Ecosystem

Ecosystem–Structure And Function

An ecosystem is a functional unit of nature, where living organisms (biotic components) interact with each other and with the physical environment (abiotic components). It encompasses all the organisms in a given area as well as the physical environment in which they live.

Examples of ecosystems range in size from a small pond to a large forest, or even the entire biosphere. Ecosystems can be natural (e.g., forests, grasslands, deserts, oceans, lakes) or artificial (e.g., crop fields, aquariums).

Structure of an Ecosystem:

The structure of an ecosystem is characterised by its components and their organisation.

• Species composition: The types and number of plant and animal species present in the ecosystem.
• Stratification: The vertical distribution of different species occupying different levels (e.g., layers of vegetation in a forest: trees, shrubs, herbs, ground cover).
• Biotic components:
  • Producers: Autotrophs (mainly photosynthetic plants, algae, some bacteria) that produce organic matter from inorganic substances using energy (light or chemical).
  • Consumers: Heterotrophs that obtain energy by feeding on other organisms. Includes herbivores (primary consumers), carnivores (secondary and tertiary consumers), and omnivores.
  • Decomposers: Heterotrophs (mainly bacteria and fungi) that break down dead organic matter (detritus) into simpler inorganic substances (mineralisation).
• Abiotic components: The non-living physical and chemical factors of the environment (e.g., temperature, water, light, soil, nutrients, gases).

*(Image shows a simplified diagram of an ecosystem illustrating sun as energy source, producers (plants), consumers (herbivores, carnivores), decomposers (bacteria, fungi), and abiotic factors (soil, water, air))*

Function of an Ecosystem:

The key functional aspects of an ecosystem involve the processes that occur within it:

• Productivity: The rate of biomass production.
• Decomposition: The breakdown of dead organic matter.
• Energy flow: The movement of energy through different trophic levels.
• Nutrient cycling (Biogeochemical cycles): The movement of essential nutrients through the ecosystem.

These processes are interconnected and essential for maintaining the balance and functioning of the ecosystem.

Ecosystems are dynamic and can change over time through ecological succession or due to disturbances.

Productivity

Productivity in an ecosystem is the rate at which biomass is produced. It is a measure of the amount of organic matter produced by producers and consumers over a certain period.

Types of Productivity:

• Primary Productivity: The rate at which producers (plants) synthesise organic matter from inorganic substances through photosynthesis or chemosynthesis. It is expressed as grams of organic matter produced per unit area per unit time ($g/m^2/year$) or as energy per unit area per unit time ($kcal/(m^2 \cdot year)$).
  • Gross Primary Productivity (GPP): The total rate of photosynthesis, including the organic matter used by plants for their own respiration ($R$).

    $ GPP = \text{Total photosynthesis} $

  • Net Primary Productivity (NPP): The organic matter remaining after producers have used some for their own respiration. It is the biomass available to herbivores.

    $ NPP = GPP - R $

• Secondary Productivity: The rate at which consumers (herbivores, carnivores) convert the organic matter they consume into their own biomass. It is the rate of formation of new organic matter by consumers.

Factors Affecting Primary Productivity:

• Light availability.
• Temperature.
• Water availability.
• Nutrient availability in the soil or water.
• Plant species present.
• Photosynthetic capacity of the plants.

Distribution of Primary Productivity:

• Primary productivity varies greatly among different ecosystems.
• Tropical rainforests and coral reefs have very high primary productivity.
• Deserts and open oceans have low primary productivity.
• The total net primary productivity of the biosphere is estimated to be about 170 billion tonnes of organic matter per year (about 115 billion tonnes from land ecosystems and 55 billion tonnes from oceans).

Productivity is a key function of an ecosystem, representing the base of the food web and the availability of energy for higher trophic levels.

Decomposition

Decomposition is the process by which decomposers (mainly bacteria and fungi) break down complex dead organic matter (detritus) into simpler inorganic substances like carbon dioxide, water, and nutrients. It is a vital process for nutrient cycling in an ecosystem.

Detritus: Dead plant remains (leaves, bark, flowers), dead animal remains, and faecal matter.

Steps in Decomposition:

1. Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller pieces. This increases the surface area for microbial action.
2. Leaching: Water-soluble inorganic nutrients seep into the soil horizon and get precipitated as unavailable salts (leaching).
3. Catabolism: Bacterial and fungal enzymes break down complex organic molecules in detritus into simpler organic and inorganic substances.
4. Humification: The accumulation of a dark-coloured, amorphous substance called humus. Humus is highly resistant to microbial decomposition and is a reservoir of nutrients. It improves soil structure and water-holding capacity.
5. Mineralisation: Further decomposition of humus by microbes, releasing inorganic nutrients into the soil (e.g., $CO_2, H_2O$, minerals). These nutrients become available for uptake by plants.

