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 i, r[0]==r0}; loesung:=NDSolve[modell,{b,r},{t,0,50}]; abb=Plot[Evaluate[Table[{b[t]}/.loesung[[1]],{i,0.95,1.05,0.02}]],{t,0,50} ,AxesLabel->{"t","b"},ImageSize->{400,300}];