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- Open Access
Control optimization and homoclinic bifurcation of a prey–predator model with ratio-dependent
- Zhenzhen Shi^{1},
- Jianmei Wang^{1},
- Qingjian Li^{2} and
- Huidong Cheng^{1}Email authorView ORCID ID profile
https://doi.org/10.1186/s13662-018-1933-z
© The Author(s) 2019
- Received: 29 July 2018
- Accepted: 13 December 2018
- Published: 3 January 2019
Abstract
In this paper, a predator–prey model with ratio-dependent and impulsive state feedback control is constructed, where the pest growth rate is related to an Allee effect. Firstly, the existence condition of the homoclinic cycle is obtained by analyzing the control parameter q. The existence, uniqueness and asymptotic stability of the periodic orbit are discussed by using the geometric theory of the differential equations, the method of successor functions and analog of the Poincaré criterion. Secondly, we formulate a control optimization with a minimal total cost in pest management, and we obtain an optimal economic threshold. Finally, we verify the main results by numerical simulation.
Keywords
- Semi-continuous dynamic systems
- Order one periodic orbit
- Homoclinic cycle
- Subsequence functions
- Optimization
MSC
- 34C25
- 34D20
- 92B05
- 34A37
1 Introduction
Differential equations can be applied extensively in many fields, including economic development, environmental protection, population ecology, infectious diseases and pharmacokinetics, and the cultivation of micro-organisms [1–6]. In [3], Cui et al. proposed the fractional differential equations and analyzed the uniqueness of solution for boundary value problems. Many researchers have obtained very good achievements in the field of stochastic differential equations [7–12].
In recent decades, many researchers have found that the occurrence of some biological phenomena and the optimal control of some life phenomena were not continuous processes, but the transient behavior of an impulse, so we should use impulsive differential equations to describe these phenomena. The theory of impulsive differential equations is proposed on the basis of continuous differential equation theory, which is difficult but valuable. It is widely applied in the harvest description [13–16], ecological resources [17–26], pest control [27–31] and epidemiologic control [32–36]. Many scholars investigated state-dependent pulse differential equations in predator–prey model to simulate the pest management including the periodic release of natural enemies [37–39] and the periodic release of natural enemies combined with periodic spraying of pesticides [40–46]. On the other hand, the bifurcation theory has been widely applied in the continuous dynamic system [47–49]. However, its application in impulsive dynamical system was little.
The organizational structure of this article is as follows. In Sect. 2, the existence of homocilinic cycle and existence, uniqueness and asymptotic stability of the periodic orbit of system (3) are proved. Furthermore, the optimization problem is formulated to reduce the total cost of pest control (i.e. the predator and the spraying agent). In Sect. 3, the numerical simulation of the concrete model is gradually carried out to verify the theoretical results. The final conclusion is drawn in Sect. 4.
2 Dynamical analysis of system (3)
2.1 Equilibria
Theorem 2.1
([51])
System (5) has five equilibria: origin \(O(0,0)\), two boundary equilibria: \(N_{1}(\alpha ,0)\) and \(N_{2}(1,0)\) and two positive equilibria: \(E_{1}(x_{1},y_{1})\) and \(E_{2}(x_{2},y _{2})\), where \(x_{1}=\frac{1+\alpha -\sqrt{(1-\alpha )^{2}-4\lambda (1-\frac{\beta }{\lambda _{1}})}}{2}\), \(y_{1}=\frac{(\lambda _{1}- \beta )x_{1}}{\beta }\), \(x_{2}=\frac{1+\alpha +\sqrt{(1-\alpha )^{2}-4 \lambda (1-\frac{\beta }{\lambda _{1}})}}{2}\), \(y_{2}=\frac{(\lambda _{1}-\beta )x_{2}}{\beta }\).
If \((H_{1})\) \(x(t_{0})=x_{0}<\alpha \), \(y(t_{0})=y_{0}>0\) holds, then the equilibrium \(O(0,0)\) of system (5) is stable.
