Dynamics of a Predator-Prey System Concerning Biological and Chemical Controls
© H. K. Kim and H. Baek. 2010
Received: 25 August 2010
Accepted: 13 November 2010
Published: 29 November 2010
We investigate an impulsive predator-prey system with Monod-Haldane type functional response and control strategies, especially, biological and chemical controls. Conditions for the stability of the prey-free positive periodic solution and for the permanence of the system are established via the Floquet theory and comparison theorem. Numerical examples are also illustrated to substantiate mathematical results and to show that the system could give birth to various kinds of dynamical behaviors including periodic doubling, and chaotic attractor. Finally, in discussion section, we consider the dynamic behaviors of the system when the growth rate of the prey varies according to seasonal effects.
In recent years controlling insects and other arthropods has become an increasingly complex issue. There are many ways that can be used to help control the population of insect pests. Integrated Pest Management (IPM) is a pest control strategy that uses an array of complementary methods: natural predators and parasites, pest-resistant varieties, cultural practices, biological controls, various physical techniques, and the strategic use of pesticides.
Chemical control is one of simple methods for pest control. Pesticides are often useful because they quickly kill a significant portion of a pest population. However, there are many deleterious effects associated with the use of chemicals that need to be reduced or eliminated. These include human illness associated with pesticide applications, insect resistance to insecticides, contamination of soil and water, and diminution of biodiversity. As a result, it is required that we should combine pesticide efficacy tests with other ways of control. Another important way to control pest populations is biological control. It is defined as the reduction of pest populations by natural enemies and typically involves an active human role. Natural enemies of insect pests, also known as biological control agents, include predators, parasites, and pathogens. Virtually all pests have some natural enemies, and the key to successful pest control is to identify the pest and its natural enemies and release them at fixed times for pest control. Biological control can be an important component of Integrated Pest Management (IPM) programs. Such different pest control tactics should work together rather than against each other to accomplish an IPM program successfully [1, 2].
where the parameters and are the periods of the impulsive immigration or stock of the predator, present the fraction of the prey which dies due to the harvesting or pesticides and so forth, and is the size of immigration or stock of the predator.
In fact, impulsive control methods can be found in almost every field of applied sciences. The theoretical investigation and its application analysis can be found in Bainov and Simeonov , Lakshmikantham et al. . Moreover, the impulsive differential equations dealing with biological population dynamics are literate in [16–21]. In particular, Zhang et al.  studied the system (1.3) without chemical control. That is, . They investigated the abundance of complex dynamics of the system (1.3) theoretically and numerically.
The main purpose of this paper is to investigate the dynamics of the system (1.3). In Section 3, we study qualitative properties of the system (1.3). In fact, we show the local stability of the prey-free periodic solution under some conditions and give a sufficient condition for the permanence of the system (1.3) by applying the Floquet theory. In Section 4 we numerically investigate the system (1.3) to figure out the influences of impulsive perturbations on inherent oscillation. Finally, in Section 5, we consider the dynamic behaviors of the system when the growth rate of the prey varies according to seasonal effects.
2. Basic Definitions and Lemmas
Before stating our main results, firstly, we give some notations, definitions and lemmas which will be useful for our main results.
It is from  that the smoothness properties of guarantee the global existence and uniqueness of solutions to the system (1.3).
Lemma 2.2 (see ).
Similar result can be obtained when all conditions of the inequalities in the Lemma 2.2 are reversed. Note that if we have some smoothness conditions of to guarantee the existence and uniqueness of the solutions for (2.4), then is exactly the unique solution of (2.4).
From Lemma 2.2, it is easily proven that the following lemma holds.
Then we introduce the following conditions.
By equality (2.6) there corresponds to the fundamental matrix and the constant matrix which we call the monodromy matrix of (2.5) (corresponding to the fundamental matrix of ). All monodromy matrices of (2.5) are similar and have the same eigenvalues. The eigenvalues of the monodromy matrices are called the Floquet multipliers of (2.5).
Lemma 2.4 (Floquet theory ).
Let conditions (H1)–(H3) hold. Then the linear -periodic impulsive equation (2.5) is
3. Mathematical Analysis
In this section, we have focused on two main subjects, one is about the extinction of the prey and the other is about the coexistence of the prey and the predator. For the extinction, we have found out a condition that the population of the prey goes to zero as time goes by via the study of the stability of a prey-free periodic solution. For example, if the prey is regarded as a pest, it is important to figure out when the population of the prey dies out. For the reason, it is necessary to consider the stability of the prey-free periodic solution. On the other hand, for the coexistence, we have investigated that the populations of the prey and the predator become positive and finite under certain conditions.
3.1. Stability for a Prey-Free Periodic Solution
is the solution of (3.1). From (3.2) and (3.3), the following results can be easily obtained without the proof.
For every solution and every positive periodic solution of the system (3.1), it follows that tend to as . Thus, the complete expression for the prey-free periodic solution of the system (1.3) is obtained .
It follows from Theorem 3.2 that the population of the prey could be controlled by using chemical or biological control parameters, , , if the other parameters are fixed. (2) Figure 2 illustrates this phenomenon.
It might be difficult to find out a necessary condition for the stability of the prey-free periodic solution . Due to this fact, it is natural to have a question what a condition that makes all species coexist is. Before answering the question, first of all, we introduce a definition which keeps the concept of coexistence of the prey and the predator.
From a biological point of view, the populations of the prey and the predator in the system (1.3) cannot increase up to infinity due to restriction of resources. To show this phenomenon for the system (1.3) mathematically, we prove that all solutions to the system (1.3) are uniformly ultimately bounded in the next proposition.
Thanks to Proposition 3.5, we have only to prove the existence of a positive lower bound for the populations of the prey and the predator to justify the system is permanent.
Suppose is any solution of the system (1.3) with . From Proposition 3.5, we may assume that , , and . Let , . So, it is easily induced from Lemma 3.1 that for all large enough. Now we shall find an such that for all large enough. We will do this in the following two steps.
4. Numerical Analysis on Impulsive Perturbations
It is from  that the system (1.3) with and has an unique limit cycle. Moreover, Figure 1 shows that the phase portrait of the system (1.3) with , , and has a limit cycle too. From Theorem 3.2, we know that the prey-free periodic solution is locally asymptotically stable provided that . A typical prey-free periodic solution of the system (1.3) is shown in Figure 2, where we observe how the variable oscillates in a stable cycle while the prey rapidly decreases to zero. On the other hand, if the amount of releasing species is smaller than , then the prey and the predator can coexist on a stable positive periodic solution (see Figure 1) and the system (1.3) can be permanent, which follows from Theorem 3.6.
In this paper, we have studied the effects of control strategies on a predator-prey system with Monod-Haldane type functional response. Conditions for the system to be extinct are given by using the Floquet theory of impulsive differential equation and small amplitude perturbation skills. Also, it is proved that the system the system (1.3) is permanent via the comparison theorem. Moreover, numerical examples on impulsive perturbations have been illustrated to substantiate our mathematical results and to show that the system we have considered in this paper gives birth to various kinds of dynamical behaviors.
And then we get the following results via similar methods used in the previous sections.
Thus, the seasonal effect on the prey may have also deeply influences on dynamics of the system (1.3).
The first author is supported by Catholic University of Daegu Research Grant. The second author was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0004426).
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