Trophic cascade after removing an apex predator
A rocky reef has four trophic levels: apex predatory fish, grazing sea urchins, fleshy macroalgae, and encrusting coralline algae. A survey records 40 apex fish, 8000 urchins, and 5% macroalgal cover. After apex fish are removed by fishing, urchin numbers rise to 30000 and macroalgal cover falls to 1%, while coralline algae expand. A manager proposes reintroducing apex fish to restore macroalgae. A critic argues the urchins will simply eat the new algae and that the reef has already shifted to a new stable state.
1. Explain the trophic cascade and evaluate the manager's proposal.
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Removing apex fish releases urchins from predation, so urchin numbers rise from 8000 to 30000. More urchins graze more macroalgae, so macroalgal cover falls from 5% to 1%, and coralline algae expand because they are less palatable or better defended. This is a top-down trophic cascade: a change at the top level propagates down through the food web. The manager's proposal is plausible but not guaranteed. Reintroducing apex fish could reduce urchin numbers and allow macroalgae to recover, but recovery depends on whether algal propagules remain, whether urchins are controlled quickly enough, and whether the reef has crossed a threshold. If urchin barrens are a stable alternative state, simply adding predators may not reverse it without also reducing urchin numbers directly.
2. Assess the critic's claim that the reef is in a new stable state.
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The claim is supported by the data but not proven. A stable state is a configuration that persists and resists change. The shift from 5% to 1% macroalgal cover, with urchins at 30000 and coralline algae expanding, is consistent with an urchin barren, a well-documented alternative state on reefs. Persistence over time and resistance to reversal would support stability. However, the data are a single snapshot, so we cannot yet show persistence. The critic is right that new algae could be grazed, but if apex fish reduce urchin density below a threshold, macroalgae may recover. The judgement should be: the claim is plausible and consistent with the evidence, but requires monitoring over time and possibly a combined intervention (predator reintroduction plus urchin removal) to test reversibility.