Discrete Dynamics in Nature and Society

Volume 2018 (2018), Article ID 6519696, 9 pages

https://doi.org/10.1155/2018/6519696

## Dynamics and Patterns of a Diffusive Prey-Predator System with a Group Defense for Prey

Correspondence should be addressed to Xuebing Zhang

Received 11 September 2017; Accepted 26 November 2017; Published 8 January 2018

Academic Editor: Chris Goodrich

Copyright © 2018 Honglan Zhu and Xuebing Zhang. 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.

#### Abstract

We study a diffusive prey-predator system with a group defense for prey. Under Neumann boundary condition, we analyze local and stability of nonnegative constant steady states and the existence and nonexistence of nonconstant steady states. These results also exhibit the critical role of the system parameters leading to the formation of spatiotemporal patterns.

#### 1. Introduction

The predator-prey system first proposed by [1, 2] is one of the fundamental ecological systems in both ecology and mathematical ecology. Based on different settings, various types of predator-prey models described by differential systems have been proposed and the dynamics of these systems are studied [3–6]. The basic form of these models is as follows:where is the intrinsic growth rate and is the environmental carrying capacity of prey population, and the function is the functional response; the constant (>0) is the ratio of biomass conversion and is the natural death rate of predator species. The simplest functional response is Lotka-Volterra function which is described as which is also called Holling type I function. However, the curve defined by the Lotka-Volterra response function is a straight line through the origin and is unbounded. Thus, more reasonable response functions should be nonlinear and bounded. In 1913, Michaelis and Menten proposed the response functionwhere denotes the maximal growth rate of the species and is the half-saturation constant. It is now referred to as a Michaelis-Menten function or a Holling type II function. Another class of response function is which is called a sigmoidal response function, while the simplificationis known as a Holling type III function. Some authors [7, 8] considered system (1) with following response function:which is called Holling type IV function. Besides, Beddington-DeAngelis type and more complicated functional response are also considered by some researchers [9, 10].

Recently, some works consider the case when animals join together in herds in order to provide a self-defense from predators. In [11], the authors argued that it is more appropriate to model the response functions of prey that exhibit herd behavior in terms of the square root of the prey population. Inspired by this thought, the authors in [12] choose response function to reflect this fact. When motion is allowed, [13] considered the spatiotemporal behavior of a prey-predator system with a group defense for prey by means of extensive computer simulations. The proposed model is as follows:where and denote, respectively, the densities of prey and predator species. is the growth rate of prey species, is its carrying capacity, is the mortality rate of predator species, is the search efficiency of predator for prey, is the biomass conversion coefficient, and represents a kind of aggregation efficiency. The local dynamics for nonspatial model was studied, such as Hopf bifurcation and existence of extinction domain. For model (7), the authors only give some numerical simulations to find some spatiotemporal features. Reference [14] considers the direction and the stability of the bifurcating periodic solutions for model (7) with under Neumann boundary conditions. Reference [15] investigated the global dynamics of nonspatial model including the nonexistence of periodic orbits and the existence and uniqueness of limit cycles. We refer readers to [16–21] as some other related works on predator-prey model with herd behavior.

It is noted that up to now no one has studied the existence and nonexistence of positive steady state solutions of (7). Therefore, the main aim of this article is to study the existence and nonexistence of nonconstant positive solutions of the following elliptic system:where is the outward unit normal vector on , and we impose a homogeneous Neumann type boundary condition, which implies that (8) is a closed system and has no flux across the boundary .

The structure of this paper is arranged as follows. In Section 2, we estimate the a priori bounds of positive solutions of (7). In Section 3, the local and global stabilities of nonnegative constant steady states of (7) are discussed. In Section 4, we give a priori estimate for the positive solutions of (8) by using maximum principle and Harnack inequality. In Section 5, we give a nonexistence result of nonconstant solutions of (8). In Section 6, we consider the existence of nonconstant positive solutions of (8). Finally, to support our theoretical predictions, some numerical simulations are given.

#### 2. Basic Dynamics and a Priori Bound

Theorem 1. *For system (7), one has the following.*(a)*If , , then system (7) has a unique solution such that , for and .*(b)*Any solution of (7) satisfies *

*Proof. *(a) Define Then and in . Hence, (7) is a mixed qusi-monotone system. Consider following system:Assume , are the unique solution to system (11). Let Obviously, and are a pair of lower-solution and upper-solution to system (7). Therefore, according to the Theorem in [22] or Theorem in [23], system (7) has a unique globally defined solution which satisfies The strong maximum principle implies that when for all .

(b) By the first equation of (7), we easily obtain the fact that in ; the first result follows easily from the simple comparison argument for parabolic problems, and thus there exists such that in for an arbitrary constant .

For the estimate of , let , ; thenMultiplying (14) by and adding it to (15), we have Integration of the inequality leads to

*3. Stability of the Nonnegative Constant Steady States of (7)*

*In this section, we will analyze the stability of nonnegative constant steady states of (7). By the direct computation, we see that the possible nonnegative constant steady states of (7) are where . Obviously, the positive constant steady state exists if holds.*

*Notation 1. *Let be the eigenvalues of on under homogeneous Neumann boundary condition. We define the following space decomposition:(i) is the space of eigenfunctions corresponding to for .(ii), where are orthonormal basis of for .(iii), and so , where .

