03 Feeding relationships and ecological pyramids
A grazing food chain in a pond: phytoplankton → zooplankton → small fish → large fish → human or osprey. A detritus chain: dead leaves → fungi and bacteria → earthworms → moles → owls. In nature the two run together as a web, so the loss of one species rarely starves another completely.
- Pyramid of energy: the energy entering each level per unit time. It is always upright, because energy can never increase along the chain.
- Pyramid of number: individuals per level; it can be inverted — one tree supports thousands of insects and a few birds — or partly inverted in a parasitic chain.
- Pyramid of biomass: the dry mass per level; usually upright on land but inverted in the sea, where phytoplankton have a small standing crop yet reproduce so fast that they support a larger mass of zooplankton.
- Bioaccumulation travels the same levels: non-biodegradable substances such as DDT or mercury increase in concentration at each step, so the top predator carries the highest dose (biomagnification).