c1=0.1;c2=0.002;c3=0.001;c4=0.15; b0=151;r0=49; modell={b'[t]==b[t] (c1 - c2 r[t]), r'[t]==r[t] (c3 b[t] - c4), b[0]==b0, r[0]==r0 (1- 0.14 i)}; loesung:=NDSolve[modell,{b,r},{t,0,100}]; abb=ParametricPlot[ Evaluate[ Table[{b[t],r[t]}/.loesung,{i,0,6}] ] ,{t,0,100} ,AxesLabel->{"b","r"},ImageSize->{400,300}];