Figure 16.7 Equilibrium population sizes of two species. (a) The equilibrium isocline for species 1, shown as a green line, represents all combinations of species 1’s population sizes (N₁) and species 2’s population sizes (N₂) that cause species 1 to experience zero growth. When species 2 has an abundance of zero, species 1 will have an abundance of K₁, which is the carrying capacity of species 1. (b) The equilibrium isocline for species 2, shown as a purple line, represents all combinations of species 1’s population sizes (N₁) and species 2’s population sizes (N₂) that cause species 2 to experience zero growth. When species 1 has an abundance of zero, species 2 will have an abundance of K₂, which is the carrying capacity of species 2. The arrows indicate how each population either increases or decreases as it moves away from the equilibrium line.