  In a critically self--organized model of punctuated equilibrium, boundaries
determine peculiar scaling of the size distribution of evolutionary avalanches.
This is derived by an inhomogeneous generalization of standard branching
processes, extending previous mean field descriptions and yielding $\nu=1/2$
together with $\tau'=7/4$, as distribution exponent of avalanches starting from
species at the ends of a food chain. For the nearest neighbor chain one obtains
numerically $\tau'=1.25 \pm 0.01$, and $\tau'_{first}=1.35 \pm 0.01$ for the
first return times of activity, again distinct from bulk exponents.