If \((H_{2})\) \(\lambda _{1}>\beta \) holds, then the equilibrium \(N_{1}\) is an unstable node and \(N_{2}\) is a saddle point.
If \((H_{3})\) \(\lambda _{1}<\beta \) holds, then the equilibrium \(N_{1}\) is a saddle point and \(N_{2}\) is a stable node.
If \((H_{4})\) \(\lambda _{1}>\beta >\frac{\alpha _{1}}{4\lambda }[4\lambda -(1-\alpha )^{2}]>0\) holds, then equilibria \(E_{1}\) and \(E_{2}\) are positive equilibria.
2.2 Existence of the order one periodic orbit and homoclinic cycle of system (3)
By using the geometric theory of differential equations and the method of successor function, the existence of the order one periodic orbit and homoclinic cycle of system (3) is investigated in this section. For the actual biological significance, we always suppose that \(\max \{\frac{1+\alpha -\sqrt{(1-\alpha )^{2}-4\lambda (1-\frac{ \beta }{\lambda _{1}})}}{2},h_{1}\}<(1-p)h<h<\min \{\frac{1+\alpha +\sqrt{(1- \alpha )^{2}-4\lambda (1-\frac{\beta }{\lambda _{1}})}}{2},h_{2}\}\) and conditions \((H_{4})\), \((H_{5})\) hold in the paper.
Case I \(0< q\leq \delta \).
Theorem 2.2
Proof
The trajectory starting from the point \(A\in N\) hits the point \(B\in M\), then point \(B\in M\) jumps to point \(B^{+}\) after impulsive effect.
(a) Firstly, we prove the existence of the order one periodic orbit of system (3) in the case of \(q=\delta \). In this case, \(B^{+}\) coincides with point A, that is, \(\widehat{AB}\) and \(\overline{BA}\) constitute the order one periodic orbit.
(b) In this case, \(B^{+}\) is above A. According to the impulsive equations of system (3), there surely has a value \(q^{*} \in (0,\delta )\) satisfying \(\varphi (y_{B_{1}},q^{*})=(1-q^{*})y_{B _{1}}+\tau =y_{A_{2}}\), that is, point \(B_{1}\) jumps to point \(A_{2}\) after impulse effect, also, there surely has a value \(q^{0}\in (q^{*},\delta )\) satisfying \(\varphi (y_{B_{1}},q^{0})=(1-q ^{0})y_{B_{1}}+\tau =y_{A}\). For any \(q\in (q^{*},q^{0})\), point \(B_{1}\) jumps to point \(B_{1}^{+}\) after impulsive effect, we have \((1-q^{0})y_{B_{1}}+\tau =y_{A}<(1-q)y_{B_{1}}+\tau =y_{B_{1}^{+}}<(1-q ^{*})y_{B_{1}}+\tau =y_{A_{2}}\), that is, \(y_{A}< y_{B_{1}^{+}}< y_{A _{2}}\). There must exist a trajectory going through point \(B_{1}^{+}\) and intersecting the line \(x=h\) at point \(C_{1}\), and \(I(C_{1})=C_{1} ^{+}\). In view of the disjointness of any two trajectories and the vector field of system (3), we have \(y_{B_{1}}< y_{C_{1}}< y _{B}\), \((1-q)y_{B_{1}}+\tau =y_{B_{1}^{+}}<(1-q)y_{C_{1}}+\tau =y_{C _{1}^{+}}\), that is, \(y_{C_{1}^{+}}>y_{B_{1}^{+}}\). Thus the successor function [54, Definition 3.3] of \(B_{1}^{+}\): \(g(B_{1} ^{+})=y_{C_{1}^{+}}-y_{B_{1}^{+}}>0\).
Theorem 2.3
If conditions \((H_{4})\) and \((H_{5})\) and \(q=q^{*}\) hold, then system (3) admits an order one homoclinic cycle; if \(q\in (0,q^{*})\), system (3) does not admit an order one periodic orbit and the pests and natural enemy population will become extinct.
