models.

Recently, we studied symbiotic interactions in a two-species CP [@scp]. This was done by allowing two CPs (species A and B), to inhabit the same lattice. The symbiotic interaction is modeled via a reduced death rate, $\mu < 1$, at sites occupied by individuals of each species. Aside from this interaction, the two populations evolve independently. We found that, as one would expect, the symbiotic interaction favors survival of a mixed population, in that the critical reproduction rate $\lambda_c$ decreases as we reduce $\mu$ [@scp].

Apart from its interest as an elementary model of symbiosis, the critical behavior of the two-species symbiotic CP (2SCP) is interesting for the study of nonequilibrium universality classes. Extinction represents an absorbing state, a frozen state with no fluctuations [@marro; @henkel; @odor07; @hinrichsen; @odor04]. Absorbing-state phase transitions have been a topic of much interest in recent decades. In addition to their connection with population dynamics, they appear in a wide variety of problems, such as heterogeneous catalysis [@zgb], interface growth [@tang], and epidemics [@bart], and have been shown to underlie self-organized criticality [@vdmz; @bjp]. Recent experimental realizations in the context of spatio-temporal chaos in liquid crystal electroconvection [@take07], driven suspensions [@pine] and superconducting vortices 