Efficiency calculations depend on energy values for each trophic level, and these are very hard to measure accurately. Results should always be treated as estimates.Limitations include:
- Measuring energy content: a dried sample must be burnt in a calorimeter. Different tissues (fat, bone, wood) contain different amounts of energy, so a small sample may not represent the whole organism.
- Destructive sampling: organisms must be killed and dried, so only small samples are taken and scaled up to the whole area. Any error in a sample is multiplied when it is scaled up.
- Feeding at several levels: many animals feed at more than one trophic level (omnivores, or a fox eating both rabbits and beetles), so their energy cannot be placed at a single level.
- Movement: animals move in and out of the area, and some leave by migration for part of the year.
- Seasonal change: productivity and population sizes vary through the year, so measurements taken in one season may not be representative.
- Respiration and decomposers: energy lost in respiration and passed to decomposers is difficult to measure directly, and is often estimated or found by subtraction.
- Mean values hide variation between individuals, sites and years.Worked example: a 0.25 m2 quadrat produces 50 g of dry plant material. Scaling up to 1 m2:50 × 4 = 200 g m−2 If this one quadrat happened to be in an unusually lush patch, the whole estimate for the field would be too high. Taking many random samples reduces this problem.Killing many organisms also raises ethical issues.
- Measuring energy content: a dried sample must be burnt in a calorimeter. Different tissues (fat, bone, wood) contain different amounts of energy, so a small sample may not represent the whole organism.
- Destructive sampling: organisms must be killed and dried, so only small samples are taken and scaled up to the whole area. Any error in a sample is multiplied when it is scaled up.
- Feeding at several levels: many animals feed at more than one trophic level (omnivores, or a fox eating both rabbits and beetles), so their energy cannot be placed at a single level.
- Movement: animals move in and out of the area, and some leave by migration for part of the year.
- Seasonal change: productivity and population sizes vary through the year, so measurements taken in one season may not be representative.
- Respiration and decomposers: energy lost in respiration and passed to decomposers is difficult to measure directly, and is often estimated or found by subtraction.
- Mean values hide variation between individuals, sites and years.Worked example: a 0.25 m2 quadrat produces 50 g of dry plant material. Scaling up to 1 m2: