# Module 6.4: PredatorPrey

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The PredatorPrey model shows the Lotka-Volterra model of predator-prey
relationships, where the predators and prey live in a somewhat
symbiotic fashion. Predators eat the prey, but when prey populations
fall, so do predator populations.

The Lotka-Volterra model consists of a pair of equations for change in
prey and predator populations (squirrels and hawks), where k is a
proportionality constant based on the number of hawks and squirrels.

Δs = (ks * s(t - Δt) - khs * ht * s(t - Δt)) * Δt

Δh = (ksh * s(t - Δt) * h(t - Δt) - kh * h(t - Δt)) * Δt

The prey and predator populations are both represented by stocks. A
prey birth fraction of 2 feeds the prey births flow, which contains
the equation prey birth fraction * prey population (initially
100). Prey deaths is a function of the term prey death proportionality
constant * predator population * prey population.

Predator population is controlled by the predator births flow. This
flow contains the predator birth fraction, multiplied by the predator
population (initially 15) and the prey population (remember, the
predators don't thrive unless there are enough prey!). Predator deaths
are controlled by the predator population * predator death
proportionality constant (1.06).

Two graphs show predator versus prey populations over time and
predator and prey populations graphed on the same axis.