Research Article | Open Access

# Hopf Bifurcation of a Predator-Prey System with Delays and Stage Structure for the Prey

**Academic Editor:**M. De la Sen

#### Abstract

This paper is concerned with a Holling type III predator-prey system with stage structure for the prey population and two time delays. The main result is given in terms of local stability and bifurcation. By choosing the time delay as a bifurcation parameter, sufficient conditions for the local stability of the positive equilibrium and the existence of periodic solutions via Hopf bifurcation with respect to both delays are obtained. In particular, explicit formulas that can determine the direction of the Hopf bifurcation and the stability of the bifurcating periodic solutions are established by using the normal form method and center manifold theorem. Finally, numerical simulations supporting the theoretical analysis are also included.

#### 1. Introduction

Predator-prey dynamics continues to draw interest from both applied mathematicians and ecologists due to its universal existence and importance. Many kinds of predator-prey models have been studied extensively [1â€“6]. It is well known that there are many species whose individual members have a life history that takes them through immature stage and mature stage. To analyze the effect of a stage structure for the predator or the prey on the dynamics of a predator-prey system, many scholars have investigated predator-prey systems with stage structure in the last two decades [7â€“15]. In [7], Wang considered the following predator-prey system with stage structure for the predator and obtained the sufficient conditions for the global stability of a coexistence equilibrium of the system: where represents the density of the prey at time . and represent the densities of the immature predator and the mature predator at time , respectively. For the meanings of all the parameters in system (1.1), one can refer to [7]. Considering the gestation time of the mature predator, Xu [8] incorporated the time delay due to the gestation of the mature predator into system (1.1) and considered the effect of the time delay on the dynamics of system (1.1).

There has also been a significant body of work on the predator-prey system with stage structure for the prey. In [12], Xu considered a delayed predator-prey system with a stage structure for the prey: where and denote the population densities of the immature prey and the mature prey at time , respectively. denotes the population density of the predator at time . All the parameters in system (1.2) are assumed positive. is the birth rate of the immature prey. is the transformation rate from immature individual to mature individuals. is the intraspecific competition coefficient of the mature prey. and are the death rates of the immature and the mature prey, respectively. is the death rate of the predator. and are the interspecific interaction coefficients between the mature prey and the predator, respectively. is the response function of the predator. And is a constant delay due to the gestation of the predator. In [12], Xu investigated the persistence of system (1.2) by means of the persistence theory on infinite dimensional systems, and sufficient conditions are obtained for the global stability of nonnegative equilibrium of the model by constructing appropriate Lyapunov function. But studies on the predator-prey system not only involve the persistence and stability, but also involve many other behaviors such as periodic phenomenon, attractivity, and bifurcation [16â€“19]. In particular, the properties of periodic solutions are of great interest [20â€“24]. Therefore, F. Li and H. W. Li [14] considered the property of periodic solutions of the following system: Motivated by the work of Xu [12] and F. Li and H. W. Li [14] and considering the intraspecific competition of the immature prey population, we consider the following system: where and denote the population densities of the immature prey and the mature prey at time , respectively. denotes the population density of the predator at time . The parameters , , , , , , , , and are defined as in system (1.3). is the intraspecific competition of the immature prey, is the feedback delay of the mature prey, and is the time delay due to the gestation of the predator.

The organization of this paper is as follows. In Section 2, by analyzing the corresponding characteristic equations, the local stability of the positive equilibrium of system (1.4) is discussed, and the existence of Hopf bifurcation at the positive equilibrium is established. In Section 3, we determine the direction of Hopf bifurcation and the stability of bifurcating periodic solutions by using the normal form theory and center manifold theorem in [20]. And numerical simulations are carried out in Section 4 to illustrate the main theoretical results. Finally, main conclusions are included.

#### 2. Local Stability and Hopf Bifurcation

From the viewpoint of biology, we are only interested in the positive equilibrium of system (1.4). It is not difficult to verify that system (1.4) has a positive equilibrium , where if the following conditions hold: , .

Let , , , and we still denote , , and by , , and . Then system (1.4) can be transformed to the following form: where Then we can get the linearized system of system (2.2) Therefore, the corresponding characteristic equation of system (2.4) is where , , , , , , , , , .

Next, we consider the local stability of the positive equilibrium and the Hopf bifurcation of system (1.4) for the different combination of and .

*Case 1. *. The characteristic equation (2.5) becomes
where , , .

It is not difficult to verify that and . Thus, all the roots of (2.6) must have negative real parts, if the following condition holds: . Namely, the positive equilibrium is locally stable in the absence of time delay, if holds.

*Case 2. *. On substituting , (2.5) becomes
where , , , , , .

Let be a root of (2.7). Then, we have
Squaring both sides and adding them up, we get the following sixth-degree polynomial equation:
where , , .

Let , then (2.9) becomes
Define
If , it is easy to know that (2.10) has at least one positive root. On the other hand, if , according to Lemma 2.2 in [25], (2.10) has positive roots if and , hold. Therefore, we give the following assumption.

: equation (2.10) has at least one positive root.

