, 9 J is lost again. The key thing to read out of these numbers is that the loss is not a fixed amount but a fixed fraction: at every step, nine-tenths disappears. Because the loss multiplies rather than subtracts, the fall is far faster than it first looks.
The 10 % figure was established by the American ecologist Raymond Lindeman in the early 1940s. Studying a small lake in Minnesota, he did something no one had done before: instead of merely listing which organism ate which, he measured how much energy was held at each feeding level and tracked how much of it actually moved upward. What he found was a steep, unavoidable tax at every step. His work on the trophic-dynamic view of ecology turned ecology from a descriptive science into a quantitative one — and it appeared in print only after his death, when he was still in his twenties.
1000 joules of sunlight through a grassland 1. 1000 J of light energy falls on the grass. Less than half of it is even photosynthetically active radiation, and the grass captures only a small percentage of that. 2. 100 J ends up as net energy stored in the grass; the other 900 J is lost to the environment — reflected, unabsorbed, or spent in the plant’s own respiration and released as heat. 3. A deer grazes. It does not eat every blade, it cannot digest every part of what it eats, and it spends energy on movement, warmth and respiration. Only 10 J becomes net energy in the deer’s body; 90 J is lost. 4. A lion eats the deer. The same losses repeat, and only 1 J becomes net energy in the lion; 9 J is gone. 5. Any predator trying to live on lions alone would inherit about 0.1 J from the original 1000 J. There is simply not enough left to keep it alive — which is why the chain ends here.
In an aquatic food chain, phytoplankton fix 8000 J of energy. Applying the 10 % Law, calculate the net energy available to the zooplankton (primary consumer), to the small fish (secondary consumer) and to the large fish (tertiary consumer). Also state how much energy is lost to the environment between the phytoplankton and the small fish.
In nature, it is possible to have different trophic levels — producer, herbivore, primary carnivore, secondary carnivore, tertiary carnivore and ultimate carnivore. But there is a hard ceiling on how many of these can actually be supported.
Beyond the secondary carnivores, however, the amount of energy available is too little. Hence, there is no tertiary carnivore that feeds exclusively on secondary carnivores — even though a secondary carnivore will many times feed on herbivores directly, dropping a level to find a larger and more reliable meal.
Q2. Why do food chains stop at four or five links instead of running on for twenty?
A. Because of the 10 % Law. Only about one-tenth of the energy at any trophic level passes on as net energy to the next; the other nine-tenths is lost, mostly as heat. This loss compounds at every step, so after four or five links almost nothing remains. A twentieth trophic level would receive an unimaginably small share of the original energy — far too little to support even one animal. It is energy, not appetite, that sets the length of a food chain.
This is precisely the reason why food chains do not exist in isolation, but are always interconnected to form a food web that maintains the stability of an ecosystem. A predator with several possible prey species can survive if one of them declines; a predator locked into a single, energy-starved chain cannot.
The same energy tax explains a familiar agricultural fact. A given field of grain can feed far more people directly than it can if the grain is first fed to poultry or cattle and the meat is then eaten, because roughly nine-tenths of the energy is lost in that extra trophic step. This is why densely populated regions have historically depended on cereal-based diets, and why the land needed to produce one kilogram of meat is many times the land needed to produce one kilogram of grain. Shortening the food chain is, quite literally, an energy-saving measure.
Think about it — what is the maximum number of trophic levels possible in a food chain, and what sets that limit?
14.3 Ecological Pyramids
We now know that organisms sit at trophic levels, and that energy shrinks sharply as it climbs from one level to the next. The obvious next question is: can we draw that relationship, so that it can be seen at a glance? The ecological pyramid is exactly such a picture.
An ecological pyramid is a graphic representation of the relationship between the organisms of various successive trophic levels, with respect to energy, biomass and number.
An ecological pyramid is a graphic representation of the relationship between the organisms of successive trophic levels of an ecosystem, in terms of their number, biomass or energy.
A pyramid is a structure which has a broader base that gradually narrows upwards, forming an upright, cone-like structure. This concept was developed by C. Elton in 1927.
The British ecologist Charles Elton, working in the 1920s, was among the first to study animal communities as feeding systems rather than as mere lists of species. From his fieldwork — including expeditions to the Arctic, where food webs are simple enough to be traced almost completely — he noticed a consistent pattern: the small plant-eating animals were extremely numerous, the animals that ate them were fewer, and the top predators were very few indeed. When this was set out as a stack of bars, the shape that emerged was a pyramid. Ecologists still call the number pyramid an “Eltonian pyramid” in his honour.
