Ecosystem structure and productivity

An ecosystem is a functional unit of nature, where living things interact with each other and with their physical surroundings. It can be as small as a pond or as large as a forest. There are terrestrial ecosystems (forest, grassland, desert) and aquatic ones (pond, lake, wetland, river, estuary). Crop fields and aquariums are man-made ecosystems.

Its structure is described by:

• the species composition: which plants and animals live there;
• stratification: the vertical layering of species at different heights. In a forest, trees form the top layer, shrubs the next, and herbs and grasses the lowest.

A pond is a simple, self-sustaining ecosystem to study:

• Abiotic (non-living) parts: the water, with its dissolved minerals, and the soil at the bottom, plus sunlight, temperature and day length.
• Producers: phytoplankton, algae and floating or rooted plants, which make food by photosynthesis.
• Consumers: zooplankton, free-swimming and bottom-dwelling animals, which eat the producers or each other.
• Decomposers: fungi, bacteria and flagellates, especially at the bottom, which break down dead matter and return its minerals.

The functions of an ecosystem are: productivity, decomposition, energy flow and nutrient cycling.

Productivity. The amount of biomass (organic matter) made by plants in a given area over a period is the primary production, and the rate of making it is the primary productivity. It is expressed as weight (g m⁻²) or energy (kcal m⁻²) per year.

• Gross primary productivity (GPP): the total rate of making organic matter by photosynthesis.
• Plants use part of it for their own respiration (R).
• What remains is the net primary productivity: NPP = GPP − R. This is the biomass available to the consumers.

The rate at which consumers make new organic matter is secondary productivity.

Productivity depends on the plant species, sunlight, temperature, nutrients and moisture. The whole biosphere makes about 170 billion tons of organic matter (dry weight) a year. Surprisingly, the oceans, though they cover about 70% of the surface, make only about 55 billion tons: most of the open ocean is short of nutrients.