  A model for large-scale evolution recently introduced by Amaral and Meyer is
studied analytically and numerically. Species are located at different trophic
levels and become extinct if their prey becomes extinct. It is proved that this
model is self-organized critical in the thermodynamic limit, with an exponent 2
characterizing the size distribution of extinction events. The lifetime
distribution of species, cutoffs due to finite-size effects, and other
quantities are evaluated. The relevance of this model to biological evolution
is critically assessed.