*(Image shows a diagram illustrating detritus being broken down by detritivores, showing leaching, microbial action (catabolism, humification), and release of inorganic nutrients (mineralisation))*

Factors Affecting Decomposition:

• Chemical composition of detritus: Decomposition rate is slower if detritus is rich in lignin and chitin, and faster if rich in nitrogen and water-soluble sugars.
• Temperature: Decomposition rate is faster at warm temperatures ($25-35^\circ C$).
• Moisture: Decomposition rate is faster in moist conditions.
• Aeration: Decomposition is faster in aerobic conditions. Anaerobic conditions (e.g., in waterlogged soil) slow down decomposition.
• pH: Neutral to slightly alkaline pH is generally optimum for bacterial decomposition. Fungi prefer slightly acidic conditions.

Decomposition is essential for recycling nutrients back into the ecosystem, making them available for producers, thus maintaining the flow of energy and matter.

Energy Flow

Energy flow in an ecosystem is the unidirectional movement of energy from the sun through the different trophic levels (feeding levels) of the ecosystem. Energy flows from producers to consumers and decomposers.

Trophic Levels:

Organisms in an ecosystem occupy different trophic levels based on their source of nutrition:

• Producers (T1): First trophic level. Synthesise their own food (e.g., plants). Capture light energy.
• Primary consumers (T2): Second trophic level. Herbivores that feed on producers.
• Secondary consumers (T3): Third trophic level. Carnivores that feed on primary consumers.
• Tertiary consumers (T4): Fourth trophic level. Carnivores that feed on secondary consumers.
• Decomposers: Obtain energy by breaking down dead organic matter from all trophic levels. Play a crucial role in releasing nutrients but are usually placed outside the main trophic level sequence in energy flow diagrams.

*(Image shows a simple food chain illustrating different trophic levels)*

Food Chain and Food Web:

• Food chain: A linear sequence of organisms where energy is transferred from one trophic level to the next when one organism eats another. Example: Grass $\rightarrow$ Deer $\rightarrow$ Lion.
• Food web: A network of interconnected food chains, reflecting the more complex feeding relationships in most ecosystems.

Laws of Thermodynamics and Energy Flow:

• Energy flow in ecosystems follows the laws of thermodynamics.
  • First Law: Energy cannot be created or destroyed, only transformed. (Solar energy is converted into chemical energy by producers).
  • Second Law: During energy transfer, some energy is lost as heat at each step. Transformations are not 100% efficient.

Energy Transfer Efficiency (Ten Percent Law):

• Only a small percentage of energy is transferred from one trophic level to the next.
• Generally, about 10% of the energy from one trophic level is available to the next trophic level. The remaining 90% is lost as heat during metabolic activities or is not consumed/assimilated.
• This 10% law, proposed by Raymond Lindeman, explains why food chains are usually short (typically 3-4 trophic levels). The amount of energy available decreases sharply at each higher level.

Example: If producers capture 10,000 units of energy, primary consumers get $\approx$ 1,000 units, secondary consumers get $\approx$ 100 units, and tertiary consumers get $\approx$ 10 units.

*(Image shows a diagram illustrating energy transfer between trophic levels (e.g., producers, primary consumers, secondary consumers) with arrows showing the direction of energy flow and indicating the approximate percentage of energy transferred at each step)*

---

Energy flow in an ecosystem is always unidirectional, from producers to consumers. Energy is not recycled in an ecosystem; there is a continuous input of solar energy (except in chemosynthetic ecosystems).

Ecological Pyramids

Ecological pyramids are graphical representations of the relationships between different trophic levels in an ecosystem. They show the amount of biomass, number of individuals, or energy present at each trophic level.

Ecological pyramids are usually upright (except for some pyramids of biomass and inverted pyramids of numbers), with the producer level at the base and successive consumer levels above.

