# Theoretical Studies on the  Effects of Dispersal Corridors on the Permanence of Discrete Predator-Prey Models in Patchy Environment

Authors: Chunqing Wu; Shengming Fan; Patricia J. Y. Wong
Journal: Abstract and Applied Analysis
 (2014)

---

## Abstract

We study two discrete predator-prey models in patchy environment, one without dispersal corridors and one with dispersal corridors. Dispersal corridors are passes that allow the migration of species from one patch to another and their existence may influence the permanence of the model. We will offer sufficient conditions to guarantee the permanence of the two predator-prey models. By comparing the two permanence criteria, we discuss the effects of dispersal corridors on the permanence of the predator-prey model. It is found that the dispersion of the prey from one patch to another is helpful to the permanence of the prey if the population growth of the prey is density dependent; however, this dispersion of the prey could be disadvantageous or advantageous to the permanence of the predator. Five numerical examples are presented to confirm the theoretical results obtained and to illustrate the effects of dispersal corridors on the permanence of the predator-prey model.


---

## Body

## 1. Introduction

The conception ofpermanence in ecological communities is first discussed in [1]. This notion of permanence is also known under different aliases, such as permanent coexistence or uniform persistence. Aptly termed,permanence of a population model reflects that all the species in the ecosystem will coexist for a relatively long time; that is, no extinction of any species will occur in the near future—this is very important in biological conservation. A permanent model may attain globally asymptotically stable positive equilibrium [2] or may have periodic solution [3, 4], and even a chaotic model can be permanent [5]. The sufficient and/or necessary conditions established to guarantee the permanence of various population models may offer helpful suggestions for environmental managers to take applicable measures for the sustainability of the ecosystems [6–8].It is common that in the real world today the habitats of the species are separated into isolated patches due to spatial heterogeneity or the development of mankind, such as the construction of highways or railways [9–16]. This patchy environment may restrict the activities of the species and lead to adverse effects on the permanence of the ecological communities; therefore, somedispersal corridors may be built to allow movement between the patches. For example, the construction of Qinghai-Tibet railway in China separates the habitats of the Tibetan antelopes to pieces of small patches. In order to reduce the impact of patchy environment on the Tibetan antelopes, several underpasses (dispersal corridors) along the railway are constructed for the Tibetan antelopes to move from one side of the railway to the other side [17]. Indeed some researches [18] have verified that the Tibetan antelopes have really made use of these underpasses to cross the railway.In the literature there are quite many papers that discuss the permanence ofcontinuous time predator-prey or competitive systems in patchy environment: for example, one can refer to [19–23] and the many references cited therein. Though some continuous time models have their discrete time analogues, it is common knowledge that these discrete time analogues may present different dynamics, which are possibly more complicated [24–26]. To our best knowledge, there are few references ondiscrete time predator-prey or competitive models in patchy environment; see, for instance, [22, 27–30]. In [28, 30], the effects of dispersion on discrete models for single species in patchy environment have been discussed; two patches have been considered in [30] while n patches have been tackled in [28]. In [27, 29], the effects of dispersion on discrete competitive models in patchy environment have been analyzed; in particular discrete Leslie-Gower type competitive model has been adopted in [27]. Motivated by the above papers as well as the importance of permanence in ecosystems, in this paper we will investigate the permanence ofdiscrete timepredator-prey models ofRicker type in patchy environment. To be specific, we will tackle two discrete predator-prey models in patchy environment; the first model has no dispersal corridors while the second model has dispersal corridors. Sufficient conditions to guarantee permanence will be established for both models. Theeffects of dispersal corridors on permanence are then analyzed by comparing the two sets of sufficient conditions. We remark that in the literature the numerous research papers on continuous/discrete predator-prey models in patchy environment have focused mainly on the permanence criteria and/or the existence of periodic solutions of the models; however, the effects of dispersal corridors on the permanence of the models are rarely studied until the present paper.The paper is organized as follows. In Section2, we construct the two discrete predator-prey models in patchy environment with/without dispersal corridors; also in this section we give some preliminaries such as the definition of permanence. In Section 3, sufficient conditions for the permanence of the two discrete predator-prey models in patchy environment with/without dispersal corridors are offered. The effects of dispersal corridors on the permanence of the models are analyzed in Section 4. In Section 5, we present five examples where numerical simulations are performed to confirm the theoretical results and to illustrate the effects of dispersal corridors. Finally, we conclude in Section 6. The mathematical proofs of the theorems in Section 3 are presented in Appendix.


