Chapter 12: Ecosystem
Introduction
An ecosystem can be visualised as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment. Ecosystems vary greatly in size from a small pond to a large forest or a sea. Ecosystems are broadly divided into two basic categories:
- Terrestrial ecosystems: Forest, grassland, and desert.
- Aquatic ecosystems: Pond, lake, wetland, river, and estuary. Crop fields and an aquarium are considered man-made ecosystems.
Components of an Ecosystem
Interaction of biotic and abiotic components results in a physical structure that is characteristic for each type of ecosystem. The vertical distribution of different species occupying different levels is called stratification (e.g., trees occupy top vertical strata, shrubs the second, and herbs/grasses the bottom layers). The basic components that function as a unit are:
- Productivity
- Decomposition
- Energy flow
- Nutrient cycling
1. Productivity
A constant input of solar energy is the basic requirement for any ecosystem to function.
- Primary production: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (\(g/m^2\)) or energy (\(kcal/m^2\)).
- Gross primary productivity (GPP): The rate of production of organic matter during photosynthesis.
- Net primary productivity (NPP): GPP minus respiratory losses (\(R\)). NPP is the available biomass for the consumption to heterotrophs (herbivores and decomposers). $$ \text{NPP} = \text{GPP} - R $$
- Secondary productivity: The rate of formation of new organic matter by consumers.
2. Decomposition
Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients. Dead plant remains and dead animals constitute detritus, which is the raw material for decomposition. Steps in decomposition:
- Fragmentation: Detritivores (e.g., earthworm) break down detritus into smaller particles.
- Leaching: Water-soluble inorganic nutrients go down into the soil horizon and get precipitated.
- Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
- Humification: Accumulation of a dark-coloured amorphous substance called humus, which is highly resistant to microbial action and acts as a reservoir of nutrients.
- Mineralisation: The humus is further degraded by some microbes, releasing inorganic nutrients. Warm and moist environments favour decomposition, whereas low temperature and anaerobiosis severely inhibit decomposition.
3. Energy Flow
Sun is the only source of energy for all ecosystems (except deep sea hydrothermal ecosystems). Of the incident solar radiation, less than 50% is Photosynthetically Active Radiation (PAR). Plants capture only 2-10% of the PAR, and this small amount of energy sustains the entire living world. Energy flow is strictly unidirectional (follows thermodynamics laws).
Food Chains:
- Grazing Food Chain (GFC): Starts with producers (plants) \(\to\) primary consumers (herbivores) \(\to\) secondary consumers (carnivores). E.g., Grass \(\to\) Goat \(\to\) Man.
- Detritus Food Chain (DFC): Begins with dead organic matter. It is made up of decomposers (fungi, bacteria). In terrestrial ecosystems, a much larger fraction of energy flows through the DFC than through the GFC.
The interconnected network of food chains forms a food web. Organisms occupy a specific place in the food chain known as their trophic level. Because energy is lost as heat at each transfer (10% law), the number of trophic levels is usually restricted to three or four.
4. Ecological Pyramids
The relationship between producers and consumers at different trophic levels can be expressed in terms of number, biomass, or energy. The base represents producers, and the apex represents top-level consumers.
- Pyramid of Number: Usually upright. In a grassland, producers vastly outnumber primary consumers. However, in a forest ecosystem, one tree (producer) can support hundreds of insects (consumers) leading to an inverted pyramid.
- Pyramid of Biomass: Usually upright. But the pyramid of biomass in a sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton.
- Pyramid of Energy: Always upright, and can never be inverted, because when energy flows from a particular trophic level to the next, some energy is always lost as heat at each step.
Competency Based Questions
Q1. In a stable grassland ecosystem, if an ecologist continuously measures the Gross Primary Productivity (GPP) over exactly one year to be roughly \(500,000 \text{ kcal/m}^2\), and the autotrophic plants strictly expend \(350,000 \text{ kcal/m}^2\) merely to actively maintain physiological respiration (\(R\)), precisely calculate the total Net Primary Productivity (NPP) physically available exclusively to primary terrestrial herbivores.
(A) \(500,000 \text{ kcal/m}^2\)
(B) \(850,000 \text{ kcal/m}^2\)
(C) \(150,000 \text{ kcal/m}^2\)
(D) \(1,000 \text{ kcal/m}^2\)
Answer and Explanation
Answer: (C) \\(150,000 \text{ kcal/m}^2\\)Explanation:
Gross primary productivity (GPP) fundamentally represents the entire gross rate of completely raw photosynthetic production. Net primary productivity (NPP) is defined precisely as the total remaining energy explicitly stored as consumable organic plant biomass physically available to heterotrophs (consumers).
The unyielding metabolic equation strictly calculates NPP by simply subtracting total respiratory respiratory energy losses (\(R\)) immediately from GPP:
$$ \text{NPP} = \text{GPP} - R $$
Given values:
$$ \text{GPP} = 500,000 $$
$$ R = 350,000 $$
$$ \text{NPP} = 500,000 - 350,000 = 150,000 \text{ kcal/m}^2 $$
The remaining \(150,000 \text{ kcal/m}^2\) is heavily utilized by grazing herbivores and eventual decomposers.
