# 10.2: Phase Space

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A good way to understand the arrows of phase spaces is to imagine raindrops falling on a curvilinear rooftop and ﬂowing across its surface. Figure \(\PageIndex{1}\) shows such a surface.

Why should thinking of raindrops on rooftops help us understand phase spaces? It is because the differential equations themselves are situated on mathematically surfaces—albeit sometimes higher-dimensional surfaces—with points ﬂowing dynamically across the surfaces, just like raindrops ﬂowing across a roof. It is not completely the same, of course, but is a useful aid to thought.

Instead of raindrops, it can also be useful to think of a marble rolling on the surface. At the bottom of the basin at Point C in Figure \(\PageIndex{1}\), a marble is trapped. The surface goes up in every direction from this point, so after any small disturbance the marble will roll to the bottom again.

Point B corresponds to the equilibrium at the origin, stable in this case, where both species are extinct. A marble resting on this surface and experiencing a small positive disturbance away from the origin must roll uphill in every direction, so it will return to that equilibrium as well. It is below the two-species Allee point.

For example, Point A divides rain ﬂowing to the left and rain ﬂowing to the right. The basin at Point C corresponds to the carrying capacity, Point B corresponds to extinction at the origin, and Point A corresponds to the unstable Allee point.

Point A, on the other hand, corresponds to the Allee point. A marble could be balanced precariously at that place, with the slightest breath of air sending it to extinction at B or carrying capacity in the basin at C, depending on miniscule variations in the breath. Marbles starting close to either of the axes roll to the origin, equilibrium B. Marbles starting farther from the axes are on the other side of a long ridge and roll to the carrying capacity at C.