Permanence and Hopf bifurcation of a delayed eco-epidemic model with Leslie–Gower Holling type III functional response

Abstract This paper is concerned with a delayed eco-epidemic model with a Leslie–Gower Holling type III functional response. The main results are given in terms of permanence and Hopf bifurcation. First of all, sufficient conditions for permanence of the model are established. Directly afterward, sufficient conditions for local stability and existence of Hopf bifurcation are obtained by regarding the delay as bifurcation parameter. Finally, properties of the Hopf bifurcation are investigated with the aid of the normal form theory and centre manifold theorem. Numerical simulations are carried out to verify the obtained theoretical results.


Introduction
One of the dominant themes in both ecology and mathematical ecology is the dynamic relationship between predators and their preys due to its universal existence and importance in population dynamics (Beretta & Kuang, 1998;Berryman, 1992). As is known, the functional response is an important factor that can affect the dynamical properties of predator-prey systems. During the last decade, many works have been devoted to the study of the effects of a disease on predator-prey systems with different functional responses. In Morozov (2012), Zhang, Jiang, Liu, and Regan (2016), Morozov et al. studied a predator-prey system with disease in the prey and Holling type I functional response. In Hu and Li (2012), Hu and Li investigated the effect of time delay on a predator-prey system with disease in the prey and Holling type I functional response. In Saifuddin, Biswas, Samanta, Sarkar, and Chattopadhyay (2016), Upadhyay and Roy (2014), Sahoo (2015), Saifuddin et al. investigated the eco-epidemiological systems with disease in the prey and Holling type II functional response. Sahoo formulated a predator-prey epidemic model supplying alternative food to predator (Sahoo, 2016). Sahoo assumed that, the interaction between predator and susceptible prey is of Holling type II functional response and that between predator and infected prey is of Holling type I functional response. In Chakraborty, Das, Haldar, and Kar (2015), Chakraborty et al. proposed a predator-prey system with disease in prey and Holling CONTACT Zi Zhen Zhang zzzhaida@163.com type III functional response based on the work in Bhattacharyya and Mukhopadhyay (2011). Chakraborty et al. studied a ratio-dependent eco-epidemiological system where prey population is subjected to harvesting (Chakraborty, Pal, & Bairagi, 2010). Shi et al. considered an eco-epidemiological model with a stage structure and disease in prey (Shi, Cui, & Zhou, 2011). In fact, as stated in the literature (Zhang, Li, & Yan, 2008), in the predator-prey system, the disease can spread not only in prey but also can spread in predator. Therefore, besides considering the spread of disease in prey, we should also consider the spread of disease in the predator. Based on this consideration, Zhang et al. investigated a delayed predator-prey eco-epidemiological system with disease spreading in predator population and Holling type I functional response (Zhang et al., 2008). Wang et al. formulated a stage-structured predator-prey system with time delay and Holling type II functional response (Wang, Xu, & Feng, 2016). The above ecoepidemiological systems assumed that only the susceptible predator have the ability to capture the prey. As we know, group living is a widespread phenomenon in nature world for animals. Many animals exhibit social behaviour and cooperate with other members of their species in order to improve their skills in defence or hunting. Thus, Hilker et al. proposed an eco-epidemiological model of pack hunting predators that suffer disease infection (Hilker, Paliaga, & Venturino, 2017). Afterwards, Francomano, Hilker, Paliaga, and Venturino (2018) identified the tipping points of the proposed eco-epidemiological model in Hilker et al. (2017).
The Leslie-Gower (LG) formulation is based on the assumption that reduction in a predator population has a reciprocal relationship with per capita availability of its preferred food (Fan, Zhang, & Gao, 2016). This interesting formulation for the predator dynamics has been discussed widely by the scholars at home and abroad (Fan et al., 2016;Gakkhar & Singh, 2012;Nindjin, Tia, & Okou, 2018;Sharma & Samanta, 2015;Wei, Liu, & Zhang, 2018;Xu, Liu, & Yang, 2017;Zhang, Chen, Gao, Fan, & Wang, 2017), and it has been also used in the ecological literature. Recently, Sarwardi et al. proposed an LG Holling type II diseased predator ecosystem in Sarwardi, Haque, and Venturino (2011). However, Holling type II functional response usually suits the invertebral predators. For the vertebral predators, we have to use Holling type III functional response to describe the relationship between the predator and the prey. Based on this consideration, Shaikh, Das, and Ali (2018) investigated a predator-prey system with disease in predator and LG Holling type III functional response.
In general, delay differential equations exhibit much more complicated dynamics than ordinary differential equations since a time delay could cause the population to fluctuate (Wang et al., 2016). Time delays of one type or another have been incorporate into dynamical systems by many researchers (Bairagi & Adak, 2016;Ghosh, Samanta, Biswas, & Rana, 2016;Ghosh et al., 2017;Hu & Li, 2012;Huang, Li, & Cao, 2019;Huang, Nie, et al. 2019;Li, Wang, Li, Shen, & Lu, 2018;Shi et al., 2011;Wang et al., 2016;Wang, Xie, Lu, & Li, 2019;Xu & Zhang, 2013;Zhang & Wan, 2017;Zhang et al., 2008). Specially, it is well known that periodic solutions can arise through the Hopf bifurcation in delayed dynamical models. Thus, it is interesting to investigate the dynamics of eco-epidemiological systems with time delay. To this end, we study a delayed ecoepidemiological system by incorporating a time delay into system (1).
The rest of this paper is structured as follows. In Section 2, the delayed eco-epidemic model with LG Holling type III functional response is formulated. In Section 3, permanence of the proposed model is investigated. Sufficient conditions for local stability and existence of Hopf bifurcation are derived in Section 4. Section 5 deals with direction of the Hopf bifurcation, stability and period of the bifurcating periodic solutions. Computer simulations of the model are performed in Section 6. Section 7, which is the last section that contains the conclusions.