Types of Ecological Pyramids:

1. Pyramid of Numbers:
   • Represents the number of individuals at each trophic level per unit area.
   • Usually upright: The number of individuals generally decreases at each successive trophic level (e.g., grassland ecosystem: large number of grasses, fewer herbivores, even fewer carnivores).
   • Can be inverted: In some cases, the pyramid of numbers can be inverted. Example: Parasitic food chain (one tree can support many birds, and each bird can support many parasites like mites).
2. Pyramid of Biomass:
   • Represents the total mass of living organisms (biomass) at each trophic level per unit area or volume. Biomass is usually expressed as dry weight.
   • Usually upright: The total biomass generally decreases at each successive trophic level on land ecosystems (e.g., total dry weight of producers is more than herbivores, which is more than carnivores).
   • Can be inverted: In some aquatic ecosystems (e.g., a pond). The biomass of phytoplankton (producers) at any given time may be less than the biomass of zooplankton (primary consumers). This is because phytoplankton reproduce and are consumed very rapidly.
3. Pyramid of Energy:
   • Represents the amount of energy available at each trophic level per unit area per unit time.
   • The energy is expressed in units like $kcal/(m^2 \cdot year)$.
   • This pyramid is always upright in any ecosystem. As energy is lost at each trophic level (10% law), the energy available at successive trophic levels always decreases. It can never be inverted.

*(Image shows diagrams of different ecological pyramids, including an upright pyramid of energy, an upright pyramid of numbers, a potentially inverted pyramid of numbers (parasitic), an upright pyramid of biomass (terrestrial), and an inverted pyramid of biomass (aquatic))*

Ecological pyramids provide a visual representation of the structure and energy/biomass transfer efficiency in an ecosystem. The pyramid of energy is the most fundamental and always depicts an upright structure due to the laws of thermodynamics.

Ecological Succession

Ecological succession is the gradual and predictable process of change in the species composition of an area over time. It is the sequence of communities that replace one another in a given area.

The entire sequence of communities that successively change in a given area is called a sere. The individual transitional communities are called seral stages or seral communities. The final community that is stable and in equilibrium with the environment is called the climax community.

Types of Succession:

• Primary succession: Occurs in areas where no living organisms have ever existed (e.g., newly cooled lava, bare rock, newly created pond or reservoir). It is a slow process as soil development is necessary. The organisms that first colonise a bare area are called pioneer species (e.g., lichens on bare rock, phytoplankton in new pond).
• Secondary succession: Occurs in areas where a community previously existed but has been removed by disturbances (e.g., abandoned farmland, burned or cut forest). Soil is already present, so it is much faster than primary succession. Pioneer species in secondary succession are often grasses or weeds.

Succession Of Plants

Ecological succession often involves changes in plant communities, which in turn influence the associated animal and microbial communities.

Succession on bare rock (Xerosere):

Example of primary succession in dry areas (xeric conditions):

1. Pioneer stage: Lichens colonise bare rock. They secrete acids that help in weathering the rock and forming a thin layer of soil.
2. Moss stage: Mosses colonise the thin soil layer, further contributing to soil formation.
3. Herb stage: Small herbaceous plants colonise as soil improves.
4. Shrub stage: Shrubs colonise as soil depth and fertility increase.
5. Forest stage (Climax): Eventually, trees colonise, leading to the formation of a stable forest community (if the climate supports it).

Succession in water (Hydrosere):

Example of primary succession in aquatic areas:

1. Pioneer stage: Phytoplankton colonise the water.
2. Submerged plant stage: Rooted submerged plants appear as sediment accumulates.
3. Floating plant stage: Rooted plants with floating leaves appear as water depth decreases.
4. Reed-swamp stage: Emergent plants (partially submerged, partially above water) colonise as water becomes shallower.
5. Marsh-meadow stage: Land plants (grasses, sedges) colonise as the area becomes saturated soil.
6. Shrub stage: Shrubs colonise as soil dries out.
7. Forest stage (Climax): Eventually, a forest may develop if the climate permits.

*(Image shows a diagram illustrating the sequence of plant communities changing over time in a specific environment (e.g., a pond filling in and turning into a forest, or bare rock developing into soil and forest))*

---

Succession is driven by changes in the environment brought about by the organisms themselves (e.g., soil formation by lichens, accumulation of sediment in a pond). The climax community is determined by the prevailing climate of the region.

Nutrient Cycling

Nutrient cycling (or biogeochemical cycles) refers to the movement of essential nutrients (like carbon, nitrogen, phosphorus) through the biotic and abiotic components of an ecosystem. Nutrients are continuously exchanged between organisms and the environment.

Unlike energy, which flows unidirectionally and is lost at each trophic level, nutrients are finite and are recycled within the ecosystem. There is also some input (from atmosphere, weathering of rocks) and output (leaching, runoff) of nutrients from an ecosystem.

Nutrient cycles can be classified into two types:

• Gaseous cycles: The reservoir for the nutrient is in the atmosphere (e.g., carbon, nitrogen).
• Sedimentary cycles: The reservoir for the nutrient is in the Earth's crust (e.g., phosphorus, sulphur, calcium).

Ecosystem – Carbon Cycle

Carbon is an essential component of all organic molecules. The carbon cycle involves the movement of carbon through the atmosphere, oceans, land, and living organisms.