How a Pyramid Is Built
The base of each pyramid represents the producers, or the first trophic level, while the apex represents the tertiary or top-level consumer. Every rung of the pyramid is a trophic level, drawn as a bar whose width stands for the quantity being measured.
There is one rule that must never be broken while constructing a pyramid: any calculation of energy content, biomass or numbers has to include all the organisms at that trophic level. You may not count only the deer and quietly ignore the rabbits, rodents and insects — every organism feeding at that level belongs in that bar.
The three ecological pyramids which are usually studied are the pyramid of biomass, the pyramid of numbers and the pyramid of energy. They differ only in what is being measured at each trophic level.
The relative number of individuals per unit area at different trophic levels constitutes the number pyramid; that of biomass per unit area is the biomass pyramid; and that of the amount of accumulated energy per unit area is the energy pyramid.
Pyramid of numbers — the relative number of individuals per unit area at each trophic level.
Pyramid of biomass — the amount of biomass per unit area at each trophic level.
Pyramid of energy — the amount of accumulated energy per unit area at each trophic level.
In most well balanced ecosystems, all the pyramids — of number, energy and biomass — are upright. That is, producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. There are exceptions to this, e.g. the oceanic ecosystem shows an inverted biomass pyramid.
The Pyramid of Biomass
Fig. 14.7 shows a pyramid of biomass for a simple grassland food chain, with the mass at each trophic level given in kilograms.
reading the biomass pyramid of Fig. 14.7 Read the bars from bottom to top: 900 kg of grasses support 100 kg of mice, which support 50 kg of snakes, which support 10 kg of owls. The mass falls as you climb, and each bar is narrower than the one below it — so the pyramid is upright. Why must this happen? Because the biomass of any level can only be built out of the food it receives from the level below, and most of that food is burnt in respiration or lost as waste rather than converted into new body mass.
Think about it — how would you explain the inverted pyramid of biomass in an oceanic ecosystem?
In the open sea, the producers are microscopic phytoplankton. At any given moment, the standing mass of these tiny cells is small — smaller than the mass of the zooplankton and fish that feed on them. Drawn as a pyramid, the base is therefore narrower than the levels above it, and the pyramid stands on its head.
The reason is turnover, not poor production. Phytoplankton multiply and are eaten extremely rapidly, so a small standing mass is replaced again and again within days. Over a year they produce an enormous quantity of food; but a snapshot taken today catches only the thin layer that happens to be alive at this instant. The pyramid of biomass measures the snapshot, and so it is misled.
The Pyramid of Numbers
The pyramid of numbers counts individuals rather than weighing them. Fig. 14.8 shows a typical upright number pyramid.
Similarly, the pyramid of numbers can also be sometimes inverted. For example, if we plot the number of insects on a single tree, the smaller birds feeding on those insects, and the parasites on those birds, we get an inverted pyramid.
Here the reason is size, not turnover. One huge producer — a single tree — can feed thousands of tiny insects. The base of the pyramid is one individual, so the level above it is bound to be wider. The pyramid of numbers ignores the size of an organism entirely, and that is precisely its weakness.
inverted pyramid of numbers vs inverted pyramid of biomass The number pyramid inverts because one large producer (a tree) supports many small consumers — a problem of organism size. The biomass pyramid inverts in the ocean because the small standing mass of phytoplankton is replaced very rapidly — a problem of turnover captured at one instant. Different causes, different examples: tree → insects → birds → parasites for numbers; the oceanic ecosystem for biomass.
Use your brain power — in the tree–insect–bird–parasite example above, what will happen to the shape of the pyramid if we substitute a larger bird of prey feeding on the small insect-eating birds?
The Pyramid of Energy
The pyramid of energy is always upright. It can never be inverted. The reason follows directly from the previous section: when energy flows from a particular trophic level, only 10 % passes on as net energy to the next trophic level, and a large amount of energy is always lost as heat at each step. Fig. 14.9 shows this as percentages.
