Inverted Pyramids of Numbers

Pyramids of numbers are not always pyramid-shaped. Because every organism counts as one, whatever its size, the shape can be distorted.Inverted pyramids of numbers
- A single large producer can support many small consumers. One oak tree may feed thousands of caterpillars, so the producer bar is narrower than the primary consumer bar.- Parasites are smaller and more numerous than their hosts. One fox may carry hundreds of fleas, so the top bar can be wider than the one below it.Worked example: 1 oak tree supports 20 000 caterpillars, which feed 100 blue tits. The producer bar is the narrowest of all, so the pyramid is inverted at its base.Limitations of pyramids of numbers
- They ignore the size of organisms: a tree and a blade of grass each count as one.
- Numbers at different levels can be so different (millions of phytoplankton, one whale) that it is impossible to draw the bars to scale.
- It can be hard to decide what counts as one individual, for example grass that spreads by runners.
- They show one moment and ignore changes in population through the year.
- Organisms that feed at more than one trophic level are hard to place.Because of these problems, ecologists prefer pyramids of biomass, and best of all pyramids of energy, when comparing ecosystems. A pyramid of energy is never inverted, because it measures energy flow over time and energy is always lost between trophic levels.