Key Processes in the Carbon Cycle:

• Photosynthesis: Producers (plants, algae, cyanobacteria) capture $CO_2$ from the atmosphere (or water) and convert it into organic compounds. This is the primary process that removes carbon from the atmosphere.
• Respiration: All living organisms (producers, consumers, decomposers) release $CO_2$ back into the atmosphere through respiration (breakdown of organic molecules to release energy).
• Decomposition: Decomposers break down dead organic matter, releasing carbon back into the atmosphere as $CO_2$ or into the soil/water.
• Combustion: Burning of organic matter (fossil fuels, biomass) releases large amounts of $CO_2$ into the atmosphere. Human activities like burning fossil fuels are significantly increasing atmospheric $CO_2$ levels.
• Ocean processes: Oceans absorb large amounts of $CO_2$. Marine organisms use carbon for shells and skeletons (calcium carbonate). Decomposition and respiration also occur in oceans.
• Geological processes: Long-term storage of carbon in rocks (e.g., limestone) and fossil fuels (coal, oil, natural gas). Volcanic activity releases $CO_2$.

The carbon cycle is heavily influenced by human activities (burning fossil fuels, deforestation), leading to concerns about climate change due to increased atmospheric $CO_2$.

*(Image shows a diagram illustrating the carbon cycle, showing carbon reservoirs (atmosphere, oceans, land biomass, fossil fuels), and processes like photosynthesis, respiration, decomposition, combustion, geological processes, and exchange between atmosphere and oceans)*

---

Ecosystem – Phosphorus Cycle

Phosphorus is an essential nutrient, a component of DNA, RNA, ATP, cell membranes, bones, and teeth. It is a sedimentary cycle, with the main reservoir in rocks.

Key Processes in the Phosphorus Cycle:

• Weathering of rocks: Phosphorus is released from rocks into the soil and water as inorganic phosphates through weathering and erosion.
• Uptake by producers: Plants absorb inorganic phosphate ions ($PO_4^{3-}$) from the soil.
• Transfer to consumers: Phosphorus moves from producers to consumers through food chains.
• Decomposition: Decomposers break down dead organic matter and waste products, returning inorganic phosphates to the soil and water.
• Sedimentation: Some phosphorus can be lost from the cycle by sedimentation in oceans, forming new rocks over geological time.
• Human activities: Mining of phosphate rock for fertilisers significantly alters the phosphorus cycle. Runoff from agricultural fields can lead to eutrophication in aquatic ecosystems.

The phosphorus cycle is slower and less complex than the carbon cycle as it does not involve a significant atmospheric gaseous phase.

*(Image shows a diagram illustrating the phosphorus cycle, showing phosphorus in rocks, soil, producers, consumers, decomposers, water, and processes like weathering, uptake, consumption, decomposition, sedimentation, human activities)*

---

Nutrient cycling is essential for sustaining life in ecosystems. Human activities can significantly impact these cycles, leading to environmental problems.

Ecosystem Services

Ecosystem services are the benefits that humans obtain from natural ecosystems. These services are essential for human survival and well-being.

Examples of essential ecosystem services provided by healthy ecosystems:

• Provisioning Services: Products obtained from ecosystems.
  • Food production (crops, livestock, fish).
  • Clean drinking water.
  • Timber and wood products.
  • Medicinal resources.
  • Fuelwood.
• Regulating Services: Benefits obtained from the regulation of ecosystem processes.
  • Climate regulation (e.g., forests absorb $CO_2$).
  • Flood control (e.g., forests and wetlands absorb excess water).
  • Disease regulation (e.g., natural enemies of pests).
  • Water purification (e.g., wetlands filter pollutants).
  • Pollination of crops (by insects and other animals).
  • Pest and disease control (by natural predators and pathogens).
  • Soil formation and erosion control.
• Cultural Services: Non-material benefits.
  • Recreational opportunities (e.g., hiking, tourism).
  • Aesthetic values (beauty of nature).
  • Spiritual and religious values.
  • Educational opportunities.
• Supporting Services: Services necessary for the production of all other ecosystem services.
  • Nutrient cycling.
  • Primary production (photosynthesis).
  • Soil formation.
  • Habitat provision for biodiversity.

*(Image shows a diagram or graphic illustrating the four categories of ecosystem services with examples for each)*

Ecosystem services are often undervalued or not assigned a monetary value in economic systems, leading to their degradation due to human activities. Recognising and valuing ecosystem services is crucial for promoting conservation and sustainable management of natural resources. For example, Robert Costanza and colleagues (1997) estimated the annual value of ecosystem services globally to be significantly higher than the global GDP.