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Food webs are tightly connected, and removing one species can ripple through the entire community. When wolves were removed from Yellowstone, the elk population grew unchecked, overgrazed willows and aspens along the riverbanks, and the loss of those plants reshaped streams and beaver populations. When wolves were brought back decades later, the willows recovered. One species can have outsized effects on the entire system, and "just adjusting" usually means another part of the web crashes.

Almost every bit of energy in a food web traces back to the sun. Producers like grass, trees, and algae capture sunlight through photosynthesis and turn it into chemical energy that the rest of the ecosystem uses. When a deer eats grass, it's getting solar energy that the grass already converted. When a coyote eats the deer, the same energy is moving up another level. Disruptions to producers (drought, fire, deforestation) cut off the energy supply for everything above them.

Disruptions move both up and down a food web. If a disease wipes out a deer population, the predators that eat them lose food, but the plants they used to graze on can grow back faster, which then changes the insects, birds, and other small animals that depend on those plants. The "domino effect" is real. The best way to see it is to draw arrows showing every connection from the disrupted species and trace what happens to each one.

Top predators often stabilize the whole ecosystem. Without them, prey populations can grow unchecked, overeat the producers, and crash the system from the bottom up. Yellowstone is the classic case. So is the relationship between sea otters and kelp forests, where otters eat sea urchins and the urchins would otherwise mow down all the kelp. Removing a top predator usually does more damage to a food web than the predator ever did.