TextCell["Pocetni model"]

rp:=1
rg:=0.0415

a:=0.1
b:=0.2

p0:=100
g0:=10
pocetnatocka:={p0,g0}

pdiff0:=rp*p0-a*g0*p0
gdiff0:=rg*p0*g0-b*g0

norma:=Sqrt[(pdiff0-p0)^2+(gdiff0-g0)^2]
norma

pt=p'[t]==rp*p[t]-a*g[t]*p[t];
gt=g'[t]==rg*p[t]*g[t]-b*g[t];
sustav:={pt,gt}
cijelisustav:=Join[sustav,{p[0]==p0,g[0]==g0}]
pocetnizec=NDSolve[cijelisustav,{p,g},{t,0,100}]
krivulje:=Flatten[pocetnizec]
zec:=p[t]/.krivulje[[1]]
lisica:=g[t]/.krivulje[[2]]

Show[Plot[{zec,lisica},{t,0,100},PlotRange->All,AxesLabel->TraditionalForm/@{Style[vrijeme,18],{Style[Zec,20],Style[Lisica,20]}},AxesOrigin->{0,0}]]

Show[ParametricPlot[{{zec,lisica}},{t,0,100},PlotRange->All,AxesLabel->TraditionalForm/@{Style[Zec,20],Style[Lisica,20]},AxesOrigin->{0,0}],Graphics[{PointSize[0.015],Point[pocetnatocka]}],Graphics[Arrow[{{p0,g0},{p0+5*(pdiff0-p0)/norma,g0+5*(gdiff0-g0)/norma}}]]]