The pyramid of energy is always upright and can never be inverted, because only about 10 % of the energy of a trophic level is passed on as net energy to the next level, while a large amount is lost as heat at each step.
reading the energy pyramid of Fig. 14.9 Take the producers as 100 %. The primary consumers hold 10 % of that, the secondary consumers 1 %, and the tertiary consumers just 0.1 %. Each bar is one-tenth of the bar below it, so the pyramid narrows sharply and simply cannot widen. A trophic level can never contain more energy than the level that feeds it — that would mean energy being created out of nothing.
Q3. A pyramid of biomass in the open ocean can stand upside down, but a pyramid of energy never can. Why is energy the one thing that refuses to invert?
A. Because numbers and biomass are snapshots, while energy is a flow. The biomass of phytoplankton measured at a single instant can be small even though the food they produce over time is enormous — so the snapshot inverts. Energy, however, is counted as it passes through, and no trophic level can pass on more energy than it received. With about 90 % lost as heat at every step, each level always holds far less energy than the one below it. An inverted energy pyramid would mean energy being created inside the ecosystem, which is impossible.
In smaller food chains, more energy is available than in the longer food chains — one more reason why nature keeps its chains short.
In a grassland, the producers trap \(1.5 \times 10^{6}\ kJ\) of energy per square metre per year. Using the pyramid of energy percentages (producers 100 %, primary consumers 10 %, secondary consumers 1 %, tertiary consumers 0.1 %), calculate the energy present at each of the four trophic levels, and find the total energy lost between the producer level and the tertiary consumer level.
Limitations of Ecological Pyramids
Useful as they are, ecological pyramids come with certain limitations, and a good student should know these as clearly as the pyramids themselves.
First, a pyramid assumes a simple food chain — something that almost never exists in nature. It does not accommodate a food web, in which an organism feeds at several levels at once.
Second, saprophytes are not given any place in ecological pyramids, even though they play a vital role in the ecosystem. The very organisms that return every nutrient to the soil do not appear anywhere in the figure.
Third, a given species may occupy more than one trophic level in the same ecosystem at the same time. For example, a sparrow is a primary consumer when it eats seeds, fruits and peas, and a secondary consumer when it eats insects and worms. A pyramid forces it into a single bar, which distorts the truth.
The three ecological pyramids | Feature | Pyramid of numbers | Pyramid of biomass | Pyramid of energy | |—|—|—|—| | What is measured | Number of individuals per unit area | Biomass per unit area | Accumulated energy per unit area | | Usual shape | Upright | Upright | Always upright | | Can it be inverted? | Yes — single tree → insects → birds → parasites | Yes — oceanic ecosystem | Never | | Main weakness | Ignores the size of organisms | Measures the standing crop at one instant, ignores turnover | Hardest of the three to measure in the field |
A useful examination note: this section is a favourite source of assertion–reason and match-the-following questions. Be ready to state which pyramid inverts, in which ecosystem, and above all why — because the reason (turnover for biomass, organism size for numbers, the 10 % Law for energy) is what carries the marks.
14.4 Nutrient Cycles
We have studied in earlier classes that all organisms need a constant supply of nutrients in order to grow and reproduce. Nutrients such as carbon, nitrogen, phosphorus and calcium are present in the soil, and their concentration varies in different kinds of ecosystems, and also on a seasonal basis within the same ecosystem.
Now recall the sharp contrast drawn earlier in this chapter. Energy enters an ecosystem, flows in one direction, and leaves it as heat. Nutrients behave in exactly the opposite way. They are never lost from the ecosystem; they are recycled indefinitely, the same atoms being used over and over by generation after generation of organisms.
The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling. Another name for nutrient cycling is the biogeochemical cycle — a word worth unpacking, because it tells you exactly what is happening: bio (living organisms), geo (rocks, soil, air and water) and chemical (the elements themselves). Here, essential elements are cycled from the abiotic world into the biotic world and back.
Nutrient cycling (biogeochemical cycle) is the movement of nutrient elements through the various components of an ecosystem, in which essential elements are cycled from the abiotic to the biotic world and back.
Energy flow vs Nutrient cycling Energy flows — one way, sun → producers → consumers → heat, and then it is gone. Nutrients cycle — round and round, abiotic → biotic → abiotic, endlessly. This is why an ecosystem needs sunlight every day, but does not need a fresh supply of carbon or phosphorus every day.
Types of Nutrient Cycles
There are two types of nutrient cycles, viz. (a) gaseous and (b) sedimentary. The difference lies entirely in where the element is mainly stored when it is not inside a living body.