## 2. Models and Preliminaries

There are many types of discrete time mathematical models, which are governed by difference equations, established to reflect the fluctuation of the single-species population [31]. Among them, the Ricker type model, given as
(1)
x
(
n
+
1
)
=
x
(
n
)
exp
⁡
⁡
[
r
-
a
x
(
n
)
]
,
is first established for some kind of fishery population [32] and subsequently used to model other kinds of single-species population [31, 33, 34]. In model (1), x
(
n
) is the population density of the species x at the nth time step, r is the intrinsic growth rate of x, a is the strength of intraspecific competition of x, and r, a are positive constants. Model (1) islogistic; that is, the fluctuation of the population of x is undergoing the effect of density dependency. One can refer to [31] for more details about (1).In the predator-prey model that we are considering in this paper, the prey isx and the predator is y, and the predator y only feeds on the prey x. Adopting model (1) to reflect the population growth of the prey x, we have the following discrete predator-prey model of Lotka-Volterra type:
(2)
x
(
n
+
1
)
=
x
(
n
)
exp
⁡
⁡
[
r
1
-
a
11
x
(
n
)
-
a
12
y
(
n
)
]
,
y
(
n
+
1
)
=
y
(
n
)
exp
⁡
⁡
[
-
r
2
+
a
21
x
(
n
)
-
a
22
y
(
n
)
]
.
In model (2), r
1 is the intrinsic growth rate of the prey x, a
11 is the intraspecific competition strength of the prey x, a
12 is the predation strength, r
2 is the death rate of the predator y if there is no prey x for y to feed on, a
21 reflects the predation efficiency or the rate of energy transferred from the prey x to the predator y through predation, and a
22 is the intraspecific competition strength of the predator y. The parameters r
i, a
i
j, i, and j
∈
{
1,2
} are all positive constants. The permanence and the global asymptotic stability of the positive equilibrium of model (2) have been discussed in [35, 36].Now, suppose that the habitats of the predator-prey system (2) are separated into two patches, patch 1 and patch 2. We denote x
i
(
n
) and y
i
(
n
) as the respective population density of the prey x and the predator y in patch i at the nth time step, i
∈
{
1,2
}. Let r
1
i be the intrinsic growth rate of the prey x in patch i and let r
2
i be the death rate of the predator y in patch i, i
∈
{
1,2
}. Also, a
i
j
(
k
) has the same biological meaning as a
i
j in model (2) except that now it is the case in patch k, i, j, k
∈
{
1,2
}. At this moment, there are no dispersal corridors between the two patches. Thus, we have the following discrete predator-prey model in patchy environment without dispersal corridors:
(3)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
⁡
[
r
11
-
a
11
(
1
)
x
1
(
n
)
-
a
12
(
1
)
y
1
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
x
1
(
n
)
-
a
22
(
1
)
y
1
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
⁡
[
r
12
-
a
11
(
2
)
x
2
(
n
)
-
a
12
(
2
)
y
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
⁡
[
-
r
22
+
a
21
(
2
)
x
2
(
n
)
-
a
22
(
2
)
y
2
(
n
)
]
