Research Article | Open Access

# The Effect of Time Delay on Dynamical Behavior in an Ecoepidemiological Model

**Academic Editor:**Shiping Lu

#### Abstract

A delayed predator-prey model with disease in the prey is investigated. The conditions for the local stability and the existence of Hopf bifurcation at the positive equilibrium of the system are derived. The effect of the two different time delays on the dynamical behavior has been given. Numerical simulations are performed to illustrate the theoretical analysis. Finally, the main conclusions are drawn.

#### 1. Introduction

During the past decades, epidemiological models have received considerate attention since the seminal SIR model of Kermack and McKendrich [1]. Great attention has been paid to the dynamics properties of the predator-prey models which have significant biological background. Numerous excellent and interesting results have been reported. For example, Bhattacharyya and Mukhopadhyay [2] studied the spatial dynamics of nonlinear prey-predator models with prey migration and predator switching, Bhattacharyya and Mukhopadhyay [3] analyzed the local and global dynamical behavior of an ecoepidemiological model, Kar and Ghorai [4] made a detailed discussion on the local stability, global stability, influence of harvesting and bifurcation of a delayed predator-prey model with harvesting, Chakraborty et al. [5] focused on the bifurcation and control of a bio-economic model of a delayed prey-predator model. For more related research, one can see [6–19].

In 2005, Song et al. [20] investigated the stability and Hopf bifurcation of a delayed ecoepidemiological model as follows: where , , represent the susceptible prey, infected prey and predator population, respectively. can be interpreted as the prey carrying capacity with an intrinsic birth rate constant . is called the transmission coefficient. The predator has a death rate constant and the predation coefficient . The death rate of infected prey is positive constant . The coefficient in conversing prey into predator is . and are the time required for mature of predator and the time required for the gestation of predator, respectively. The more detail biological meaning of the coefficients of system (1.1), one can see [20].

For the sake of simplicity, Song et al. [20] rescales time , then system (1.1) can be transformed into the following form: where , , , , , , , , .

We would like to point out that although Song et al. [20] investigated the local stability and Hopf bifurcation of system (1.2) under the assumption and obtained some good results, but they did not discuss what the different time delay and have effect on the stability and Hopf bifurcation behavior of system (1.2). Thus it is important for us to deal with the effect of time delay on the dynamics of system (1.2). There are some work which deal with this topic [21–24]. In this paper, we will further investigate the stability and bifurcation of model (1.2) as a complementarity. We will show that the two different time delay and have different effect on the stability and Hopf bifurcation behavior of system (1.2).

The remainder of the paper is organized as follows. In Section 2, we investigate the stability of the positive equilibrium and the occurrence of local Hopf bifurcations. In Section 3, numerical simulations are carried out to illustrate the validity of the main results. Some main conclusions are drawn in Section 4.

#### 2. Stability and Local Hopf Bifurcations

In this section, we will study the stability of the positive equilibrium and the existence of local Hopf bifurcations.

If the following condition: holds, then system (1.2) has a unique equilibrium point , where

Let , , and still denote , , by , , , respectively, then (1.2) reads as The characteristic equation of (2.3) is given by That is where where The following lemma is important for us to analyze the distribution of roots of the transcendental equation (2.5).

Lemma 2.1 (see [13]). *For the transcendental equation
**
as vary, the sum of orders of the zeros of in the open right half plane can change, and only a zero appears on or crosses the imaginary axis. *

In the sequel, we consider four cases.

*Case 1. *, (2.5) becomes
All roots of (2.9) have a negative real part if the following condition holds:
Then the equilibrium point is locally asymptotically stable when the conditions and are satisfied.

*Case 2. *, , (2.5) becomes
For , be a root of (2.11), then it follows that
which is equivalent to
Let , then (2.13) takes the form
where , , . Denote
Let , where , . There are three cases for the solutions of (2.15).(i)If , (2.15) has a real root and a pair of conjugate complex roots. The real root is positive and is given by
(ii)If , (2.15) has three real roots, of which two are equal. In particular, if , there exists only one positive root, ; If , there exists only one positive root, for , and there exist three positive roots for , , .(iii)If , there are three distinct real roots, , , , where . Furthermore, if , there exists only one positive root. Otherwise, if , there may exist either one or three positive real roots. If there is only one positive real root, it is equal to .