The reservoir for a gaseous type of nutrient cycle (e.g. the nitrogen cycle, the carbon cycle) is the atmosphere. The reservoir for a sedimentary cycle (e.g. the phosphorus cycle) is the Earth’s crust — that is, rocks and sediments.
The function of the reservoir is to meet the deficit which occurs due to an imbalance in the rate of influx and the rate of efflux in any ecosystem. In simpler words, if an ecosystem uses up an element faster than it returns it, the reservoir quietly makes up the shortfall — it works like a bank balance that the ecosystem can draw upon.
Gaseous vs Sedimentary nutrient cycles | Feature | Gaseous cycle | Sedimentary cycle | |—|—|—| | Reservoir | Atmosphere | Earth’s crust (rocks, sediments) | | Examples | Carbon cycle, nitrogen cycle | Phosphorus cycle | | Speed of recycling | Relatively fast — the reservoir is freely mixed and everywhere accessible | Slow — the element must first be released by weathering of rock | | Availability to organisms | Generally not in short supply | Often in short supply, and hence limiting |
Carbon Cycle
All life forms on earth are carbon based, because carbon is the main component of all the organic compounds of protoplasm — carbohydrates, proteins, lipids and nucleic acids are all built on a carbon skeleton. Carbon constitutes 49 % of the dry weight of organisms; that is, roughly half of what remains of any living thing after its water is removed is carbon.
If we look at the total quantity of global carbon, we find that 71 % of it is found dissolved in the oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere — when atmospheric CO₂ rises, the sea absorbs more of it; when it falls, the sea gives some back. The whole cycle is shown in Fig. 14.10.
49 % and 71 % Two numbers define the carbon cycle. The first, 49 %, tells you why carbon matters to life: half of every dry organism is carbon. The second, 71 %, tells you where the carbon actually is: not in the air, and not in living bodies, but dissolved in the oceans. Read together, they explain why the sea — and not the atmosphere — is the true regulator of carbon dioxide on this planet.
Sinks: Carbon Taken Out of Circulation
Carbon that is a part of rocks and of fossil fuels like oil, coal and natural gas has been away from the rest of the carbon cycle for a very long time. These long-term storage places are known as “sinks”.
A sink, however, is not a permanent grave. When fossil fuels are burned, carbon that had been safely underground for millions of years is released back into the air as carbon dioxide — and a cycle that had been paused suddenly resumes, at a speed nature never intended.
The element carbon is a part of seawater, the atmosphere, rocks such as limestone and coal, soils, as well as all living things. Let us now follow it along each of its routes.
The Routes of Carbon
- Carbon as CO₂ moves from the atmosphere into plants. Through the process of photosynthesis, carbon dioxide is pulled from the air to produce food. This is the single doorway through which carbon enters the living world:
\[6CO_2 + 6H_2O \xrightarrow{\text{light energy}} C_6H_{12}O_6 + 6O_2\]
- Carbon moves from plants to animals, through food chains — the carbon present in plants moves into the bodies of the animals that eat them.
- Carbon moves from living things back to the atmosphere. Each time you exhale, you are releasing carbon dioxide gas (CO₂) into the atmosphere. Respiration is essentially photosynthesis run backwards, releasing the energy stored in the carbon skeleton.
- Decomposers also contribute substantially to the CO₂ in the atmosphere, by their processing of waste materials and of the dead organic matter of land and oceans.
- When fossil fuels burn to power factories, power plants and motor vehicles, most of the carbon quickly enters the atmosphere as carbon dioxide gas. Each year, 5.5 billion tons of carbon is released through the combustion of fossil fuels. Of this massive amount, 3.3 billion tons stays in the atmosphere. Most of the remainder is dissolved in seawater and deposited as calcium or magnesium carbonate compounds, which make up the shells of marine animals.
- Burning of wood, forest fire, and the combustion of organic matter and of fossil fuel, along with volcanic activity, are additional sources for releasing CO₂ into the atmosphere.
- Carbon moves from the atmosphere to the oceans. The oceans and other water bodies absorb some carbon in the form of CO₂ from the atmosphere, and this carbon is dissolved into the oceanic water. Some amount of the fixed carbon is lost to sediments and removed from circulation.
- Fossil fuels represent a reservoir of carbon. Carbon cycling occurs through the atmosphere, the ocean, and through living and dead organisms. Human activities have significantly influenced the carbon cycle.
- Rapid deforestation and the massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.