.Next, we introduce dispersal corridors to the patchy model (3). It is assumed that the prey x can migrate from one patch to another via these dispersal corridors, but the predator y cannot use the dispersal corridors to migrate. We make some remarks to this assumption. Firstly, this assumption of only prey (but not predator) dispersal is usual in continuous/discrete time predator-prey models in patchy environment; see, for example, [22, 23, 37, 38]. Moreover, this assumption fits the common definition ofprey refuge if the predator is fixed in one patch, and intensive studies have been done; refer to [39] and the references cited therein. Our assumption here is that the prey can move among patches while the predator is fixed in each patch—an example of such a system can be found in [40], where the prey (mayflies) is mobile among different patches, but the predator (charr) is confined to its patch since the river is fenced into patches. Indeed, systems of this type may exist naturally in swamp fields, where the prey (such as flying insects) can migrate to different ponds while the predator (such as fish) is fixed in each pond. Denote d
i
j as the dispersal rate of the prey x migrating from patch i to patch j, i, j
∈
{
1,2
}, i
≠
j. We have the following discrete predator-prey model in patchy environment with dispersal corridors:
(4)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
⁡
[
r
11
-
a
11
(
1
)
x
1
(
n
)
-
a
12
(
1
)
y
1
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
a
11
(
1
)
-
d
12
x
1
(
n
)
+
d
21
x
2
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
x
1
(
n
)
-
a
22
(
1
)
y
1
(
n
)
+
α
d
21
x
2
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
[
r
12
-
a
11
(
2
)
x
2
(
n
)
-
a
12
(
2
)
y
2
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
a
11
(
2
)
+
d
12
x
1
(
n
)
-
d
21
x
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
⁡
[
-
r
22
+
a
21
(
2
)
x
2
(
n
)
-
a
22
(
2
)
y
2
(
n
)
+
β
d
12
x
1
(
n
)
]
.
Here, α and β represent the predation rates of the predator on the prey that migrates from another patch. Note that model (4) is reduced to model (3) if the dispersal rates d
12 and d
21 are both zero. Model (4) may be viewed as a discrete analogue of the continuous time predator-prey reaction-diffusion model for patchy environment proposed by Allen [41]. In [41] anextinction result, whereby some sufficient conditions are established to ensure the extinction of the species, has been developed. We remark that, from a practical standpoint, permanence results that guarantee species survival are more useful than extinction results; as such, we will focus on the permanence of the discrete model.In view of the biological background, we let the initial values of models (3) and (4) be
(5)
x
1
(
0
)
=
x
1
>
0
,
x
2
(
0
)
=
x
2
>
0
;
y
1
(
0
)
=
y
1
>
0
,
y
2
(
0
)
=
y
2
>
0
.
Also, throughout the paper we assume the following: (H
1
)
r
i
j, a
i
j,  a
i
j
(
k
), i, j, k
∈
{
1,2
} and α, β are all positive constants, and d
12, d
21 are nonnegative constants.It is obvious that all the solutions of either model (3) or model (4) with initial values (5) are positive, which corresponds well to the biological meanings of x
i
(
n
), y
i
(
n
), i
∈
{
1,2
}, noting that population density of any species is always nonnegative.The permanence results of models (3) and (4) will be presented in Section 3. To be precise, we state the definition ofpermanence as follows.Definition 1.
Model (3) (or (4)) with initial values (5) is said to be permanent if there exist positive constants m
i, m
i
*, M
i, and M
i
*, i
∈
{
1,2
}, such that
(6)
m
i
≤
liminf
⁡
n
→
+
∞
x
i
(
n
)
≤
limsup
⁡
n
→
+
∞
x
i
(
n
)
≤
M
i
,
i
=
1,2
,
m
i
*
≤
liminf
⁡
n
→
+
∞
y
i
(
n
)
≤
limsup
⁡
n
→
+
∞
y
i
(
n
)
≤
M
i
*
,
i
=
1,2
,
hold for each solution of model (3) (or (4)) with initial values (5).


