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International Journal of Engineering Research and Development
e-ISSN: 2278-067X, p-ISSN: 2278-800X, www.ijerd.com
Volume 6, Issue 5 (March 2013), PP. 01-05

Dynamics in a Discrete Prey-Predator System
M.ReniSagayaRaj1, A.George Maria Selvam2, M.Meganathan3
1, 2, 3
Sacred Heart College,Tirupattur - 635 601, S.India.

Abstract:- The stability analysis around equilibrium of a discrete-time predator prey system is considered
in this paper. We obtain local stability conditions of the system near equilibrium points. The phase
portraits are obtained for different sets of parameter values. Also limit cycles and bifurcation diagrams
are provided for selective range of growth parameter. It is observed that prey and predator populations
exhibit chaotic dynamics. Numerical simulations are performed and they exhibit rich dynamics of the
discrete model. 2010 Mathematics Subject Classification. 39A30, 92D25

Keywords and phrases:- Difference equations, predator – prey system, fixed points, stability.

I. INTRODUCTION
Dynamics of interacting biological species has been studied in the past decades. The first models were put
forward independently by Alfred Lotka (an American biophysicist, 1925) and Vito Volterra (an Italian
Mathematician, 1926). Volterra formulated the model to explain oscillations in fish populations in Mediterranean.
The model is based on the following assumptions:
(a) Prey population grow in an unlimited way when predator is absent
(b) Predators depend on the presence of prey to survive
(c) The rate of predation depends up on the likelihood that a predator encounters a prey
(d) The growth rate of the predator population is proportional to rate of predation.
The Lotka-Volterra model is the simplest model of predator-prey interactions, expressed by the following equations
[2, 4].
𝑥 ′ = 𝑎𝑥 − 𝑏𝑥𝑦
′
𝑦 = −𝑐𝑦 + 𝑑𝑥𝑦
where x, yare the prey and predator population densities and a, b, c,d are positive constants.

II. MODEL DESCRIPTION AND EQUILIBRIUM POINTS
The discrete time models described by difference equations are more appropriate when the populations
have non overlapping generations. Discrete models can also provide efficient computational models of continuous
models for numerical simulations. The maps defined by simple difference equations can lead to rich complicated
dynamics [1,3,5,7]. The paper [1] discusses the local stability of fixed points, bifurcation, chaotic behavior,
Lyapunov exponents and fractal dimensions of the strange attractor associated with (1). This paper considers the
following system of deference equations which describes interactions between two species and presents the various
nature of fixed points and numerical simulations showing certain dynamical behavior.
x(n  1)  rx(n)[1  x(n)]  ax(n) y (n)
(1)
y (n  1)   cy (n)  bx(n) y (n)
 r 1 
where r,a,b,c>0 The system (1) has the equilibrium points E0=(0,0), E1   ,0  and
 r 
 1  c r b  1  c  1 
E2   ,   . The trivial equilibrium point E0 corresponds to extinction of prey and predator
 b ab a
species, E1 corresponds to presence of prey and absence of predator and E2 corresponds to coexistence of both
b
species. The equilibrium point E2 is an interior positive equilibrium point provided r  .
b  1  c 

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III. EIGEN VALUES AND STABILITY
Nonlinear systems are much harder to analyze than linear systems since they rarely possess analytical
solutions. One of the most useful and important technique for analyzing nonlinear systems qualitatively is the
analysis of the behavior of the solutions near equilibrium points using linearization. The local stability analysis of
the model can be carried out by computing the Jacobian corresponding to each equilibrium point. The Jacobian
matrix J for the system (1) is
 r  2rx  ay ax 
J ( x, y )   
 by bx  c 
The Jacobian at E0 is of the form
r 0 
J  E0    
 0 c 
The eigen values are 1  r and 2   c . Stability is ensured if 1,2 1 which implies r < 1 and c < 1.The
Jacobian matrix for E1 is
 1 r  
2 r a  
  r  
J  E1  
  r 1 
 0 b c
  r  
 r 1
The eigen values are 1  2  r and 2  b    c .The interior equilibrium point E2 has the Jacobian
 r 
 r 1  c  1 c 
 1 a  
J  E2   
b  b 
 r b  1  c   b 
 1 
 a 
r 1  c  1  c  r  b  2  c   c .
Computation yields Tr  2  and Det 
b b

IV. CLASSIFICATION OF EQUILIBRIUM POINTS
The following lemma [8] is useful in the study of the nature of fixed points.
Lemma1.Let p( )    B  C andλ1, λ2 be the roots of p( )  0 . Suppose that p(1)  0 Then we have
2

(i) 1  1 and 2 1 if and only if p(1)  0 and C<1.
(ii) 1  1 and 2 1 (or 1 1 and 2 1 ) if and only if p(1)  0 .
(iii) 1 1 and 2 1 if and only if p(1)  0 and C>1.
(iv) 1   1 and 2  1 if and only if p(1)  0 and B  0, 2 .
(v) 1 and 2 are complex and 1  2 if and only if B2-4C < 0 and C=1.
 
The characteristic roots 1 and 2 of p( )  0 are called eigen values of the fixed point x , y . The fixed point

 x , y  is a sink if
 
1,2 1 . Hence the sink is locally asymptotically stable. The fixed point  x , y  is a source if


1,2  1 . The source is locally unstable. The fixed point  x , y  is a saddle if 1 1 and 2 1 (or 1 1 and
2 1 ). Finally  x , y  is called non hyperbolic if either 1 1or 2 1 . For the system (1), we have the
following results.

