On the Dynamics of a Nonautonomous Predator-Prey Model with Hassell-Varley Type Functional Response

and Applied Analysis 3 In this paper, letC denote all continuous functions on the real line. Given f ∈ C, we denote f M = sup t≥0 f (t) , f m = inf t≥0 f (t) . (4) If f is ω-periodic, then the average value of f on a time interval [0, ω] can be defined as


Introduction
In the natural world, no species can survive alone.While species spread the disease, compete with the other species for space or food, or are predated by other species, predator-prey relationship can be important in regulating the number of preys and predators.And the dynamic relationship between predators and their preys has long been and will continue to be one of the dominant themes in both ecology and mathematical ecology due to its universal existence and importance [1].Since the pioneering work of Hadeler and Freedman (1989) of describing a predator-prey model, where the prey is infected by a parasite and in turn infects the predator with the parasite [2], more and more mathematical models for predator-prey behavior are carried out in the following decades; see [3][4][5][6][7] and the references cited therein.
Migratory birds play an important role in the outburst of a new disease and the reintroduction of a disease to a place that was totally washed away from that place on various cases of infectious diseases [8].For example, the epidemic of Eastern Equine Encephalomyelitis (EEE) which broke in Jamaica in 1962 was suspected to result from transportation of the virus by birds from the continental United States [9,10].As another example, the West Nile Virus was introduced to the Middle East by migrating white storks.Therefore, to control and eradicate infectious diseases spread by the migratory birds has been the key issue in the world as well as in the study of mathematical epidemiology [11].
When investigating biological phenomena, there are many factors which affect dynamical properties of biological and mathematical models.One of the familiar nonlinear factors is functional response.There are many significant functional responses in order to model various different situations.As to predator-prey model, the phenomenon that predators have to share or compete for food is common.Therefore, most of the functional responses, which are assumed to depend on the prey numbers only in most models, are not realistic in the real situation and the predators functional response (i.e., the rate of prey consumption by an average predator) is one of the significant elements which have influence on the relationship between predator and prey [12,13].The three classical predator-dependent functions are Crowley-Martin type [14], Beddington-DeAngelis type by Beddington [15] and DeAngelis et al. [16], and Hassell-Varley type [17].A general predator-prey model with Hassell-Varley type functional response may take the following form: ) ,  ∈ (0, 1) ,  (0) > 0,  (0) > 0, where  is called Hassell-Varley constant.In a typical predator-prey interaction, where predators do not form groups, one can assume that  = 1, producing the so-called ratiodependent predator-prey dynamics.For terrestrial predators that form a fixed number of tight groups, it is often reasonable to assume that  = 1/2.For aquatic predators that form a fixed number of tights groups,  = 1/3 may be more appropriate [18].There are a lot of excellent works on predator-prey models with Hassell-Varley type functional response; for example, see [19][20][21][22] and the references therein.Motivated by these factors, a new nonautonomous predator-prey model with Hassell-Varley type functional response and the saturation incidence rate is proposed to give a more appropriate result and better understanding of the role of migratory birds in pathophoresis.Moreover, under quite weak assumptions, sufficient conditions for the permanence and extinction of the disease are established.In addition, the existence of globally attractive periodic solutions of the system is proposed by discussion and numerical simulation.
The rest of the paper is structured as follows.In the next section, we will introduce the new model.In Section 3, some useful lemmas for one-dimensional nonautonomous equation are proposed.And we establish the sufficient conditions on the permanence and extinction of the disease.Also, by constructing a Lyapunov function, we obtain the global attractivity of the model.Moreover, as applications of the main results, some corollaries are introduced.Particularly, the periodic model is discussed.In Section 4, numerical simulations that verify our qualitative results and a discussion which is about the new model (2) are given.The paper ends with a conclusion.