*Let be a nonnegative constant steady state of (7); then the linearization of (7) at a constant solution can be expressed bywhere , , and In view of Notation 1, we can induce the eigenvalues of system (19) confined on the subspace . If is an eigenvalue of (19) on , it must be an eigenvalue of the matrix for each . It is easy to see that satisfies the characteristic equation*

*Theorem 2. (i) The trivial equilibrium is unstable.(ii) If , then is globally asymptotically stable.(iii) If , then is locally asymptotically stable.*

*Proof. *(i) For , the eigenvalues are Obviously, is unstable.

(ii) For , the eigenvalues are If , then and are all negative. Therefore is locally asymptotically stable. Indeed, is globally asymptotically stable.

On account of Theorem 1, we have , and thus there exists such that, for an arbitrary constant , It follows from the second equation of (7) that Therefore, , and there exists such that It follows from the first equation of (7) that On account of and the arbitrariness of , we have . This combined with allows us to derive Hence, is globally asymptotically stable when .

(iii) When exists, the corresponding characteristic equation is as follows:Obviously, we have If , then and . Hence, all the roots of (29) have negative real part which means that is locally asymptotically stable when .

*4. The Prior Estimate*

*In this section, we will give some a priori estimates of positive solutions to (8). Firstly, we give two known lemmas.*

*Lemma 3 (Harnack inequality (cf. [24])). Let be a positive classical solution to Then there exists a positive constant such that *

*Lemma 4 (maximum principle (cf. [25])). Suppose that .(i) Assume that satisfies If , then .(ii) Assume that satisfies If , then .*

*Lemma 5. For any positive solution of system (8), *

*Proof. *Form Lemma 4, and from the strong maximum principle for all . Multiplying the first equation of (8) by and adding it to the second equation, we have Then the maximum principle implies that Hence, .

*In the following, we estimate the positive lower bound of positive solution of (8).*

*Theorem 6. Let be a bounded smooth domain in . There exist two positive constants depending possibly on , , , , , , and , such that such that any positive solution of system (8) satisfies *

*Proof. *From Lemma 5, we obtain where depends on , , , , , and .

From Lemma 3, we obtain the fact that there exists a positive constant such thatOn the contrary, suppose the result is false. Then there exists a sequence of positive solutions to system (8) such thatBy the regularity theory for elliptic equations, there exists a subsequence of , which will be denoted again by , such that in as . Observe that and, from (41), either or . Therefore, we have the following two cases:

(i) , ; or , .

(ii) , .

Since is a positive solution of (8), one can obtain the following integral equation by integrating (8) for and over : (i) In this case, ; then uniformly as and ; then for sufficiently large , we have which is a contradiction.

(ii) If , , then this implies that satisfies (8). So for large . Thus for large since , which derives a contradiction again to the second integral equation of (42). This completes the proof.

*5. Nonexistence of Nonconstant Positive Steady States*

*In this section, we can show the nonexistence of nonconstant positive solutions to system (8) when the diffusion coefficients and are large.*

*Theorem 7. There exists a positive constant such that elliptic problem (8) has no nonconstant positive solution if .*

*Proof. *Suppose that is a nonconstant positive solution of system (8). Denote , . Then Define for . Indeed, we can prove that and . In fact, notice Let and we have which implies that for . Therefore, we obtain the fact that .

Furthermore, multiplying the first equation of (8) by , adding it to the second equation, and integrating over , we getand then the Neumann boundary conditions lead to ThusMultiplying the first equation in (8) by , we haveMultiplying the second equation in (8) by , we haveFrom (54) and (55) and the Poincaré inequality, we obtain where Hence, if then and must be a constant solution.

*6. Existence of Nonconstant Positive Steady States*

*In this subsection, we discuss the existence of nonconstant positive solutions to system (8) when the diffusion coefficients and vary while the parameters , , , , , and are fixed by using the Leray-Schauder degree theory. Throughout this section, we assume that the positive constant steady state exists.*

*For simplicity, denote and Thus, (8) can be written asand, obviously, is a positive solution of (61) if and only ifwhere is the inverse of with the homogeneous Neumann boundary condition. As is a compact perturbation of the identity operator, the Leray-Schauder degree is well-defined from Theorem 6. By direct computation, we have If is invertible, the index of is defined as where is the number of negative eigenvalues of . Note that is an eigenvalue of on if and only if it is an eigenvalue of the matrix Thus is invertible if and only if, for all , the matrix is nonsingular. Writingwe have that if , then if and only if the number of negative eigenvalues of in is odd. The following lemma gives the explicit formula of calculating the index.*

*Lemma 8. If for all , then where is the algebraic multiplicity of .*

*To facilitate our computation of , we only need consider the sign of . The direct calculation givesObviously, nonnegative roots of (68) exist if and only if and . Assume that and are the two roots of (68), we have the following conclusion.*

*Theorem 9. Assuming that , and there exist , such that and is odd, then (8) has at least one nonconstant positive solution.*

*Proof. *For , we define where is defined in Theorem 7.

The positive solutions of the problemare contained in . Note that is a positive solution of system (8) if and only if it is a positive solution of (71) with . is the unique positive constant solution of (71) for any . According to the choice of in Theorem 7, we have which is the only fixed point of .Since and if (8) has no other solutions except the constant one , then we have On the other hand, by the homotopy invariance of the topological degree,which is a contradiction. Therefore, there exists at least one nonconstant solution of (8).

*7. Numerical Simulation*

*7.1. Global Stability of Equilibrium *

*Consider system (7) with following parameters: , and . According to the discussions in Section 3, the steady state is globally asymptotically stable; see Figure 1.*