Particles may be removed from a lattice by murder, coalescence, mutual
annihilation and simple death. If the particle system is not to die o
ut, the removed particles must be replaced by births. This letter show
s that coalescence can be counteracted by arbitrarily small birth-rate
s and contrasts this with the situations for annihilation and pure dea
th where there are critical phenomena. The problem is unresolved for m
urder.