Without loss of generality, we assume that it has three positive roots which are denoted as , , and . Thus, (2.9) has three positive roots , . The corresponding critical value of time delay is where , , , , , .

Let , , .

To verify the transversality condition of Hopf bifurcation, differentiating the two sides of (2.7) with respect to , and noticing that is a function of , we can obtain Thus, Therefore, From (2.9), we can get Then, we have where .

Therefore, if holds. Notice that and have the same sign. Then we have if holds. In conclusion, we have the following results.

Theorem 2.1. * Suppose that the conditions and hold. The positive equilibrium of system (1.4) is asymptotically stable for and unstable when . Further, system (1.4) undergoes a Hopf bifurcation when . *

*Case 3. *. On substituting , (2.5) becomes
where , , , , , .

Let be a root of (2.18). Then, we get
which follows that
where , , .

Let , then (2.20) becomes
Define
Similar as in case (2), we give the following assumption.

: equation (2.21) has at least one positive root.

Without loss of generality, we assume that it has three positive roots denoted by , , and . Thus, (2.20) has three positive roots .

The corresponding critical value of time delay is where , , , , , .

Let , , .

Similar as in case (1), next, we suppose that the condition holds, where . Then we have . By the above analysis, we have the following results.

Theorem 2.2. *Suppose that the conditions and hold. The positive equilibrium of system (1.4) is asymptotically stable for and unstable when . Further, system (1.4) undergoes a Hopf bifurcation when . *

*Case 4. *.

For , (2.5) can be rewritten in the following form:
where , , , , , , , .

Multiplying on both sides of (2.24), it is obvious to get
Let be the root of (2.25). Then, we have
where , , , , , .

It follows that
where , , , , , , , , .

From (2.27), we can get
where , , , , , .

Let , then (2.28) becomes
Suppose that (2.29) has at least one positive root, and, without loss of generality, we assume that it has six positive roots which are denoted as , , , , , and . Then, (2.28) has six positive roots .

The corresponding critical value of time delay is
Let , , .

Next, we verify the transversality condition. Differentiating (2.25) regarding and substituting , we get
where
Thus, if the condition holds, the transversality condition is satisfied.

Theorem 2.3. * Suppose that the conditions and hold. The positive equilibrium of system (1.4) is asymptotically stable for and unstable when . Further, system (1.4) undergoes a Hopf bifurcation when . *

*Case 5. *.

We consider (2.5) with in its stable interval, and is considered as a parameter.

Let be the root of (2.5). Then, we have
where
From (2.33), we can get the following transcendental equation:
where , , , , , , , , .

In order to give the main results, we suppose that (2.35) has finite positive root. We denote the positive roots of (2.35) as . For every , the corresponding critical value of time delay is
Let , and is the corresponding root of (2.35) with .

In the following, we differentiate the two sides of (2.5) with respect to to verify the transversality condition.

Taking the derivative of with respect to in (2.5) and substituting , we get
where
Obviously, if the condition holds, the transversality condition is satisfied. Through the above analysis, we have the following results.

Theorem 2.4. *Suppose that the conditions and hold and . The positive equilibrium of system (1.4) is asymptotically stable for and unstable when . Further, system (1.4) undergoes a Hopf bifurcation when . *

*Case 6. *.

We consider (2.5) with in its stable interval, and is considered as a parameter.

Substitute into (2.5). Then, we get
where , , , , , , , , .

Similar as in case (5), we give the following assumption. : (2.39) has finite positive root.

The positive roots of (2.39) are denoted as , ,,. For every , the corresponding critical value of time delay is
where

Let , and is the corresponding root of (2.39) with .

Then, we suppose that holds. By the general Hopf bifurcation theorem for FDEs in Hale [26], we have the following results.

Theorem 2.5. *Suppose that the conditions and hold and . The positive equilibrium of system (1.4) is asymptotically stable for and unstable when . Further, system (1.4) undergoes a Hopf bifurcation at when .*

#### 3. Direction and Stability of Bifurcated Periodic Solutions

In Section 2, we have obtained the conditions under which a family of periodic solutions bifurcate from the positive equilibrium of system (1.4) when the delay crosses through the critical value. In this section, we will determine the direction of Hopf bifurcation and stability of bifurcating periodic solutions of system (1.4) with respect to for by using the normal form method and center manifold theorem introduced by Hassard et al. [20]. It is considered that system (1.4) undergoes Hopf bifurcation at . Without loss of generality, we assume that , where .

Let , , , , . We still denote by . Then, system (1.4) can be transformed into the following system: where and , are given, respectively, by where Hence, by the Riesz representation theorem, there exists a matrix function whose elements are of bounded variation such that In fact, we choose For , we define and Then, system (3.1) can be transformed into the following operator equation: For , where is the 3-dimensional space of row vectors, we define the adjoint operator of : For and , we define a bilinear inner product: where .

By the discussion in Section 2, we know that are eigenvalues of . Thus, they are also eigenvalues of .

Suppose that is the eigenvector of corresponding to and is the eigenvector of corresponding to . By direction computation, we can get