Model formulation
The proposed eco-epidemic model with LG Holling type III functional response in Shaikh et al. (2018) is as follows where x(t), y(t) and z(t) denote the densities of the prey, the susceptible predator and the infected predator at time t, respectively. a 1 , a 2 and a 3 are the intrinsic growth rate of the prey, the susceptible predator and the infected predator, respectively; b 1 is the intra-specific competition rate of the prey; c 1 is the predation rate of susceptible predator; c 2 and c 3 are the death rates due to intraspecific competition of the susceptible predator and the infected predator, respectively; θ is the disease incidence rate; k 1 is the half saturation constant of the prey; k 2 measures the alternative food of the susceptible predator and the infected predator; p is a constant that lies between 0 and 1. Due to crowding, dynamics of the susceptible predator and the infected predator can be delayed by their negative feedback delay. Time delay can cause a stable equilibrium to become unstable and cause the population to fluctuate, and it can cause interesting dynamics such as bifurcation and periodic solutions. Based on this consideration, we investigate the following delayed eco-epidemic model subject to the initial condition where τ is the negative feedback delay of the susceptible predator and the infected predator. It should be pointed out that we assume that the feedback delay of the susceptible predator is the same as that of the infected predator throughout the present paper.

Permanence
One can check that the system (2) has a positive solution with the positive initial condition given in (3). Now we show that the system (2) has permanence. Before proving, we first state the definition of permanence and a lemma which will be used to prove our statement given in Theorem 3.1.
Proof: With the positive initial condition (x(0), y(0), z(0)), it is easy to see that the solution (x(t), y(t), z(t)) of (2) is positive. From the first equation of (2), we can write Integrating both sides of (4) within the interval [0, t] and using the Lemma 3.1, we get where Second equation of (2), we can write dy dt ≤ a 2 y.
Integrating both sides of (6) within 't − τ ' to 't' we get By using Lemma 3.1, we can write following the previous steps, the third equation of (2) can be written as: and integrating within 't − τ ' to 't', we get Again from third equation of (2), we can write ∴ Using the Lemma 3.1, we get Again from the first equation of (2), we can write and using the Lemma 3.1 we get (15) Assume The second equation of (2) can be written as Integrating both sides of (16) within [t − τ , t], we get ∴ From the second equation of (2), we get Using the Lemma 3.1, we get where Again, from the third equation of (2) we get, Integrating both sides within [t − τ , t], we get Thus from the third equation of (2) we get, Hence Lemma 3.1 gives where (5) & (15), (9) & (20) and (13) with the positive initial condition Thus, we conclude that the system (2) has permanence.

Local stability and existence of Hopf bifurcation
Based on the analysis in Shaikh et al. (2018), we know that system (2) has coexistence equilibrium E * (x * , y * , z * ), where and x * is the positive root of the following equation where The Jacobi matrix of system (2) about the coexistence equilibrium E * (x * , y * , z * ) is given by The characteristic equation of that matrix (29) can be obtained as follows: In order to give the main results in this section, we make some assumptions as follows: where v 0 = ω 2 0 and the expression of ω 0 is in the following of this section.

Properties of Hopf bifurcation
For system (2), we have the following results.

Conclusions
In this paper, a delayed eco-epidemic model with LG Holling type III functional response is proposed by incorporating the negative feedback delay of the susceptible predator and the infected predator into the model formulated in the literature (Shaikh et al., 2018). Compared with the work by Shaikh et al. (2018), we mainly focus on the effect of the time delay on the model and the model in the present paper is more general.
It has been shown that, under some conditions, the system (2) is permanent. Moreover, we find that the negative feedback delay of the predator may destabilize the coexistence equilibrium of the eco-epidemiological system and cause the population to fluctuate if the given conditions are satisfied. Particularly, there is a threshold τ 0 for the time delay such that below it the coexistence equilibrium is locally asymptotically stable. In this case, the disease spreading among the predators can be controlled. However, if the delay is greater than the threshold τ 0 , a Hopf bifurcation arises. This implies that the disease spreading among the predators becomes periodically endemic and the disease among the predators is out of control. Furthermore, properties of the Hopf bifurcation such as direction and stability are investigated. Finally, we present some numerical simulations to support our theoretical results.
However, we only consider the existence of the local Hopf bifurcation of the system (2). We leave the existence of the global Hopf bifurcation for our next work. In addition, we neglect the negative feedback delay of the prey in the system (2). Therefore, we will investigate the Hopf bifurcation of the following system with multiple delays by considering the different combinations of the delays as the bifurcation parameter: dy(t) dt = y(t) a 2 − c 2 (y(t − τ 2 ) + z(t − τ 2 )) k 2 + x(t − τ 2 ) − θ z(t) , where τ 1 , τ 2 and τ 3 are the negative feedback delay of the prey, the susceptible predator and the infected predator, respectively.