Chapter 1.1, Problem 21E

For the following predator-prey systems, identify which dependent variable,x or y, is the prey population and which is the predator population. Is the growth of the prey limited by any factors other than the number of predators? Do the predators have sources of food other than the prey? (Assume that the parameters $\alpha ,\beta ,\gamma ,\delta$ and N are all positive.)
(a) $\begin{array}{l}\frac{dx}{dt}=-\alpha x+\beta xy\\ \frac{dy}{dt}=\gamma y-\delta xy\end{array}$     (b) $\begin{array}{l}\frac{dx}{dt}=\alpha x-\alpha \frac{x^2}{N}+\beta xy\\ \frac{dy}{dt}=\gamma y+\delta xy\end{array}$