Q2. Unlike the constantly fluctuating dynamic pyramids of generic biological numbers or sheer organic biomass, mathematically explain why a formal Ecological Pyramid of Energy can physically “never” present itself entirely in a heavily inverted geometric shape within any standard functional ecosystem universally.
Answer and Explanation
Answer: Due to the Second Law of Thermodynamics and the 10% energy transfer rule.Explanation:
The rigid physical laws of thermodynamics govern biological energy flow continuously. The Second Law of Thermodynamics completely strictly dictates that any generic biological energy transfer unavoidably results in massive degradation, mostly escaping outwards uselessly as metabolic heat.
Consequently, as caloric energy flows sequentially upward from green autotrophic producers up to hungry primary consumers and so forth, merely an average of approximately 10% of the usable energy is successfully transferred efficiently at each specific trophic step. Because each higher tier is mathematically restricted to receiving only a tiny physically diminished fraction of the massive energy heavily captured unconditionally by the tier positioned immediately below it, the geometric physical base (Producers) inherently always contains vastly more total energy than the shrinking apex, flawlessly forcing the geometric visual diagram mathematically to remain permanently physically upright.
Q3. If roughly 1,000,000 Joules of generic incident solar energy directly successfully strikes a large field of green terrestrial vegetation daily, calculate carefully how many Joules of energy generally ultimately definitively transfer upward to be successfully stored within the tissues of a tertiary carnivore occupying the fourth biological trophic level (Level 4), assuming ideal basic 1% PAR capture and standard 10% transfer laws continuously.
Answer and Explanation
Answer: 10 Joules.Explanation:
First, carefully ascertain the exact functional energy successfully captured and fundamentally utilized solely by the Level 1 Autotrophic Producers:
Green plants typically effectively harness only roughly 1% of the total raw incident solar radiation hitting their delicate leaves to physically drive the entire biological GPP photosynthetic engine.
$$ \text{Energy captured by primary Producers} = 1\% \text{ of } 1,000,000 \text{ J} = 10,000 \text{ Joules}. $$
Next, stringently apply Lindeman’s classic standard 10% biological efficiency rule uniformly sequentially upwards:
- From standard Producers (Level 1) to Primary Consumers (Level 2): $$ 10\% \text{ of } 10,000 \text{ J} = 1,000 \text{ Joules}. $$
- From Primary Consumers to Secondary Consumers (Level 3): $$ 10\% \text{ of } 1,000 \text{ J} = 100 \text{ Joules}. $$
- From Secondary Consumers to Tertiary Carnivores (Level 4): $$ 10\% \text{ of } 100 \text{ J} = 10 \text{ Joules}. $$
Q4. During normal robust decomposition within a dense, deeply shaded forest, how does heavily dropping raw environmental temperature below critical 10°C thresholds physically affect the formal biochemical rates of detritus catabolism usually performed seamlessly heavily by generic soil microbial bacteria?
Answer and Explanation
Answer: Severely reduces biochemical decomposition rates due to cold-induced enzymatic inhibition.Explanation:
Generic soil bacteria and highly active terrestrial fungi universally rely on a series of heavily temperature-dependent functional extracellular hydrolytic enzymes to relentlessly chemically degrade complex tough rigid dead detritus into simple inorganic basic nutrients.
Sharply lowering ambient environmental temperatures heavily slows completely down these critical thermodynamic heavily kinetic enzymatic reactions drastically. Extremely cold environments physically inhibit microbial growth and firmly suppress crucial necessary extracellular enzyme functional biological activity. Consequently, generic decomposition rates fundamentally drop practically uniformly to near complete physical standstills, heavily allowing enormous rigid organic detritus to stack relentlessly.
Q5. Explain exactly why grazing terrestrial herbivores structurally heavily rely absolutely on fully active gut cellulolytic methanogens, when the herbivores themselves exclusively ingest purely cellulose-rich plant diets.
Answer and Explanation
Answer: Herbivores inherently lack enzymes to biochemically digest complex plant cellulose; the symbiotic methanogens definitively possess the required cellulases.Explanation:
Terrestrial herbivores structurally consume heavy terrestrial plant matter, which fundamentally consists almost entirely of rigid complex cellulose cell walls. Absolutely no higher vertebrate mammal natively precisely encodes or naturally synthesizes the exact required digestive enzyme, cellulase, directly in its own genome.
Therefore, herbivores strictly universally rely on a heavy dense distinct strictly maintained symbiotic population of highly active cellulolytic methanogens (like Methanobacterium) exclusively thriving completely safely deep within the anaerobic core of their specialized gut rumens. These archaea perfectly produce massive amounts of active extracellular cellulase, which strictly successfully ferments the incredibly tough rigid plant cellulose efficiently down into usable simple volatile short-chain fatty acids explicitly structurally absorbed directly by the host mammal for foundational energy.