occupied sites for $D=100$, in comparison with $25\%$ for $D=5$.

In the three-dimensional parameter space space of $\lambda$, $\mu$, and $D$, there is a critical surface separating the active and absorbing phases. On this surface, a [*tricritical line*]{} separates regions exhibiting continuous and discontinuous phase transitions (see Fig. \[crsurf\]). The mean-field theory of Eqs. (1)-(4) yields a tricritical line that begins at $\lambda=1$, $\mu=1/2$ (for $D=0$), and then tends, for increasing $D$, to ever smaller values of $\mu$ (asymptotically, $\mu = 1/D$, with $\lambda=1$ all the while). Simulations show a somewhat different picture, with the tricritical line approaching the point $\lambda=\mu=D=0$, and then curving toward larger $\mu$ and $\lambda$ values for small but nonzero $D$, before doubling back towards $\mu=0$, as shown in Fig \[crsurf\]. This means that for a given, nonzero value of $\mu$, the transition is discontinuous (if at all), only within a restricted range of $D$ values. For example, our simulations reveal that for $\mu=0.25$, the transition is discontinuous for $3 < D < 10$, but becomes continuous for $D \geq 100$. We defer a full mapping of the tricritical line to future work.

Conclusions
===========

We present a detailed study of the phase diagram of the 