# Rosenzweig-MacArthur predator-prey model [duplicate]

The predator-prey model is governed by the following system of ode's. \begin{eqnarray} &&\displaystyle{\frac{dx}{dt}=r x\left(1 - \frac{x}{K}\right) - \frac{s y x}{1 + s \tau x}},\\[0.1cm] &&\displaystyle{\frac{dy}{dt}=-c y +d \frac{s x y}{1 + s \tau x}}, \end{eqnarray} where $r,K,c,d>0$, $s$ is the predator search rate and $\tau$ is time. The background of this model can be found here.

Using appropriate scaling as discussed here, the above the system can be written in the dimensionless form,

\begin{eqnarray} &&\displaystyle{\frac{dx}{dt}=x\left(1 - \frac{x}{k}\right) - \frac{m y x}{1 + x}},\\[0.1cm] &&\displaystyle{\frac{dy}{dt}=-c y + \frac{m x y}{1 + x}}. \end{eqnarray}

How to get solution of the above system?

How one can get phase portrait of the system?

Any help is highly appreciated.

• the second equation should begin with $dy(x)/dt$ ? – Julien Kluge Feb 18 '17 at 14:25
• Please write the equations in Mathematica syntax as well, and show what you tried. I assume you tried at least DSolve, and that if it couldn't solve it, you tried a numerical solution using NDSolve. What went wrong when you tried these? Explain where the difficulty is. – Szabolcs Feb 18 '17 at 14:36
• yes, I am sorry, edited. – Sk Sarif Hassan Feb 18 '17 at 14:37
• What is the origin of this model? – zhk Feb 18 '17 at 15:04
• @SkSarifHassan Here it is mathematica.stackexchange.com/questions/45922/… – zhk Feb 18 '17 at 15:12

Note

In my attempt to answer the OP's question, I have presented all most all the visuals/graphs which are important for the analysis of such models. If there is something missing or physically incorrect then please feel free to edit and correct?

Credit goes to @ChrisK for pointing me in the right direction, which made me able to carry out correct graphical analysis of the model.

The equations and values for the different parameters are take from the document cited in the question.

Solution & Plot

To solve such nonlinear systems, the best choice, in almost all cases is to use NDSolve to get numerical solutions.

c = 1; m = 3;
sol = With[{k = 3}, First@NDSolve[{x'[t] == x[t] (1 - x[t]/k) - m*x[t]*y[t]/(1 + x[t]),
y'[t] == m*x[t]*y[t]/(1 + x[t]) - c*y[t], x == 1,
y == 1}, {x, y}, {t, 0, 50}]]

Plot[Evaluate[{x[t], y[t]} /. sol], {t, 0, 50},
PlotStyle -> {Red, Directive[Green, Dashed]}, Frame -> True,
PlotLegends -> LineLegend[{Red, Directive[Green, Dashed]}, {"prey", "predator"}]] Nullclines

nk1 = With[{k = 3}, ContourPlot[{((1 - x/k) - m*y/(1 + x)), (m*x/(1 + x) - c),
y}, {x, -1.5, 3.5}, {y, -0.5, 1},
Epilog -> {Text[Style["k>(m+c)/(m-c)", 12], Scaled[{0.7, 0.9}]]}]];

nk2 = With[{k = 1}, ContourPlot[{((1 - x/k) - m*y/(1 + x)), (m*x/(1 + x) - c),
y}, {x, -1.5, 3.5}, {y, -0.5, 1},
Epilog -> {Text[Style["k<(m+c)/(m-c)", 12], Scaled[{0.7, 0.9}]]}]];

GraphicsGrid[{{nk1, nk2}}, ImageSize -> Large] Phase Portrait

sol1[k_?NumericQ, x0_?NumericQ] :=
First@NDSolve[{x'[t] == x[t] (1 - x[t]/k) - m*x[t]*y[t]/(1 + x[t]),
y'[t] == m*x[t]*y[t]/(1 + x[t]) - c*y[t], x == x0,
y == x0}, {x, y}, {t, 0, 200}];

ppk1 = ParametricPlot[
Evaluate[{x[t], y[t]} /. sol1[1, #] & /@ Range[0, 1, 0.2]], {t, 0,
200}, Frame -> True,
Epilog -> {Text[Style["k=1", 20], Scaled[{0.8, 0.8}]]},
ImageSize -> 200, PlotRange -> {{0, 2.5}, {0, 1.6}}];

ppk3 = ParametricPlot[
Evaluate[{x[t], y[t]} /. sol1[3, #] & /@ Range[0, 1, 0.2]], {t, 0,
200}, Frame -> True,
Epilog -> {Text[Style["k=3", 20], Scaled[{0.8, 0.8}]]},
ImageSize -> 200, PlotRange -> {{0, 2.5}, {0, 1.6}}];

GraphicsGrid[{{ppk1, ppk3}}] sp1 = With[{k = 1},
StreamPlot[{x*(1 - x/k) - m*x*y/(1 + x), m*x*y/(1 + x) - c*y}, {x,
0, 2.5}, {y, 0, 1.5}]];

sp2 = With[{k = 3},
StreamPlot[{x*(1 - x/k) - m*x*y/(1 + x), m*x*y/(1 + x) - c*y}, {x,
0, 2.5}, {y, 0, 1.5}]];

GraphicsGrid[{{Show[ppk1, sp1, nk2], Show[ppk3, sp2, nk1]}}] • This is a nice start. For more interesting results, you should increase c relative to d (otherwise the predators can't persist no matter how much prey they have) and then use prey carrying capacity k as a bifurcation parameter. This is the well-known Rosenzweig-MacArthur predator-prey model. As you increase k there is a Hopf bifurcation where a predatory-prey limit cycle is born. – Chris K Feb 18 '17 at 16:36
• @MMM, how to achieve the bifurcation diagram of the model with respect to the parameter k? – Sk Sarif Hassan Feb 19 '17 at 4:08
• @SkSarifHassan Check my edited response. – zhk Feb 19 '17 at 6:39
• @SkSarifHassan Is this what you wanted? or I am missing something? – zhk Feb 19 '17 at 13:23
• @MMM, yes it is. I was thinking from complex variable point of view. What about bifurcation diagram for k? – Sk Sarif Hassan Feb 19 '17 at 14:35