Thus, the entire carbon cycle is run by five basic processes, viz. photosynthesis, respiration, decomposition, sedimentation and combustion. Learn these five, and the whole cycle can be reconstructed from them.
the five processes that run the carbon cycle P–R–D–S–C: Photosynthesis, Respiration, Decomposition, Sedimentation, Combustion. The first three are done by living organisms, the fourth by the Earth, and the fifth mostly by us.
the fate of 5.5 billion tons of carbon Of the 5.5 billion tons of carbon released each year by burning fossil fuels, 3.3 billion tons — well over half — simply stays in the atmosphere. Most of the remainder is dissolved in seawater and locked into carbonate compounds. Notice what these two figures together imply: the natural sinks are removing only a part of what we add, so the rest accumulates year after year. A cycle that stayed balanced for millions of years can be pushed out of balance within a century.
Each year, 5.5 billion tons of carbon is released by the combustion of fossil fuels, of which 3.3 billion tons remains in the atmosphere. Calculate (a) the mass of carbon that does not remain in the atmosphere, and (b) the percentage of the released carbon that stays in the atmosphere.
a single carbon atom Zoom in: a carbon atom sits inside a glucose molecule, inside a leaf cell, inside a tree. Zoom out: that same atom may next be inside a deer, then in the air you breathe, then dissolved in the sea, then locked inside a limestone rock for a million years. The atom never changes — only its address does.
The oceanic part of this cycle carries a hidden cost. As the sea absorbs more of the carbon dioxide we release, part of it forms carbonic acid, and the seawater becomes slightly more acidic. Corals, molluscs and many planktonic organisms build their shells and skeletons out of calcium carbonate, and this becomes harder to do as acidity rises. The very reservoir that shields the atmosphere by soaking up our carbon dioxide is therefore itself being altered by it — a reminder that in an ecosystem, no compartment can be changed in isolation.
Phosphorus Cycle
The cyclic movement of phosphorus through the hydrosphere, the lithosphere and the biosphere constitutes the phosphorus cycle. Notice at once what is missing from that list — the atmosphere. That single absence explains almost everything that is peculiar about this cycle.
The phosphorus cycle is the cyclic movement of phosphorus through the hydrosphere, the lithosphere and the biosphere.
Phosphorus is a major constituent of biological membranes, of nucleic acids, and of cellular energy transfer systems. Many animals also need large quantities of this element to make shells, bones, hooves and teeth.
the phosphate group Phosphate carries a strong negative charge and forms stable ester bonds → it links the sugar units of DNA and RNA into a backbone, forms the water-facing head of every membrane phospholipid, and holds the energy-rich bonds of ATP → without phosphorus there can be no heredity, no cell membrane, and no usable cellular energy.
How Phosphorus Moves
The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates. When rocks are weathered, minute amounts of these phosphates dissolve in the soil solution and are absorbed by the roots of the plants. The full route is shown in Fig. 14.11.
Herbivores and other animals obtain this element from plants. The waste products and the dead organisms are decomposed by phosphate-solubilizing bacteria, releasing phosphorus back into the soil.
Unlike the carbon cycle, there is no respiratory release of phosphorus into the atmosphere. No organism breathes out phosphorus. This is the fundamental difference between a sedimentary and a gaseous cycle, and every other difference follows from it.
There are two major differences between the carbon and the phosphorus cycles. First, the atmospheric inputs of phosphorus through rainfall are much smaller (meagre) than the carbon inputs. Second, the exchanges of phosphorus between the organism and the environment are negligible as compared with those of carbon.
Carbon cycle vs Phosphorus cycle | Feature | Carbon cycle | Phosphorus cycle | |—|—|—| | Type of cycle | Gaseous | Sedimentary | | Main reservoir | Atmosphere (with a large oceanic store) | Rock, in the form of phosphates | | Respiratory release | Yes — CO₂ released by all respiring organisms | None — no respiratory release into the atmosphere | | Atmospheric input through rainfall | Large | Much smaller (meagre) | | Exchange between organism and environment | Rapid and large | Negligible | | Availability | Generally sufficient | Always in short supply — often the limiting factor |
Phosphorus as a Limiting Factor
Unlike carbon and nitrogen, phosphorus is always in short supply, and hence it acts as a limiting factor for plant growth. A limiting factor is the resource that runs out first, and it alone sets the ceiling on growth — no matter how plentiful everything else may be. A plant with abundant light, water, carbon dioxide and nitrogen still cannot grow if there is no phosphate for its DNA and its ATP.
