This function calculates the density-independent rate of offspring production for each species. The rate of offspring production is given by $$R_{di}(t) =\frac{\epsilon}{2w_0}\int N(w,t)q(w,t)r(w,t)dw$$ where \(q(w,t)\) is the gonadic mass of an individual, \(r(w,t)\) is the mass-specific gonad release rate, \(N(w,t)\) is the abundance density, \(\epsilon\) is the reproductive efficiency and \(w_0\) is the weight of an offspring.