## 3. Permanence of the Models

In this section, we will establish the permanence of models (3) and (4) with initial values (5). The mathematical proofs of Theorems 2, 4, and 5 will be presented in Appendix.


### 3.1. Permanence without Dispersal Corridors

We consider model (3) together with initial values (5). Notice that in (3) the first two equations are independent of the last two equations; therefore, we only need to consider the permanence of the model comprising just the first two equations of (3); namely,
(7)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
⁡
[
r
11
-
a
11
(
1
)
x
1
(
n
)
-
a
12
(
1
)
y
1
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
x
1
(
n
)
-
a
22
(
1
)
y
1
(
n
)
]
with initial values
(8)
x
1
(
0
)
=
x
1
>
0
,
y
1
(
0
)
=
y
1
>
0
.Theorem 2.
Suppose that(9)
r
11
-
a
12
(
1
)
M
1
>
0
,
(10)
-
r
21
+
a
21
(
1
)
m
1
>
0
are satisfied, where
(11)
M
1
=
1
a
22
(
1
)
exp
⁡
⁡
(
-
r
21
+
a
21
(
1
)
A
1
-
1
)
,
A
1
=
1
a
11
(
1
)
exp
⁡
⁡
(
r
11
-
1
)
,
m
1
=
r
11
-
a
12
(
1
)
M
1
a
11
(
1
)
exp
⁡
⁡
(
r
11
-
a
12
(
1
)
M
1
-
a
11
(
1
)
A
1
)
.
Then, model (7) with initial values (8) is permanent.Remark 3.
(a) The permanence or global asymptotical stability for the positive equilibrium of model (7) has been studied in [35, 36, 42]. In [35] it is shown that, for sufficiently small r
11 and r
21, model (7) has a positive equilibrium which is globally attractive, and as a consequence model (7) is permanent. So the conditions for the permanence of (7) are not directly derived in [35]. On the other hand, based on the study of a nonautonomous version of (7), it is proved in [36] (Corollary 4) that the positive equilibrium of (7) is globally asymptotically stable if r
11
≤
1 together with other conditions. Further, in [42] it is proved that the positive equilibrium of (7) can also be globally asymptotically stable if r
11
>
1. It is known that a system is permanent if its positive equilibrium is globally asymptotically stable [8]. Hence, in [36, 42] once again permanence is a consequence but the conditions to ensure it are indirectly obtained.
(b) Our method to obtain the permanence of (7) (or (3)) is different from that in [35, 36, 42]. In particular, the inequalities m
1
≤
M
1 and m
1
*
≤
M
1
* in Definition 1 can be verified here (refer to (A.17), (A.18), (A.26), and (A.27) in Appendix). In most cases if it is proved that each solution of a model is bounded both below and above by some positive constants, then the model is considered to be permanent. However, it isnot easy to show the inequalities such as (A.17) and (A.26).Using Theorem2, we give the following theorem on the permanence of model (3).Theorem 4.
Suppose that conditions (9) and (10) are satisfied, and
(12)
r
12
-
a
12
(
2
)
M
2
>
0
,
(13)
-
r
22
+
a
21
(
2
)
m
2
>
0
hold, where
(14)
M
2
=
1
a
22
(
2
)
exp
⁡
⁡
(
-
r
22
+
a
21
(
2
)
A
2
-
1
)
,
A
2
=
1
a
11
(
2
)
exp
⁡
⁡
(
r
12
-
1
)
,
m
2
=
r
12
-
a
12
(
2
)
M
2
a
11
(
2
)
exp
⁡
⁡
(
r
12
-
a
12
(
2
)
M
2
-
a
11
(
2
)
A
2
)
.
Then, model (3) with initial values (5) is permanent.


### 3.2. Permanence with Dispersal Corridors

The following theorem gives the sufficient conditions to guarantee the permanence of predator-prey model (4) in patchy environment with dispersal corridors.Theorem 5.
Suppose that(15)
r
11
-
a
12
(
1
)
S
1
>
0
,
(16)
-
r
21
+
a
21
(
1
)
s
1
>
0
,
(17)
r
12
-
a
12
(
2
)
S
2
>
0
,
(18)
-
r
22
+
a
21
(
2
)
s
2
>
0
are satisfied, where
(19)
S
1
=
1
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
+
d
12
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
,
s
1
=
r
11
-
a
12
(
1
)
S
1
a
11
(
1
)
+
d
12
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
S
1
-
exp
⁡
⁡
(
r
11
-
1
)
]
,
S
2
=
1
a
22
(
2
)
exp
⁡
⁡
[
-
r
22
+
a
21
(
2
)
a
11
(
2
)
+
d
21
exp
⁡
⁡
(
r
12
-
1
)
-
1
]
,
s
2
=
r
12
-
a
12
(
2
)
S
2
a
11
(
2
)
+
d
21
exp
⁡
⁡
[
r
12
-
a
12
(
2
)
S
2
-
exp
⁡
⁡
(
r
12
-
1
)
]
.
Then, model (4) with initial values (5) is permanent.Remark 6.
Unlike the proof of Theorem4, in the proof of Theorem 5 (refer to Appendix) the inequalities m
1
≤
M
1, m
1
*
≤
M
1
*, m
2
≤
M
2, and m
2
*
≤
M
2
* in Definition 1 cannot be verified. This also reinforces Remark 3(b) where we indicate that such inequalities arenot easy to prove in most cases.


