Chapter 12: Ecosystem

Factors and Biosphere Productivity

• Variation: Primary productivity depends on plant species, environmental factors, nutrient availability and plants’ photosynthetic capacity; therefore, it differs among ecosystems.
• Whole biosphere: Annual NPP is about 170 billion tons of dry organic matter.
• Ocean and land: Oceans cover about 70% of Earth’s surface but contribute 55 billion tons of annual NPP; the remaining 115 billion tons comes from land.

Detritus and the role of decomposition

• Detritus includes dead plant parts (leaves, bark and flowers), dead animals and their remains, and faecal matter.
• Role Decomposition disposes of detritus, recycles biogeochemicals and creates space for new generations of organisms.
• Earthworms are called farmers’ friends because they break down organic matter and loosen the soil.
• Main steps Fragmentation → leaching → catabolism → humification → mineralisation.

Detritus composition and climate

• Control Decomposition rate depends on the chemical composition of detritus and climatic conditions.
• Slower decomposition Detritus rich in lignin and chitin decomposes slowly under a given climate.
• Faster decomposition Detritus rich in nitrogen and water-soluble substances, such as sugars, decomposes quickly.
• Environmental factors Temperature, moisture and pH affect the rate of decomposition.

• How it occurs Detritus is pulverised as it passes through the digestive tracts of detritivores.
• Result Smaller particles have a greatly increased surface area.

• Decomposers Bacteria and fungi secrete digestive enzymes over detritus.
• Assimilation Decomposers take up some of the broken-down food; this material becomes immobilised in them.

Properties and importance of humus

• Composition Humus is rich in lignin, cellulose and other resistant substances.
• Persistence It resists microbial action and decomposes extremely slowly; its colloidal nature helps it retain nutrients.
• Soil benefits Humus acts as a nutrient reservoir and helps maintain soil moisture and aeration.

• Rate Mineralisation is slow because humus is trapped and some nutrients are immobilised in decomposers.

Energy Flow in Ecosystems

• Energy source: The Sun supplies energy to all ecosystems except deep-sea hydrothermal ecosystems.
• Photosynthetically active radiation (PAR): Less than \(50\%\) of incident solar radiation is PAR; plants and photosynthetic bacteria capture only \(2\text{–}10\%\) of it.
• Dependence on producers: All organisms depend on producers for food, either directly or indirectly; the small amount of captured energy supports the living world.
• Direction: Energy flows unidirectionally from the Sun to producers and then to consumers, in accordance with the first law of thermodynamics.
• Energy loss: The second law of thermodynamics means ecosystems need a constant energy supply to build required molecules and counter increasing disorder; energy is lost as heat.

Trophic Levels and Standing Crop

• Food-chain pattern: Usually a straight sequence of populations related by eating and being eaten; a chain commonly has \(3\text{–}6\) trophic levels.
• Trophic level: A feeding position in a food chain, identified by how an organism obtains food; the number of levels equals the number of steps in the chain.
• Multiple levels: Some organisms, such as humans, can occupy more than one trophic level.
• Progressive decrease: Available energy, biomass and number of individuals generally decrease as trophic level rises; much energy is lost as heat at each transfer.
• Standing crop: The living biomass or number of organisms per unit area in an ecosystem or trophic level; dry weight is preferred because fresh weight varies with seasonal moisture.
• Support: Producers sustain food chains, while decomposers use dead organic matter and help sustain the system.

Energy Transfer Between Trophic Levels

• Parasites: They do not occupy one fixed trophic level because they may feed on producers, herbivores or carnivores; examples include aphids, ticks, mites, leeches and mosquitoes.
• After death: Energy does not remain trapped at a trophic level; it passes to a higher level through feeding or becomes available to detritivores and decomposers.
• Energy use by herbivores: Some food is lost through digestion and assimilation; some assimilated food is broken down to release energy for body activities, and only a small portion becomes body tissue.
• Transfer onward: Herbivores may be eaten by primary carnivores, then by secondary carnivores and higher consumers; a large share of energy is lost as heat at each step.

Implications of the 10% Law

• Producer biomass: A large amount of biomass at the producer level is needed to support organisms at higher trophic levels.
• Chain length: With a given initial energy supply, shorter food chains can support greater biomass and are more efficient; chains with fewer links are favoured.

Detritus Food-Chain Alternative

• Another pathway: Detritus → Earthworm → Frog → Snake → Peacock.

Food-Web Connections and Stability

• Grassland example: If rabbits are absent, mice may eat grass; hawks may eat mice directly or eat snakes that have eaten mice.
• Stability: Alternative feeding links in a food web help maintain ecosystem stability.

• Structure: Producers form the base; tertiary or top-level consumers occupy the apex.
• Origin: Ecological pyramids were developed by Charles Elton in 1927; they are also called Eltonian pyramids.
• Calculations: Include all organisms at a trophic level when calculating their number, biomass or energy.
• Trophic level: A functional feeding position, not a species identity; one species may occupy multiple levels.
• Example: A sparrow is a primary consumer when eating seeds or fruits, but a secondary consumer when eating insects or worms.

Why pyramid shapes differ

• Numbers—tree food chain: One tree can support many herbivores; each herbivore may support several parasites, which may support many hyperparasites. Numbers increase toward consumers, producing an inverted pyramid.
• Numbers—forest: The pyramid is spindle-shaped.
• Biomass—aquatic ecosystem: Small producers such as phytoplankton have low standing biomass but support a larger zooplankton biomass; the pyramid is inverted.
• Energy: Energy declines from producers to consumers at every successive trophic level, so the pyramid cannot be inverted.

• Multiple trophic levels: Species occupying two or more trophic levels are not represented adequately.
• Oversimplified food chains: Pyramids treat feeding relationships as simple food chains, unlike complex natural food webs.
• Saprophytes: They are not given a place in ecological pyramids.