Proof
When \(q\in (0,q^{*})\), we have \((1-q)y_{B_{1}}+\tau =y_{B_{1}^{+}}>(1-q ^{*})y_{B_{1}}+\tau =y_{A_{2}}\) and the trajectory of system (3) starting from \(B_{1}^{+}\) will tend to origin when \(t\rightarrow +\infty \), and it has no impulse effect. Thus system (3) has no order one periodic orbit and the pest and natural enemy population will become extinct. This completes the proof. □
Case II \(\delta < q<1\).
Theorem 2.4
If the conditions \((H_{4})(H_{5})\) and \(\delta < q<1\) hold, then the system (3) admits the order one periodic orbit.
Proof
2.3 Stability of the order one periodic orbit of system (3)
Next, we study the stability of the order one periodic orbit of system (3).
Theorem 2.5
Proof
Let \(P(x,y)=x(x-\alpha )(1-x)-\frac{\lambda xy}{x+y}\), \(Q(x,y)=\frac{ \lambda _{1}xy}{x+y}-\beta y\), \(\varPhi (x,y)=-px\), \(\varPsi (x,y)=-qy+ \tau \), \(\varUpsilon (x,y)=x-h\).
Therefore \(\vert \mu _{2} \vert <1\). According to the analog of the Poincaré criterion [55, Theorem 2.3], we know that the order one periodic orbit of system (3) is orbitally asymptotically stable. This completes the proof. □
2.4 Determination of optimal pest economic threshold
In order to determine the optimum release amount of natural enemies and the optimal frequency of spraying chemical pesticide, we formulate the following optimization problem and find the optimal economic threshold.
Solving the objective function yields the optimum economic threshold \(h^{*}\), which results in the optimum release amount of the predator \(\tau ^{*}=\tau (h^{*})\), the optimal chemical control strength \(p^{*}=p(h^{*})\) and the optimal control period of chemical control \(T^{*}=T(\tau ^{*},p^{*})\). However, it is important to be noted that the optimal economic threshold \(h^{*}\) is dependent on the ratio of \(\omega =\frac{l_{2}}{l_{1}}\).
3 Simulations and optimization
In order to verify the theoretical results obtained in this paper, a specific example is presented in this section. Let \(\alpha =0.1\), \(\lambda =0.5\), \(\lambda _{1}=0.8\), \(\beta =0.55\), \(h_{1}=0.35\), \(h_{2}=0.76\), \(p _{\max }=0.3\), \(\tau _{\max }=0.1\), \(\tau _{\min }=0.01\), \(q_{\max }=0.8\). By a simple calculation, the saddle point and locally asymptotically stable point are \(E_{1}(0.3349,0.1522)\) and \(E_{0}(0.7651,0.3478)\), respectively. We carry out simulations by changing the main parameter h and fixing all other parameters. The control parameters p, q, τ are calculated by (4).
3.1 Numerical simulations
3.2 Optimum pest control level
4 Conclusion
In this paper, a predator–prey model with ratio-dependent and impulsive state feedback control is proposed. We take control measures that combine biological control and chemical control. The aim of this study is to get optimum pest control level and minimize the cost of pest control.
Based on the above analysis, both the predator and the prey are extinct when \(0< q< q^{*}\), that is, the order one periodic orbit does not exist when \(0< q< q^{*}\), system (3) admits an order one homoclinic cycle for \(q=q^{*}\); we obtain a unique and stable order one periodic orbit according to the geometric theory of the differential equations and the method of subsequence functions in the case of \(q^{*}< q<\delta \); and system (3) admits the order one periodic orbit in the case of \(\delta < q<1\).
In order to reduce the total cost of pest management, an optimization problem is formulated, and the optimal economic threshold and the minimum control cost are obtained. Finally, numerical simulation is carried out to verify the theoretical results.
Declarations
Funding
The paper was supported by the National Natural Science Foundation of China (No. 11371230), Shandong Provincial Natural Science Foundation, China (No. S2015SF002), SDUST Research Fund (2014TDJH102), and Joint Innovative Center for Safe and Effective Mining Technology and Equipment of Coal Resources, Shandong Province of China.
Authors’ contributions
All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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