What Really Happens to a Fallen Leaf on the Forest Floor

A pond is more than water with fish in it. Its plants, animals, microbes, sunlight, water and minerals all interact, cycling nutrients and passing energy along. That working unit is an ecosystem.

This part covers the structure of an ecosystem, its functions, productivity, and decomposition.

An ecosystem is a functional unit of nature in which living organisms interact with one another and with their physical environment; its structure comes from its abiotic and biotic components, the species present, and their vertical arrangement into layers, called stratification.

Components:

- Abiotic — water, sunlight, soil, minerals, temperature and other physical factors
- Biotic — producers, consumers and decomposers

Structural features:

- Species composition — the identity and number of plant and animal species
- Stratification — vertical layering; in a forest, trees form the top layer, shrubs the second, and herbs and grasses the bottom layers

A pond as an example:

- Abiotic — water with dissolved substances and a rich layer of soil at the bottom
- Producers — phytoplankton, algae, and floating, submerged and marginal plants
- Consumers — zooplankton and free-swimming and bottom-dwelling animals
- Decomposers — fungi, bacteria and flagellates, especially abundant at the bottom
- The pond is a self-sustaining unit in which energy flows and nutrients cycle

An everyday example. A village pond with lotus, water plants, fish and frogs is a complete ecosystem on a small scale.

The substance. An aquarium is a man-made ecosystem that is not self-sustaining — people must feed it and clean it.

Every ecosystem carries out four functional processes — productivity, decomposition, energy flow and nutrient cycling — through producers that make food by photosynthesis, consumers that feed on other organisms, and decomposers that break down dead matter.

The four functions:

- Productivity — the rate of biomass production
- Decomposition — the breakdown of dead organic matter
- Energy flow — the one-way movement of energy from the sun through living organisms
- Nutrient cycling — the movement of nutrients between living and non-living parts

Roles of organisms:

- Producers — green plants and phytoplankton that turn solar energy into chemical energy
- Consumers — herbivores, or primary consumers, eat plants; carnivores, or secondary and tertiary consumers, eat other animals
- Decomposers — bacteria and fungi that break down dead organic matter and release nutrients

An everyday example. Grass, a grazing cow, and the dung beetles and fungi that break down cow dung are producer, consumer and decomposers in an Indian field.

The substance. Energy flows through an ecosystem once, but nutrients cycle again and again — the key difference between these two functions.

Gross primary productivity (GPP) is the rate at which producers make organic matter in photosynthesis, net primary productivity (NPP) is what remains after their respiration losses, and secondary productivity is the rate at which consumers form new organic matter; primary productivity depends on the plant species, environmental factors, nutrients and photosynthetic capacity.

Key terms:

- Primary production — biomass made by plants per unit area over a period, measured as weight or energy
- Productivity — the rate of biomass production per unit area per unit time
- GPP — the total rate of organic matter production in photosynthesis
- NPP — GPP minus respiration losses (R); this is the biomass available to consumers
- Secondary productivity — the rate at which consumers form new organic matter

Worked example. A grassland plot has a GPP of $2400$ kcal m${}^{-2}$ over a season and loses $1000$ kcal m${}^{-2}$ in respiration:

$$\text{NPP}=\text{GPP}-\text{R}=2400-1000=1400{\text{ kcal m}}^{-2}$$

Factors limiting primary productivity:

- The plant species in the area
- Environmental factors such as light, temperature and water
- The availability of nutrients
- The photosynthetic capacity of plants

Aquatic ecosystems. Productivity is limited by light, which fades with depth, and by nutrients; the open oceans cover most of Earth's surface yet have low productivity because nutrients are scarce there.

An everyday example. A well-watered, fertilised paddy field produces far more biomass than a dry, rocky hillside of the same area.

The substance. Only NPP is available to herbivores — the energy plants spend on their own respiration never reaches consumers.

Decomposition is the breakdown of complex organic matter by decomposers into inorganic substances such as carbon dioxide, water and nutrients; it proceeds through fragmentation, leaching, catabolism, humification and mineralisation, and it is faster when detritus is rich in nitrogen and sugars and when conditions are warm and moist.

Detritus — dead plant remains such as leaves, bark and flowers, and dead animal remains including faecal matter; it is the raw material for decomposition.

Steps of decomposition:

- Fragmentation — detritivores such as earthworms break detritus into smaller particles
- Leaching — water-soluble inorganic nutrients seep into the soil and are precipitated as unavailable salts
- Catabolism — bacterial and fungal enzymes break detritus into simpler inorganic substances
- Humification — forms humus, a dark, amorphous substance that resists microbes, decomposes very slowly and stores nutrients
- Mineralisation — microbes break down humus, releasing inorganic nutrients

Factors controlling the rate:

- Chemical composition of detritus — slow when rich in lignin and chitin; fast when rich in nitrogen and water-soluble sugars
- Climate — warm, moist conditions speed decomposition
- Oxygen — decomposition is largely aerobic, so low temperature and lack of oxygen slow it and organic matter builds up

An everyday example. A compost pit in a garden turns kitchen waste and leaves into dark, crumbly, humus-rich manure.

The substance. Humus decomposes very slowly by design — its resistance to microbes is what lets it store nutrients in the soil for long periods.