Ecosystem productivity

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Ecosystem productivity

Every ecosystem has a level of productivity, which helps discover the potential of an ecosystem for food production.

Primary productivity: Plant productivity.

Secondary productivity: Animal productivity.

Gross Primary Productivity: The measure of all photosynthesis that occurs in an ecosystem.

Net productivity: energy left after losses as a result of respiration, growth, excreta.

Net primary productivity (NPP): Amount of energy made available by plants to animals, only at the herbivore level, and is expressed as kg/m2/yr.

This means that once the rate of primary productivity in an ecosystem is established it is possible to compare ecosystems, as a figure for the potential of each ecosystem for food production can be found. The NPP of an ecosystem depends on the levels of heat, moisture, nutrients available, competition, amount of sunlight, age and health of plants. In broad terms NPP increases towards the equator and decreases away from it, towards the poles.

Examples of Plant primary productivity and biomass for various ecosystems:

Ecosystem: Area (million km2): Mean NPP g/m2/yr : World net primary productivity (billion tonnes per year): Mean biomass (kg/m2):
Tropical Rain Forest 17.0 2200 37.4 45
Savannah 15.0 900 13.5 4
Tundra and Alpine 8.0 140 1.1 0.6
Temperate Grassland 9.0 600 5.4 1.6

Reasons for decrease in energy at each level within an ecosystem:

Energy is lost at each stage in an ecosystem (at each transfer).

The following diagram shows how energy is lost within an ecosystem:

Energy loss

The temperate deciduous forest

It could be argued that the temperate deciduous forest is not a true example of a high productivity ecosystem, but in the UK it is one of the most productive. The table and map below highlights its main characteristics:

Map

Productivity High NPP, 1.2kg/m2/year, result of high summer temps and large amounts of daylight. Large amount of biomass as a result of woody material.
Vegetation Varies with soil type acidic = birch and rowan trees, alkaline = box and maple, elm common on clay, willow on gleyed soils. Oak often dominant due to tolerance of wider Ph range. More ground vegetation beneath oak trees, as small leaves allow more light to ground.
Climate Mild / wet up to 1500mm per year, more in winter often from depressions. More precipitation than evapotranspiration. Temps above freezing in winter. Average summer temperature = 15 - 20.
Soils Fertile brown earths, with a mildly acidic mull humus. Wide range of flora and fauna in litter layer, soil mixing encouraged by earthworms. Blurred soil horizons due to worms.
Nutrient cycle Fast rates of leaching balanced by fast rates of weathering. Many nutrients in soil as a result of slow winter growth and low density of vegetation.
Animals Adaptations occur in the winter because of low temperatures. Many animals either migrate or hibernate.
Human interference Few natural areas are left, and many areas have been cleared for agriculture, or recreation.

Temperate coniferous (boreal) forest

Although found in the UK the boreal forest is not as common as the deciduous, and is more common on Canada and Eastern/Central Europe as shown on the map:

Map

Productivity Low, NPP = 0.8kg/m2/year. High biomass from the woody material.
Vegetation Trees 20 - 30m high, mainly pine, larch and spruce. Evergreen to allow photosynthesis all year. Needle leaves reduce evapotranspiration, conical shape removes snow easily.
Climate Either cool temperate or cold continental. Small amounts of rainfall (below 500mm per year). Summer frosts, much winter snowfall, precipitation above evapotranspiration. Reduced growing season, but 16 - 20 hours of sunlight in summer increases photosynthesis.
Soils Normally podsols. Leaching is a result of snowmelt. Humus is acidic (pH 4.5 - 5.5). Iron pans may form. Few earthworms lead to distinct horizons. A thick litter layer.
Nutrient cycle Controlled by the low temperatures which limit rates of weathering in transfer, resulting in many nutrients in the litter.
Animals Sparse as little available food.
Human interference In the UK many are planted and used for forestry.

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