Tukuk-OSEnergy flows, nutrients cycle. A plant captures a few percent of light; a herbivore eats the plant and keeps roughly a tenth of the energy it consumed — the rest is metabolism, movement and heat — and a predator gets about a tenth of that tenth. This ten-percent arithmetic is why food chains rarely exceed four or five links, why there are always more grass than gazelle than cheetah, and why eating lower on the chain feeds more people per hectare: the pyramid is not a policy, it is physics.
Remove one link and the cascade reorganises the map. The textbook case: sea otters eat sea urchins, urchins eat kelp. Where otters were hunted out, urchin barrens scraped the kelp forests off the seabed — fish nurseries, storm buffers and carbon sinks lost together; where otters returned, the forest returned with them. Wolves in Yellowstone changed willow and stream banks after their reintroduction reduced overbrowsing. “Keystone species” names exactly this: not the biggest animal, but one whose removal rearranges far more than its own share.
The quiet link everyone forgets is the disposal crew: fungi and decomposer bacteria close every loop, turning dead matter back into plant nutrient, and without them the world would be buried in its own leftovers. Dung beetles were imported to Australia precisely because European cattle dung sat undissolved for years — a living demonstration that even nutrient cycling needs appointed staff.