c1=1; c2= 1; c3=2; c4=0.1; c5=0.5; c6=1; b0=0.6;r0=0.2;l=200; modell={b'[t]==b[t] (c1 (1 - b[t]/c2) - (c3 r[t])/(b[t] + c4) ), r'[t]==r[t] c5 ( 1 - (c6 r[t])/b[t] ), b[0]==b0, r[0]==r0}; loesung=NDSolve[modell,{b,r},{t,0,l}]; abb=ParametricPlot[Evaluate[{b[t],r[t]}/.loesung],{t,0,l} ,PlotRange->All ,AxesLabel->{"b","r"} ,ImageSize->{400,300}];