Obviously, the number of positive real roots of (2.15) depends on the sign of . If , (2.15) has only one positive real root. Otherwise, there may exist three positive roots.

Without loss of generality, we assume that (2.14) has three positive roots, defined by , , , respectively. Then (2.13) has three positive roots,
By (2.12), we have
Thus, if we denote
where ; , then are a pair of purely imaginary roots of (2.11) with . Define

Based on above analysis, we have the following result.

Lemma 2.2. *If and hold, then all roots of (1.2) have a negative real part when and (1.2) admits a pair of purely imaginary roots when ; . *

Let be a root of (2.11) near , and , and . Due to functional differential equation theory, for every ; , there exists such that is continuously differentiable in for . Substituting into the left hand side of (2.11) and taking derivative with respect to , we have We can easily obtain where . Thus, we have Since , , we can conclude that the sign of is determined by that of .

The analysis above leads to the following result.

Theorem 2.3. *Suppose that and , where is defined by (2.15). Then
**
and the of is consistent with that of .*

In the sequel, we assume that According to above analysis and the results of Kuang [25] and Hale [26], we have the following.

Theorem 2.4. *For , if and hold, then the positive equilibrium of system (1.2) is asymptotically stable for . In addition to the conditions and , we further assume that holds, then system (1.2) undergoes a Hopf bifurcation at the positive equilibrium when , ; .*

*Case 3. *, , (2.5) takes the form
For , be a root of (2.26), then it follows that
which is equivalent to
Let , then (2.13) takes the form
where , , . Denote
Let , where , . For (2.15), Similar analysis on the solutions of system (2.30) as that in Case 2. Here we omit it.

Without loss of generality, we assume that (2.30) has three positive roots, defined by , , , respectively. Then (2.29) has three positive roots
By (2.27), we have
Thus, if we denote
where ; , then are a pair of purely imaginary roots of (2.26) with . Define

The above analysis leads to the following result.

Lemma 2.5. *If and hold, then all roots of (1.2) have a negative real part when and (1.2) admits a pair of purely imaginary roots when ; . *

Let be a root of (2.26) near , and , and . Due to functional differential equation theory, for every , ; , there exists such that is continuously differentiable in for . Substituting into the left hand side of (2.26) and taking derivative with respect to , we have We can easily obtain where . Thus, we have Since , we can conclude that the sign of is determined by that of .

From the analysis above, we obtain the following result.

Theorem 2.6. *Suppose that and , where is defined by (2.30). Then
**
and the of is consistent with that of .*

In the sequel, we assume that Based on above analysis and in view of Kuang [25] and Hale [26], we get the following result.

Theorem 2.7. *For , if and hold, then the positive equilibrium of system (1.2) is asymptotically stable for . In addition to the condition and , one further assumes that holds, then system (1.2) undergoes a Hopf bifurcation at the positive equilibrium when , ; .*

*Case 4. *, . We consider (2.5) with in its stable interval. Regarding as a parameter. Without loss of generality, we consider system (1.2) under the assumptions and . Let be a root of (2.5), then we can obtain
where
Denote
Assume that
It is easy to check that if holds and . We can obtain that (2.42) has finite positive roots . For every fixed , , there exists a sequence , such that (2.42) holds. Let
When , (2.5) has a pair of purely imaginary roots for .

In the following, we assume that
Thus, by the general Hopf bifurcation theorem for FDEs in Hale [26], we have the following result on the stability and Hopf bifurcation in system (1.2).

