Parrotfish are major grazers of the algae that grow on coral reefs. Protecting them from over-fishing may be decisive for the survival of reefs in the Caribbean. Sea urchins can also do the trick. That’s the take-home message of a Nature article, most of which consists are rather hard to follow mathematical modelling – ecology is not all (or even at all) about tree hugging! There are some very tough sums involved!