A limiting factor is the resource that is in shortest supply relative to demand, and which therefore sets the ceiling on the growth or productivity of an organism or an ecosystem.
This very scarcity is what makes phosphorus dangerous when it is suddenly supplied in bulk. A sudden influx of phosphorus, in the form of agricultural runoff or of industrial effluents rich in phosphate content, leads to eutrophication in water bodies.
Eutrophication is due to the overgrowth of algae, brought on by the high phosphorus dissolved in the water. The overgrowth of algae kills or harms the aquatic life.
Eutrophication is the excessive enrichment of a water body with nutrients (especially phosphates), leading to an overgrowth of algae which kills or harms aquatic life.
The chain of events is worth following carefully, because it shows how a nutrient turns into a pollutant. The algae multiply explosively and form a dense mat over the surface, cutting off light from the plants below. When this huge mass of algae dies, decomposers multiply to break it down — and since decomposition requires oxygen, they strip the water of the dissolved oxygen that fish and other aquatic animals need in order to breathe. The water ends up rich in nutrients and empty of life.
Q4. A little fertiliser washing off a field can turn a clear lake green and kill its fish. Which single nutrient is usually behind this, and why is it so powerful in such small amounts?
A. Phosphorus. Because phosphorus is normally in very short supply, it is the limiting factor that holds algal growth in check — algae are not held back by any lack of light, water or carbon, but by a lack of phosphate. The moment agricultural runoff or phosphate-rich effluent removes that limit, the brake comes off and the algae multiply explosively. This is eutrophication. The resulting algal mat cuts off light, and when it dies, decomposers use up the dissolved oxygen and the fish suffocate. A small amount of the limiting nutrient does what a large amount of any other nutrient never could.
The scale of this can be enormous. Where a large river carries fertiliser runoff from vast farmlands into the sea — as the Mississippi does into the Gulf of Mexico — the nutrient enrichment triggers algal blooms every summer, and the decay that follows leaves a “dead zone” of oxygen-starved water covering thousands of square kilometres, in which fish and shellfish cannot survive. The same process fouls urban lakes that receive sewage and detergent-rich waste water, choking them with algae and weeds. The remedy is not to add anything to the lake, but to stop the phosphate from reaching it.
Internet my friend — find out what the short-term and the long-term effects of eutrophication in water bodies are.
14.5 Ecological Succession
Earlier in this chapter we studied spatial patterns — stratification and zonation — which describe how organisms are arranged in space. Succession is the pattern that appears when we add the fourth dimension: time.
Succession is a spatial pattern which occurs over time. Time is the crucial factor here. Watch a patch of bare ground for a week and nothing seems to happen; watch it for a century and it may have become a forest.
An area newly opened to life is initially occupied by the pioneer species — the first colonisers, tough and undemanding. As time passes, species diversity increases, giving way to more complex organisms. Eventually this leads to a climax community, and the climax community does not evolve further.
The gradual and predictable change in the species composition of a given area is called ecological succession. The change is sequential — one community follows another in a definite order — and it is environmentally regulated, meaning that the conditions of soil, water and light at each stage decide who can live there next.
Ecological succession is the gradual and predictable change in the species composition of a given area over time; the change is sequential and environmentally regulated.
A climax community is the final, stable community of a succession, which does not evolve or change further as long as the environment remains unchanged.
The Steps of Succession
The process of succession involves sequential steps like Nudation, Invasion, Ecesis, Aggregation, Competition and co-action, Reaction and Stabilization. These steps are set out in order in FC 14.8.
The single most important step to understand is reaction. At each stage, the organisms living there change the habitat itself — they add humus, hold moisture, cast shade. In doing so they make the site less suitable for themselves and more suitable for the community that will replace them. Succession is therefore driven from within: each community prepares the bed of its own successor.
the seven steps of succession, in order “Nine Insects Eat All Crops Rather Slowly” — Nudation, Invasion, Ecesis, Aggregation, Competition and co-action, Reaction, Stabilization.
During succession, some species colonize an area and their populations become more numerous, whereas the populations of other species decline and even disappear. Succession is thus not a simple addition of species; it is a continuous exchange of one set of species for another.
Seres and Seral Communities
The entire sequence of communities that successively change in a given area constitutes what is called a sere (plural: seres). Alternatively, it is the entire gradient of organisms from the pioneer stage to the climax stage.
