Botany · Ch 7 — Ecosystem
Ecological Pyramids
Ecological Pyramids
An ecological pyramid is a graphic device — first introduced by Charles Elton in 1927, and hence also called an Eltonian pyramid — for representing the trophic structure and function of an ecosystem at each successive trophic level. Three types are recognised. A pyramid of number graphs how many organisms occupy each trophic level, and its shape varies with ecosystem type: it is always upright in grassland and pond ecosystems, since the number of organisms steadily falls from producers through primary, secondary and finally tertiary consumers; it is spindle-shaped in a forest ecosystem, because the base (T1) is occupied by a comparatively small number of very large trees, herbivores (T2, such as fruit-eating birds, elephants and deer) actually outnumber the producers, and tertiary consumers (T4, such as lions) are again fewer in number than the secondary consumers (T3, such as foxes and snakes) below them; and it is always inverted in a parasitic ecosystem, because it starts from a single host organism (such as one tree) and the number of organisms then increases at each successive level as smaller and more numerous parasites and hyperparasites are added. A pyramid of biomass graphs the amount of organic material present at each level instead of raw numbers: grassland and forest ecosystems show a gradual decrease in biomass from producers up to top carnivores and so are upright, but a pond ecosystem is the opposite — its base is occupied by tiny, low-biomass producers such as phytoplankton, so biomass actually rises toward the tip …
What this figure shows. Four side-by-side panels, each stacking trophic levels T1 (Producers) at the base through T2 (Herbivores), T3 (Secondary consumers) to T4 (Tertiary consumers) at the top, but drawn in four different outline shapes: Panel A (Grassland ecosystem) is a simple upright triangle, narrowing steadily from a wide T1 base to a narrow T4 tip; Panel B (Pond ecosystem) is also upright, narrowing the same way; Panel C (Forest ecosystem) is spindle-shaped — narrow at T1 (a few large trees), bulging outward at T2 (more numerous herbivores such as birds/elephants/deer), then narrowing again through T3 and T4; Panel D (Parasite ecosystem) is inverted — narrowest at T1 (a single host organism) and widening upward through T2, T3 and T4 as progressively smaller and more numer …
What this figure shows. Three side-by-side panels stacking T1 through T3/T4 by biomass (dry weight) rather than headcount: Panel A (Grassland ecosystem) is upright, biomass falling steadily from producers to top carnivores; Panel B (Forest ecosystem) is also upright, following the same falling pattern; Panel C (Pond ecosystem) is inverted — its producer base (phytoplankton) holds the LEAST biomass of any level despite being most numerous, so the pyramid narrows at the bottom and widens toward the tip, the opposite of the other two panels. …
What this figure shows. A single upright pyramid diagram tracing energy (in Joules, reusing the same 1000→100→10→1→0.1 figures as the ten percent law diagram) flowing from Sunlight into Producers at the widest base, then narrowing by roughly ten-fold at each successive step up through Primary consumers, Secondary consumers and Tertiary consumers at the tip — visually reinforcing that, unlike the pyramid of number or biomass, the pyramid of energy is always upright in every ecosystem type without excep …