### Hastings & Powell 1991
### Model equations

### Define parameters and equations
f(u, a, b) = a*u/(1 + b*u)

function parameterized_model!(du, u, p, t)
    x, y, z = u
    a1, a2, b1, b2, d1, d2 = p
    du[1] = dx = x*(1-x) - f(x, a1, b1)*y
    du[2] = dy = f(x, a1, b1)*y - f(y, a2, b2)*z - d1*y
    du[3] = dz = f(y, a2, b2)*z - d2*z
end