The individual transitional communities along the way are termed seral communities. In the successive seral stages there is a change in the diversity of species of organisms, an increase in the number of species and of organisms, and also an increase in the total biomass.
A sere is the entire sequence of communities that successively occupy a given area, from the pioneer stage to the climax stage.
Seral communities are the individual transitional communities that appear one after another during a sere.
Sere vs Seral community The sere is the whole journey — the complete series from pioneer to climax. A seral community is one stop along that journey. The sere is the film; a seral community is a single frame of it.
Primary Succession
The present-day communities of the world have come to be what they are because of succession that has occurred over millions of years, since life started on earth. Succession is hence a process that can start where no living organisms were present before — like on a newly formed volcanic island. This is called primary succession.
Other examples of areas where primary succession occurs are newly cooled lava, bare rocks, and a newly created pond or reservoir. Notice what all these have in common: no life, and — crucially — no soil.
The establishment of a new biotic community is generally very slow. Before a biotic community of diverse organisms can become established, there must be soil. Depending mostly on the climate, it takes natural processes several hundred to several thousand years to produce fertile soil on bare rock.
Primary succession is the succession that begins in an area where no living organisms were present before and where no soil exists — such as bare rock, newly cooled lava, a newly formed volcanic island, or a newly created pond or reservoir.
In 1883, the volcanic island of Krakatoa in the Indonesian archipelago was torn apart by one of the most violent eruptions in recorded history. What remained of the island was buried under thick, scorching ash, and every living thing on it was killed — leaving a completely sterile surface. For naturalists, this became an unrepeatable natural experiment. Visitors in the following years found first a film of blue-green algae, and a few ferns whose spores had drifted in on the wind. Grasses followed; then shrubs, from seeds carried by birds and by the sea; and within a few decades a young woodland was rising on the ash. Krakatoa showed, on a scale no laboratory could match, that primary succession is not a theory — it is something the Earth does, patiently, whenever it is given bare ground and time.
Secondary Succession
Secondary succession begins in areas where natural biotic communities have been destroyed — such as abandoned farm lands, burned or cut forests, and lands that have been flooded.
The crucial difference here is that some soil or sediment is already present. That soil holds nutrients, moisture, buried seeds and other propagules. Because the slowest step of all — the making of soil — has already been completed, succession is much faster than in primary succession, and the climax is reached more quickly.
Secondary succession is the succession that begins in an area where a natural biotic community has been destroyed but soil or sediment is still present — e.g. abandoned farmland, a burned or cut forest, or flooded land.
Descriptions of ecological succession usually focus on changes in vegetation. However, these vegetational changes in turn affect the food and the shelter available to various types of animals. Thus, as succession proceeds, the numbers and types of animals and of decomposers also change — the animal community quietly follows the plant community.
At any time during primary or secondary succession, natural or human-induced disturbances (fire, deforestation, and so on) can convert a particular seral stage of succession back to an earlier, preceding stage. Such disturbances also create new conditions that encourage some species and discourage or eliminate others.
Fig. 14.14 shows the sequence of stages 1 to 8 in a forest ecosystem after fire — a clear case of secondary succession.
Primary vs Secondary succession | Feature | Primary succession | Secondary succession | |—|—|—| | Starting area | No living organisms present before | A biotic community existed but was destroyed | | Soil at the start | Absent — it must be formed first | Already present, with seeds and propagules | | Speed | Very slow — several hundred to several thousand years | Much faster; climax reached more quickly | | Pioneer species | Crustose lichens on rock; phytoplankton in water | Herbs and grasses growing from surviving seeds | | Examples | Newly cooled lava, bare rock, newly formed volcanic island, newly created pond or reservoir | Abandoned farmland, burned or cut forest, flooded land |
Succession of Plants
Based on the nature of the habitat — whether it is water (or very wet land) or a very dry area — the succession of plants is called hydrarch (hydrosere) or xerarch (xerosere) respectively.
Hydrarch succession takes place in wetter areas, and the successional series progresses from hydric to mesic conditions. As against this, xerarch succession takes place in dry areas, and the series progresses from xeric to mesic conditions.
Hence both hydrarch and xerarch successions lead to medium water conditions (mesic) — neither too dry (xeric) nor too wet (hydric). This is a striking result, and an important one: two successions that begin as opposites end at the same destination.