## 3.1. Permanence without Dispersal Corridors

We consider model (3) together with initial values (5). Notice that in (3) the first two equations are independent of the last two equations; therefore, we only need to consider the permanence of the model comprising just the first two equations of (3); namely,
(7)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
⁡
[
r
11
-
a
11
(
1
)
x
1
(
n
)
-
a
12
(
1
)
y
1
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
x
1
(
n
)
-
a
22
(
1
)
y
1
(
n
)
]
with initial values
(8)
x
1
(
0
)
=
x
1
>
0
,
y
1
(
0
)
=
y
1
>
0
.Theorem 2.
Suppose that(9)
r
11
-
a
12
(
1
)
M
1
>
0
,
(10)
-
r
21
+
a
21
(
1
)
m
1
>
0
are satisfied, where
(11)
M
1
=
1
a
22
(
1
)
exp
⁡
⁡
(
-
r
21
+
a
21
(
1
)
A
1
-
1
)
,
A
1
=
1
a
11
(
1
)
exp
⁡
⁡
(
r
11
-
1
)
,
m
1
=
r
11
-
a
12
(
1
)
M
1
a
11
(
1
)
exp
⁡
⁡
(
r
11
-
a
12
(
1
)
M
1
-
a
11
(
1
)
A
1
)
.
Then, model (7) with initial values (8) is permanent.Remark 3.
(a) The permanence or global asymptotical stability for the positive equilibrium of model (7) has been studied in [35, 36, 42]. In [35] it is shown that, for sufficiently small r
11 and r
21, model (7) has a positive equilibrium which is globally attractive, and as a consequence model (7) is permanent. So the conditions for the permanence of (7) are not directly derived in [35]. On the other hand, based on the study of a nonautonomous version of (7), it is proved in [36] (Corollary 4) that the positive equilibrium of (7) is globally asymptotically stable if r
11
≤
1 together with other conditions. Further, in [42] it is proved that the positive equilibrium of (7) can also be globally asymptotically stable if r
11
>
1. It is known that a system is permanent if its positive equilibrium is globally asymptotically stable [8]. Hence, in [36, 42] once again permanence is a consequence but the conditions to ensure it are indirectly obtained.
(b) Our method to obtain the permanence of (7) (or (3)) is different from that in [35, 36, 42]. In particular, the inequalities m
1
≤
M
1 and m
1
*
≤
M
1
* in Definition 1 can be verified here (refer to (A.17), (A.18), (A.26), and (A.27) in Appendix). In most cases if it is proved that each solution of a model is bounded both below and above by some positive constants, then the model is considered to be permanent. However, it isnot easy to show the inequalities such as (A.17) and (A.26).Using Theorem2, we give the following theorem on the permanence of model (3).Theorem 4.
Suppose that conditions (9) and (10) are satisfied, and
(12)
r
12
-
a
12
(
2
)
M
2
>
0
,
(13)
-
r
22
+
a
21
(
2
)
m
2
>
0
hold, where
(14)
M
2
=
1
a
22
(
2
)
exp
⁡
⁡
(
-
r
22
+
a
21
(
2
)
A
2
-
1
)
,
A
2
=
1
a
11
(
2
)
exp
⁡
⁡
(
r
12
-
1
)
,
m
2
=
r
12
-
a
12
(
2
)
M
2
a
11
(
2
)
exp
⁡
⁡
(
r
12
-
a
12
(
2
)
M
2
-
a
11
(
2
)
A
2
)
.
Then, model (3) with initial values (5) is permanent.


## 3.2. Permanence with Dispersal Corridors

The following theorem gives the sufficient conditions to guarantee the permanence of predator-prey model (4) in patchy environment with dispersal corridors.Theorem 5.
Suppose that(15)
r
11
-
a
12
(
1
)
S
1
>
0
,
(16)
-
r
21
+
a
21
(
1
)
s
1
>
0
,
(17)
r
12
-
a
12
(
2
)
S
2
>
0
,
(18)
-
r
22
+
a
21
(
2
)
s
2
>
0
are satisfied, where
(19)
S
1
=
1
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
+
d
12
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
,
s
1
=
r
11
-
a
12
(
1
)
S
1
a
11
(
1
)
+
d
12
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
S
1
-
exp
⁡
⁡
(
r
11
-
1
)
]
,
S
2
=
1
a
22
(
2
)
exp
⁡
⁡
[
-
r
22
+
a
21
(
2
)
a
11
(
2
)
+
d
21
exp
⁡
⁡
(
r
12
-
1
)
-
1
]
,
s
2
=
r
12
-
a
12
(
2
)
S
2
a
11
(
2
)
+
d
21
exp
⁡
⁡
[
r
12
-
a
12
(
2
)
S
2
-
exp
⁡
⁡
(
r
12
-
1
)
]
.
Then, model (4) with initial values (5) is permanent.Remark 6.
Unlike the proof of Theorem4, in the proof of Theorem 5 (refer to Appendix) the inequalities m
1
≤
M
1, m
1
*
≤
M
1
*, m
2
≤
M
2, and m
2
*
≤
M
2
* in Definition 1 cannot be verified. This also reinforces Remark 3(b) where we indicate that such inequalities arenot easy to prove in most cases.


