Figure 2.

Graphical representation of competition for blue and red light between a green alga (green lines) and a cyanobacterium (blue lines). (A) The cyanobacterium is a stronger competitor for both blue and red light than the green alga (i.e., the critical light intensities $I_{\text{blue}}^{\ast }$ and $I_{\text{red}}^{\ast }$ are both lower for the cyanobacterium). In this case, the zero isoclines (solid lines) of the species do not intersect and the cyanobacterium always wins. (B) The cyanobacterium is a stronger competitor for red light, whereas the green alga is a stronger competitor for blue light. Intersection of the two zero isoclines indicates a coexistence equilibrium, which may be either stable or unstable. In panels C and D, the coordinates are transformed to light transmission I _out on a logarithmic scale. Coexistence is feasible for all supply points located in the conical region bounded by the two absorption vectors (dashed lines). (C) The coexistence equilibrium is stable if the cyanobacterium has a steeper absorption vector than the green alga. (D) The coexistence equilibrium is unstable if the green alga has a steeper absorption vector than the cyanobacterium; in this case, the system displays alternative stable states where the winner depends on the initial abundances of the species.