BioTopics

Site author Richard Steane

The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

Loss of biomass and energy between trophic levels

For additional information, leave your mouse pointer over the pictures (or text).
Clicking on the small photos below might bring up a bigger copy.

These Hereford cattle are feeding on the lush grass of the valley floor, which contrasts with the steep hillside behind.
It also reminds us that production of meat relies on a considerable area of ground in order for plant growth

This information could be used in the consideration of the (ecological) efficiency of production of different food items, or of meat production compared with arable farming.
If humans eat meat, they are acting as secondary consumers (the animal eats plants like grass or feed like grain derived from plants), whereas if the humans eat plant material (not necessarily grass!) they are primary consumers. The same area of ground could be used to feed many more people this way. By shortening the food chain there is less energy and biomass lost in the conversion from one trophic level to the next.

Some themes that have been suggested include:
	meat versus wheat,
	rabbits vs carrots,
	beef vs beans,
	chicken vs corn etc

Sheep vs grass could be considered as a different theme, since humans do not eat grass, and (some varieties of) sheep can survive and grow on very poor land - mostly upland areas - which is unsuitable for crop production. But some sheep are still fed on lowland grass and concentrated food from other vegetable sources.

The lifestyle of each trophic level in an ecosystem has specific features which affect the efficiency of transfer of energy and biomass to the next level.

Producers

Take in only some of the light energy shining on them - perhaps only 1%

Loss is due to:

Light reflected off ( green colour)
Plants respiring

Advantages	Disadvantages
of this trophic level
Light supply is fairly reliable	Relies on sun+water+soil etc
Life style requires little energy	Consumers take some productivity

Primary Consumers [herbivores]

Cellulose (main polysaccharide carbohydrate - plant cell walls) is difficult to digest

Loss is due to:

Animals respiring
Energy needed to find and process food, and escape from predators
Incomplete digestion
Faeces still contain much energy
Not all of producer is edible e.g.roots/branches/etc

Advantages	Disadvantages
of this trophic level
Large amount of plant food material usually available	Takes long time to digest food
Easy to find plant material for grazing/browsing	Takes long time (all day?) to collect food So :Vulnerable to predators
	Cellulose wears away teeth
	Food plants availability may change with seasons

Secondary, (also tertiary, quaternary) Consumers [carnivores]

Animals' flesh (high in protein) is easy to digest

Only about 10% of the energy in producers is passed on to primary consumers, and only about 10% of the energy in primary consumers is passed on to secondary consumers, and so on.
In practice, this limits the length of actual food chains to a maximum of 4 links.

What percentage of the energy already absorbed by producers is passed on to secondary consumers?
> 1%

Loss is due to:

Animals respiring
Energy needed to find and catch prey, {and escape from predators?}
Incomplete digestion - faeces still contain some energy
Not all of primary consumer is edible e.g.bones/skin/etc

Advantages	Disadvantages
of this trophic level
Takes short time to digest protein	Time taken to hunt for prey may not be successful: prey animals may escape!
	Prey availability may change with seasons
	Well maintained teeth, claws etc essential

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