Theorem 2.8. *For system (1.2), suppose , , , , and are satisfied, and , then the positive equilibrium is asymptotically stable when , and system (1.2) undergoes a Hopf bifurcation at the positive equilibrium when .*

*Case 5. *, . We consider (2.5) with in its stable interval. Regarding as a parameter. Without loss of generality, we consider system (1.2) under the assumptions and . Let be a root of (2.5), then we can obtain
where , , , and are defined by (2.41). Denote
Obviously, if holds and . We can obtain that (2.47) has finite positive roots . For every fixed , , there exists a sequence , such that (2.47) holds. Let
When , (2.5) has a pair of purely imaginary roots for .

In the following, we assume that
In view of the general Hopf bifurcation theorem for FDEs in Hale [26], we have the following result on the stability and Hopf bifurcation in system (1.2).

Theorem 2.9. *For system (1.2), assume that , , , , and are satisfied and , then the positive equilibrium is asymptotically stable when , and system (1.2) undergoes a Hopf bifurcation at the positive equilibrium when .*

#### 3. Computer Simulations

In this section, we present some numerical results of system (1.2) to verify the analytical predictions obtained in the previous section. Let us consider the following system: which has a positive equilibrium . We can easily obtain that are satisfied. When , using Matlab 7.0, we obtain , . The positive equilibrium is asymptotically stable for and unstable for which is shown in Figure 1. When , (3.1) undergoes a Hopf bifurcation at the positive equilibrium , that is, a small amplitude periodic solution occurs around when and is close to which is shown in Figure 2.

**(a)**

**(b)**

**(c)**

**(d)**

**(a)**

**(b)**

**(c)**

**(d)**

Let and choose as a parameter. We have , Then the positive equilibrium is asymptotically when . The Hopf bifurcation value of (3.1) is (see Figures 3 and 4).

**(a)**

**(b)**

**(c)**

**(d)**

**(a)**

**(b)**

**(c)**

**(d)**

When , using Matlab 7.0, we obtain , . The positive equilibrium is asymptotically stable for and unstable for which is shown in Figure 5. When , (3.1) undergoes a Hopf bifurcation at the positive equilibrium , that is, a small amplitude periodic solution occurs around when and is close to which is illustrated in Figure 6.

**(a)**

**(b)**

**(c)**

**(d)**

**(a)**

**(b)**

**(c)**

**(d)**

Let and choose as a parameter. We have . Then the positive equilibrium is asymptotically stable when . The Hopf bifurcation value of (3.1) is (see Figures 7 and 8).

**(a)**

**(b)**

**(c)**

**(d)**

**(a)**

**(b)**

**(c)**

**(d)**

#### 4. Conclusions

In this paper, we have investigated local stability of the positive equilibrium and local Hopf bifurcation of an ecoepidemiological model with two delays. It is shown that if some conditions hold true, and , then the positive equilibrium is asymptotically stable when , when the delay increases, the positive equilibrium loses its stability and a sequence of Hopf bifurcations occur at the positive equilibrium , that is, a family of periodic orbits bifurcates from the the positive equilibrium . We also showed if a certain condition is satisfied and , then the positive equilibrium is asymptotically stable when , when the delay increases, the positive equilibrium loses its stability and a sequence of Hopf bifurcations occur at the positive equilibrium . Some numerical simulations verifying our theoretical results is performed. In addition, we must point out that although Song et al. [20] have also investigated the the existence of Hopf bifurcation for system (1.2) with respect to positive equilibrium , it is assumed that . But what effect different time delay has on the dynamical behavior of system (1.2)? Song et al. [20] did not consider this issue. Thus we think that our work generalizes the known results of Song et al. [20]. In addition, we can investigate the Hopf bifurcation nature of system (1.2) by choosing the delay or as bifurcation parameter. We will further investigate the topic elsewhere in the near future.

#### Acknowledgments

This work is supported by National Natural Science Foundation of China (no. 11261010 and no. 11101126), Soft Science and Technology Program of Guizhou Province (no. 2011LKC2030), Natural Science and Technology Foundation of Guizhou Province (J2100), Governor Foundation of Guizhou Province (53), and Doctoral Foundation of Guizhou University of Finance and Economics (2010).

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#### Copyright

Copyright © 2012 Changjin Xu and Yusen Wu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.