Fig. 1

Stable or unstable coexistence of two species A and B in the PR-plane. The drawn lines are the zero isoclines. At the intersection point both consumer densities are stationary. The dashed arrows are consumption vectors, indicating in what proportion the two resource densities decrease when a single consumer density increases. The dotted arrows in the shaded area give the combined resource density change; a motion towards the intersection, and hence a stable equilibrium point (left), or away from it, and hence a saddle point (right)