Hydrarch vs Xerarch succession | Feature | Hydrarch (hydrosere) | Xerarch (xerosere) | |—|—|—| | Habitat where it begins | Water, or very wet land | Very dry area, such as bare rock | | Direction of change | Hydric → mesic | Xeric → mesic | | Pioneer species | Phytoplankton | Crustose lichens | | Final condition | Mesic climax (forest) | Mesic climax (forest) |
Hydric, Xeric and Mesic Hydric = too much water. Xeric = too little water. Mesic = the moderate, medium-moisture condition in between. Both hydrarch and xerarch succession travel towards mesic — one by drying out, the other by building moisture up.
Xerarch Succession: From Bare Rock to Forest
The species that invade a bare area are called pioneer species. In primary succession on rocks these are usually crustose lichens, which are able to secrete acids that dissolve the rock, thereby helping in the weathering of rock and in soil formation.
Pioneer species are the first species to invade and colonise a bare area, preparing it for the communities that follow.
crustose lichens as pioneers They grow flat against bare rock and need no soil → they secrete acids that dissolve the rock surface and trap dust and dead matter → this produces the first thin film of soil, allowing the next community to take hold. A pioneer is not the strongest organism; it is the one that can live where nothing else can, and change the place while doing so.
These lichens pave the way for bryophytes — mosses that are able to take hold in the small amount of soil now available. They are, with time, succeeded by herbaceous plants, and after several more stages, ultimately a stable climax forest community is formed. Fig. 14.12 shows this sequence.
The climax community remains stable as long as the environment remains unchanged. That last condition matters — a climax forest is not immortal, it is merely at rest, and a changed climate or a major disturbance can set the whole sequence going again.
Hydrarch Succession: From Pond to Forest
In the primary succession of an aquatic habitat (steps 1 to 7 in Fig. 14.13), the pioneers are the small phytoplankton. They are replaced with time by rooted-submerged plants (e.g. Hydrilla), then rooted-floating angiosperms (e.g. Lotus), followed by free-floating plants (e.g. Pistia), then reed swamp (e.g. Typha), marsh-meadow (e.g. Cyperus), scrub (e.g. Alnus) and finally the trees (e.g. Quercus). The climax again would be a forest.
With the passage of time, the water body is converted into land. This is the mechanism that drives the whole hydrosere: at every stage the plants trap silt, and their dead remains settle to the bottom, so the pond becomes shallower. Each community, simply by living there, fills in the very water it depends on — and thereby hands the site over to the next, drier community.
a pond becoming a forest 1. Phytoplankton colonise the open water and, on dying, sink and begin to build up organic matter on the pond floor. 2. As the bottom rises and light reaches it, rooted-submerged plants such as Hydrilla take root; their dead remains raise the floor further. 3. The water is now shallow enough for rooted-floating angiosperms such as Lotus, and for free-floating plants such as Pistia, which shade the water below. 4. Silt and dead matter accumulate until the margins are marshy, and reed-swamp plants such as Typha stand in the shallows. 5. The site is now waterlogged soil rather than open water, and marsh-meadow plants such as Cyperus take over. 6. The soil dries further, and shrubs such as Alnus invade at the scrub stage. 7. Trees such as Quercus finally establish themselves, and a climax forest stands where a pond once was — the water conditions having travelled from hydric all the way to mesic.
Q5. A pond left undisturbed for a few centuries can slowly become a forest. What drives this change, and can the forest itself keep changing forever?
A. What drives it is reaction — the habit every community has of altering its own habitat. Phytoplankton, then submerged plants, then floating plants and reeds all die and settle, and together with trapped silt they steadily fill the basin. The pond grows shallower until it is marsh, then meadow, then scrub, and finally forest — a hydrarch succession moving from hydric to mesic conditions. And no, the change does not continue forever. Once the mesic climax forest is reached, the community is stable and does not evolve further, so long as the environment remains unchanged. The forest is where succession comes to rest.
Two Final Points
It is important to understand that succession, particularly primary succession, is a very slow process — it may require thousands of years for the climax to be reached. The lichen that begins the process will never see the forest that ends it.
Another important fact to understand is that all successions, whether taking place in water or on land, proceed to a similar climax community — the mesic. Wherever it begins, and however long it takes, succession is a journey towards the same balanced middle.
Collect information and prepare a chart of the sequential steps involved in ecological succession, explaining each step in your own words.