The Basic Mathematical Model
In this paper, we propose a predator-prey system, where the predator population  is assumed to be present in the system and the prey population  =  +  migrates into the system.Before we introduce the model, we would like to present a brief sketch of the construction of the model.This may indicate the biological relevance of it.
In the diseases like WNV and avian influenza, it was found that direct transmission in the bird-to-bird transmission of diseases is possible and birds get recovered from the disease, but the duration and variability of immunity among the WNV survivor are essentially unknown [5]; thus, it is reasonable to assume that all the recruitment in the bird population is in the susceptible class () and the infective prey population () is generated through infective of susceptible prey ().Also, as time passes, some of the prey population is recovered from the disease at a rate of () and goes to the susceptible class.Furthermore, ()/(1 + ()) is the saturation incidence, where () > 0 and () > 0 measure the force of infective (contact rate) and the force of the inhibition effect at time , respectively.
In the absence of the prey, it is assumed that there exists some alternative food source for the growth of the predator population.The predators eat the susceptible and infected prey with Hassell-Varley type functional response.The growth rate of the predator population is assumed to be () at time .As after the predation of the infective prey, either the infected predators die immediately and thus are removed from the system, or they are dead-end host of the disease like mammals (such as cats) in the case of WNV and in the transmission of many diseases from the migratory birds to their predators, such as highly pathogenic avian influenza (HPAI) virus (H5N1) [23,24].Then we assume that the disease is only spread among the prey population and the disease is not genetically inherited and also the predator becomes infected but the infection does not spread in the predator population.
The above considerations motivate us to introduce the nonautonomous model for the study of the migrating birds under the framework of the following set of nonlinear ordinary differential equations: Obviously, the set Ω = {(, , ) ∈  3 :  > 0,  > 0,  > 0} is a positively invariant set of system (2).
In this paper, let  denote all continuous functions on the real line.Given  ∈ , we denote If  is -periodic, then the average value of  on a time interval [0, ] can be defined as

Theorem 1. Suppose that assumptions (H1) and (H2) hold and there is a constant
where the constant  2 = [( +  3 )/]  is the upper bound of the prey population.Then the prey population () = ()+() and the predator population () are permanent.
On the other hand, from the first and second equations of (2), we get By Lemma 2.1 in [25] and the comparison theorem, there are constants 0 <  1 < 1 and  3 >  2 , such that Further, from the third equation of system (2), According to the comparison theorem, condition (7), and conclusion (a) of Lemma 1 (see [26]), there exist constants 0 <  2 < 1 and  4 >  3 such that Therefore, from ( 9)-( 15), we obtain that The proof is completed.
Remark 2. Suppose that assumptions (H1) and (H2) hold for system (2), and   > 0, (/)  > 0, then we can choose the constants given in the above theorem as follows: Let  0 () be some fixed solution of system and  0 () is a fixed solution of the following nonautonomous Logistic equation: where  0 =  1 +  2 .Then we have the following theorem.
Theorem 3. Suppose that assumptions (H1) and (H2) hold.If there is a constant  > 0, such that then the infective prey  is permanent.
As consequences of Theorems 3 and 4, we have the following corollaries.
Lastly, we will give the discussion on the global attractivity of model (2) as follows: Theorem 7. Suppose that assumptions (H1) and (H2) hold and  ∈ (0, 1) is a rational number.If there are constants and  1 ,  2 are the constants obtained in Theorem 1, then model (2) is globally attractive.

Numerical Simulation
In this section, we present some numerical simulations to substantiate and augment our analytical findings of system (2) by means of the software Matlab.
Finally, it is well known that the disease plays an important role in a predator-prey system.Anderson and May [28] pointed that invasion of a resident predator-prey system by a new strain of parasites could cause destabilization and exhibits limit cycle oscillation.Thus to keep the system stability we have to make the system disease free.In the following, we will perform some numerical simulations to show the importance of recovery rate  for controlling disease in an eco-epidemiological system.For system (2), we will discuss the effect of the mean value of recovery rate, , in the dynamics of the system.We choose () = 0.5 + 0.2 sin 2,  1 () = 0.18 + 0.1 cos , Λ() = 0.6 + 0.3 sin 2,  2 () = 0.5+ 0.1 sin , () = 0.6 + 0.2 sin , () = 0.5 + 0.4 sin , () = 0.1 + 0.08 sin , () = 0.2 + 0.01 sin , () = 0.8 + 0.1 sin , and the period  = 2.As  varies in [0, 9], we obtain the graph for the relation of the basic reproduction ratio to  (see Figure 5).This figure shows that increasing the amplitude of periodically recovery rate reduces the risk of epidemic prevalence and the recovery rate on the system is also an important parameter for controlling disease in an eco-epidemiological system.

Figure 1 :Figure 2 :
Figure 1: (a) The movement paths of , , and  as functions of time .The graph of the trajectory in (, , )-Space is shown in (b). * = 0.9898 < 1.The disease is extinct.

Figure 3 :
Figure 3: The graph of the upper threshold value  * versus .
Note that  is a rational number, which yields that there exist two mutually prime numbers  and  with  >  and  = /, such that