c1=0.1;c2=0.002;c3=0.001;c4=0.16;
b0=170;r0=40;
modell={b'[t]==b[t] (c1 - c2 r[t]),
        r'[t]==r[t] (c3 b[t] - c4),
        b[0]==b0, r[0]==r0};
loesung=NDSolve[modell,{b,r},{t,0,100}];
abb=Plot[Evaluate[{b[t],r[t]}/.loesung],{t,0,100}
                   ,PlotStyle->{{},{Dashing[{0.01}]}}
                   ,AxesLabel->{"t","b,r"},ImageSize->{400,300}];