Nullclines
It is difficult to give a general outline for the determination of nullclines. The key is to set each RHS equal to zero and find all curves on which the resulting equation is satisfied.
x-nullclines
y-nullclines
The complete nullcline plot is the union of the plots of the - and -nullclines.
> NULLplot:=display( [ xNULLplot, yNULLplot ], view=[ -5 .. 15, -5 .. 25 ], title=`Nullclines` ):
> NULLplot;
>