A field has 640 mice at the start of a month…

In predation, the predator gains and the prey is harmed. Predators carry energy, control the prey and keep species diversity. Prey defend with camouflage, poison and warning colours, thorns and chemicals. The balance point of the Lotka-Volterra equations $\dfrac{dx}{dt} = ax - bxy$ and $\dfrac{dy}{dt} = cxy - dy$ is $x = d/c$ and $y = a/b$, and the swings have the period $2\pi/\sqrt{ad}$.