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Proposition 2. The fixed point E0 is a
 Sink if r 1 and c 1 . Source if r 1 and c 1 .
 Saddle if r 1 and c 1 . Non hyperbolic if r 1 and c 1 .
r 1  c  r 1  c 
 Sink if 1 r  3 and b  . Source if r  3 and b  .
r 1 r 1
r 1  c 
 Saddle if 1 r  3 and b  .
r 1
b  c  3 b
 Sink if r  .
1  c  b   3  c   b   2  c
b  c  3 b b  c  3
 Source if r  and r  . Saddle if r  .
1  c   b   3  c   b  2  c 1  c   b   3  c  
V. NUMERICAL SIMULATIONS
In this section, we provide the numerical simulations to illustrate some results of the previous sections.
Mainly, we present the time plots of the solutions x and y with phase plane diagrams (around the positive
equilibrium point) for the predator-prey systems. Dynamic natures of the system (1) about the equilibrium points
with different sets of parameter values are presented. Existence of limit cycles for selective set of parameters is
established through phase planes in Figures-3, 4. Also the bifurcation diagram, Figure-5, indicates the existence of
chaos in both prey and predator populations.
Example1.For the values r = 2.89, a = 0.099, b = 3.075, c = 1.09, we obtain E1=(0.65, 0) which is an axial fixed
point. Eigen values are 1 = -0.89 and 2 = 0.9209 so that 1,2 1 . Hence the fixed point is stable. The time plot
and the phase diagramillustrate the result, see Figure - 1.

Figure 1. Stability at E1

Example 2. In this example, we take r = 2.41, a = 1.19, b = 3.91 and c = 0.45. Computations yield E 2 = (0.37,
0.43). The eigen values are 1,2 = 0.5531 ± i0.7409 and 1,2 = 0.9246 < 1. Hence the criteria for stability are
satisfied [6]. The phase portrait in Figure - 2 shows a sink and the trajectory spirals towards the fixed point E 2.

Figure 1. Stability at E2

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Example 3. The parameters are r = 2.41, a = 1.43, b = 3.91, c = 0.25. The initial conditions on the populations of
the species are x(0) = 0.2 and y(0) = 0.3. The trajectory spirals inwards but does not approach a point. The
trajectory finally settles down as a limit cycle, see Figure-3.

Figure 3. Limit Cycle-1

The model (1) with parameters r = 2.31, a = 1.43, b = 3.91, c = 0.15 and initial conditions x(0) = 0.3,
y(0) = 0.4 exhibits another form of limit cycle. In this case the trajectory moves out in growing spirals and finally
approaches the limit cycle. The existence of limit cycles for selective range of parameters shows the oscillating
nature of the populations, see Figure-4.

VI. Figure 4. Limit Cycle – 2

Studies in population dynamics focuses on identifying qualitative changes in the long-term dynamics
predicted by the model. Bifurcation theory deals with classifying, ordering and studying the regularity in the
dynamical changes. Bifurcation diagrams provide information about abrupt changes in the dynamics and the values
of parameters at which such changes occur. Also they provide information about the dependence of the dynamics on
a certain parameter. Qualitative changes are tied with bifurcation.

Figure 5. Bifurcation Diagram

Example 4. The parameters are assigned the values a = 1.43, b = 3.91, c = 0.25 and the bifurcation diagram is
plotted for the growth parameter in the range 2 - 3.9. Both prey and predator population undergoes chaos,Figure-5.

This paper, dealt with a 2-dimensional discrete predator - prey system. Fixed points are found and stability
conditions are obtained. The results are illustrated with suitable hypothetical sets of parameter values. Numerical
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simulations are presented to show the dynamical behavior of the system (1). Finally, bifurcation diagrams for both
species are presented.

REFERENCES
[1]. Abd-Elalim A. Elsadany, H. A. EL-Metwally, E. M. Elabbasy, H. N. Agiza, Chaos and bifurcation of a
nonlinear discrete prey-predator system, Computational Ecology and Software, 2012, 2(3):169-180.
[2]. Leah Edelstein-Keshet, Mathematical Models in Biology, SIAM, Random House, New York, 2005.
[3]. Marius Danca, Steliana Codreanu and Botond Bako, Detailed Analysis of a Nonlinear Prey-predator
Model, Journal of Biological Physics 23: 11-20, 1997.
[4]. J.D.Murray, Mathematical Biology I: An Introduction, 3-e, Springer International Edition, 2004.
[5]. Robert M.May, Simple Mathematical Models with very complicated dynamics,Nature, 261, 459 –
67(1976).
[6]. Saber Elaydi, An Introduction to Difference Equations, Third Edition, Springer International Edition,
First Indian Reprint, 2008.
[7]. L.M.Saha, Niteesh Sahni, Til Prasad Sarma, Measuring Chaos in Some Discrete Nonlinear Systems, IJEIT,
Vol. 2, Issue 5, Nov- 2012.
[8]. Sophia R.J.Jang, Jui-Ling Yu, Models of plant quality and larch bud moth interaction, Nonlinear Analysis,
doi:10.1016/j.na.2009.02.091.
[9]. Xiaoli Liu, Dongmei Xiao, Complex dynamic behaviors of a discrete-time predator prey system, Chaos,
Solutions and Fractals 32 (2007) 8094.

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