## 4. Effects of Dispersal Corridors

In Section3, sufficient conditions for the permanence of models (3) and (4) with initial values (5) are offered in Theorems 4 and 5, respectively. In this section, we will analyze the effects of dispersal corridors on the permanence of discrete predator-prey system by comparing the two sets of sufficient conditions obtained. Recall that the sufficient conditions to guarantee the permanence of predator-prey systemwithout dispersal corridors in patchy environment are (9)–(13), while (15)–(18) are the sufficient conditions to guarantee the permanence of the similar systemwith dispersal corridors. From the proofs of Theorems 4 and 5 (see Appendix), these conditions can be classified in Table 1.Table 1
Sufficient conditions for the permanence of models without/with dispersal corridors.
Concerned with the permanence
Without dispersal corridors
With dispersal corridors
The preyx in patch 1
(9)
(15)
The predatory in patch 1
(10)
(16)
The preyx in patch 2
(12)
(17)
The predatory in patch 2
(13)
(18)


### 4.1. Effects of Dispersal Corridors on the Permanence of the Prey

We begin with conditions (9) and (15), which are mainly concerned with the permanence of the prey x in patch 1 under the situation of without or with dispersal corridors. Rewrite (9) as
(20)
r
11
>
a
12
(
1
)
M
1
=
a
12
(
1
)
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
≜
B
1
.
Also, rewrite (15) as
(21)
r
11
>
a
12
(
1
)
S
1
=
a
12
(
1
)
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
+
d
12
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
≜
B
2
.On the left sides of (20) and (21), r
11 is the intrinsic growth rate of the prey x in patch 1, which makespositive contribution to the population growth of the prey since it is greater than 0. The right sides of (20) and (21) can be understood as thenegative contribution, caused by the intracompetition of species x in patch 1, the predation by the predator y in patch 1, and also the dispersion to patch 2 if dispersal corridors exist, to the population growth of the prey. Thus, (
r
11
-
B
1
) and (
r
11
-
B
2
) can be interpreted as thenet reproduction rates of the prey x in patch 1 without or with dispersal corridors, respectively. Consequently, the biological interpretation of (20) or (21) is that the prey x in patch 1 is permanent if its net reproduction rate is positive.The only difference between (20) and (21) is whether the dispersal rate d
12 is involved or not. Recall that d
12
(
>
0
) is the dispersion rate of the prey x from patch 1 to patch 2. It is obvious that
(22)
B
1
>
B
2
,
or equivalently
(23)
r
11
-
B
1
<
r
11
-
B
2
.
This inequality shows that the net reproduction rate of the prey in patch 1 is larger when dispersal corridors are introduced. Hence, the existence of dispersal corridors for the prey x to migrate from patch 1 to patch 2 ishelpful to the permanence of the prey in patch 1. Moreover, if r
11
-
B
1
>
0, then r
11
-
B
2
>
0 always holds; but if r
11
-
B
2
>
0, then we are not certain whether r
11
-
B
1
>
0. In other words, with the intrinsic growth rate r
11 of the prey x in patch 1 keeping the same value regardless of the existence or nonexistence of dispersal corridors, if the prey in patch 1 can be permanent without dispersal corridors, then it is also permanent in patch 1 with dispersal corridors; but if the prey in patch 1 is permanent with dispersal corridors, it may or may not be permanent without the dispersal corridors. In fact, the population growth of the prey in patch 1 is density dependent; hence, an appropriate dispersion from patch 1 to patch 2 may counterbalance the effect of density dependency. Certainly only the dispersion rate d
12 in anappropriate range is helpful to the permanence of the prey in patch 1. If the dispersion rate is too large, then the residence of the prey in patch 1 will decrease to a very small number, which will lead to a decrease in the intrinsic growth rate r
11 and will consequently increase the value of a
22
(
1
), the intraspecific competition strength of the predator in patch 1, due to limited food sources. Hence, in the case of large dispersion rate d
12, the value of B
2 will decrease. If the decrease in r
11 is larger than the decrease in B
2, then the net reproduction rate of the prey in patch 1 may decrease and (21) may not hold; that is, the permanence of the prey in patch 1 may not be guaranteed.In summary, an appropriate degree of dispersion of the prey from patch 1 to patch 2, measured by the dispersion rated
12, ishelpful to the permanence of the prey in patch 1. Similar conclusion can be drawn in patch 2 by comparing the condition (12) with (17). An appropriate dispersion rate d
21 of the prey from patch 2 to patch 1 ishelpful to the permanence of the prey in patch 2.


