The concept of the ecological pyramid is one of the most important models described in ecology. Classically defined, biomass and energy pyramids are wider at the bottom. This indicates that there are more prey items than predators in a given environment, with cumulative biomass and energy content being greater in prey than predators. Inverted pyramids have predators stably outnumber prey, with the most biomass locked up in the highest rung.
This inverted scheme goes against the convention generally taught in basic biology, and for good reason. They’re rare, and, in fact, their existence is debatable, depending upon semantics.
A popular example of biomass inversion comes from aquatic environments (particularly closed off, nutrient poor lakes). Here, phytoplankton, the primary producers, can be much reduced in biomass compared to the planktivores that eat them.
This is usually explained by the high turnover of plankton numbers. Their rapid synthesis and mortality ensures that, although their overall biomass is less, because of the fast rates of death and reproduction, more ENERGY is going through these lower rungs. So the energy throughput of the phytoplankton is greatest amongst pyramid levels. Conversely, predators live longer, grow slower, and are predated less often, so it may appear as though there is more biomass there, but the energy flowing through the top is reduced.
This shows that energy pyramids are always bottom up and can never be inverted. Nature is inefficient. Due to requirements of organisms to “waste” energy maintaining homeostasis (for starters), not all the energy taken from the lower food level gets transferred to the next one up.
Another familiarly published inverted system describes coral reef habitats, where shark populations see significantly more accumulated biomass than those of smaller reef fish species.
Problems with the inversion concept include mobile predators with access to multiple ecosystems, inconsistent ratios between predator/prey mass [i.e., in a lake, there may be less algae than small fish (inverted scheme), but there are more small fish than top predators (normal), resulting in only a partially inverted biomass pyramid], and just a lack of understanding of the full complexities of food chains.