Energy Transfer and Productivity

Energy Transfer and Productivity

Food Chains and Energy

• Food chains represent the flow of energy through a sequence of organisms, from primary producers (like green plants) to primary consumers (herbivores), secondary consumers (carnivores), and so on.

• Each level in the food chain is referred to as a trophic level. The energy flow between trophic levels is never 100% efficient, with energy being lost at each stage, primarily through respiration and waste production.

Gross Primary Productivity (GPP) and Net Primary Productivity (NPP)

• All green plants use photosynthesis to capture light energy and convert it into chemical energy in glucose. The rate at which they produce organic matter (biomass) through photosynthesis is called gross primary productivity (GPP).

• Not all of this energy is available to the subsequent trophic levels. Plants use some of the glucose produced during photosynthesis in their own cellular respiration. The amount of energy left after this has been deducted is the net primary productivity (NPP).

• The NPP is the energy that’s available for growth and reproduction of the plants and for any herbivores (and consequently, carnivores) that eat them.

Secondary Productivity

• When a herbivore eats a plant, only a fraction of the plant’s energy becomes part of the herbivore. This energy then becomes available to any animal that eats the herbivore. This energy transfer is referred to as secondary productivity.

• Some energy is lost at each stage of a food chain. Thus, food chains are usually relatively short; energy availability limits the number of trophic levels.

Efficiency of Energy Transfer

• The efficiency of energy transfer from one trophic level to the next can be calculated with a simple formula: (Energy available after the transfer ÷ Energy available before the transfer) x 100%.

• The efficiency of energy transfer between trophic levels is typically low, often less than 10%. This is due, in part, to the energy losses through respiration, waste production, and non-digestible plant material.

• Successful natural adaptation often involves methods to maximise energy uptake and minimise energy loss, to make the most use of the available energy.

Ecological Pyramids and Biomass

• The efficiency of energy transfer through an ecosystem can be represented through ecological pyramids, such as pyramids of numbers, biomass, or energy.

• A pyramid of numbers shows the number of organisms at each trophic level of a food chain, while a pyramid of biomass shows the dry mass of all organisms at each trophic level.

• A pyramid of energy shows the flow of energy through each trophic level in a given time period. This is the most accurate of the three types, as it takes into account not only the mass of organisms, but also how quickly they convert energy in their bodies to biomass.