  Collective actions, from city marathons to labor strikes, are often
mass-driven and subject to the snowball effect. Motivated by this, we study
evolutionary advantages of conditional punishment in the spatial public goods
game. Unlike unconditional punishers who always impose the same fines on
defectors, conditional punishers do so proportionally with the number of other
punishers in the group. Phase diagrams in dependence on the punishment fine and
cost reveal that the two types of punishers cannot coexist. Spontaneous
coarsening of the two strategies leads to an indirect territorial competition
with the defectors, which is won by unconditional punishers only if the
sanctioning is inexpensive. Otherwise conditional punishers are the victors of
the indirect competition, indicating that under more realistic conditions they
are indeed the more effective strategy. Both continuous and discontinuous phase
transitions as well as tricritical points characterize the complex evolutionary
dynamics, which is due to multipoint interactions that are introduced by
conditional punishment. We propose indirect territorial competition as a
generally applicable mechanism relying on pattern formation, by means of which
spatial structure can be utilized by seemingly subordinate strategies to avoid
evolutionary extinction.
