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}];