### 4.2. Effects of Dispersal Corridors on the Permanence of the Predator

We begin with conditions (10) and (16), which are mainly concerned with the permanence of the predator y in patch 1 under the situation of without or with dispersal corridors. Rewrite (10) as
(24)
r
21
<
a
21
(
1
)
m
1
=
a
21
(
1
)
r
11
-
a
12
(
1
)
M
1
a
11
(
1
)
×
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
M
1
-
exp
⁡
⁡
(
r
11
-
1
)
]
≜
B
3
,
and rewrite (16) as
(25)
r
21
<
a
21
(
1
)
s
1
=
a
21
(
1
)
r
11
-
a
12
(
1
)
S
1
a
11
(
1
)
+
d
12
×
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
S
1
-
exp
⁡
⁡
(
r
11
-
1
)
]
≜
B
4
.
For convenience, we also list the expressions of M
1 and S
1 (refer to Theorems 2 and 5) as follows:
(26)
M
1
=
1
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
,
S
1
=
1
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
+
d
12
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
.On the left sides of (24) and (25), r
21 is the death rate of the predator y in patch 1. The right sides of (24) and (25) can be understood as thepositive contribution to the population growth of the predator in patch 1 due to the predation. Hence, (
B
3
-
r
21
) and (
B
4
-
r
21
) can be interpreted as thenet reproduction rates of the predator y in patch 1 without or with dispersal corridors, respectively. Consequently, the biological interpretation of (24) or (25) is that the predator y in patch 1 is permanent if its net reproduction rate is positive.There are two “places” in (25) where the dispersal rate d
12 appears; one is in the denominator of B
4, while another is in the term S
1. We will discuss the biological implications of them. First, it is clear that the d
12 that is explicit in the denominator of (25) has a diminishing effect on B
4. Therefore, this dispersal rate in the denominator isdisadvantageous to the net reproduction rate of the population growth, further the permanence, of the predator in patch 1. In fact, the migration of the prey from patch 1 to patch 2, while the predator remains fixed to patch 1, causes the decrease in the food sources of the predator, which in turn is disadvantageous to the permanence of the predator in patch 1.Next, we consider the effect of the dispersal rated
12 in the term S
1. It is obvious from (26) that S
1
<
M
1; thus
(27)
r
11
-
a
12
(
1
)
S
1
>
r
11
-
a
12
(
1
)
M
1
,
or equivalently (see (20) and (21))
(28)
r
11
-
B
2
>
r
11
-
B
1
,
which is exactly (23). We have observed earlier that the net reproduction rate of the prey in patch 1 with dispersal corridors is larger than that without dispersal corridors. In the following, we continue to analyze how this larger net reproduction rate of the prey in patch 1 impacts the net reproduction rate of the predator in patch 1.Consider the functiong
(
x
)
=
x
exp
⁡
(
x
-
a
) where a
>
0 in the interval (
0
,
+
∞
). The derivative of g
(
x
) is g
′
(
x
)
=
(
1
+
x
)
exp
⁡
(
x
-
a
)
>
0, so g
(
x
) is monotonically increasing in (
0
,
+
∞
). Thus, noting (27) we have
(29)
(
r
11
-
a
12
(
1
)
S
1
)
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
S
1
-
exp
⁡
(
r
11
-
1
)
]
>
(
r
11
-
a
12
(
1
)
M
1
)
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
M
1
-
exp
⁡
(
r
11
-
1
)
]
.
Noting the definitions of B
3 and B
4, inequality (29) shows that the dispersal rate d
12 which appeared in S
1 isadvantageous to the increase of the net reproduction rate of the predator in patch 1, which in turn ishelpful to the permanence of the predator in patch 1. In fact, from (27) (or (23)) this dispersion rate in S
1 increases the net reproduction rate of the prey in patch 1, which consequently increases the food supply for the predator in patch 1. As a result, this contributes positively to the net reproduction rate of the predator in patch 1, which further helps to sustain the permanence of the predator in patch 1.In summary, the effects of the dispersion of the prey from patch 1 to patch 2, measured by the dispersal rated
12, on the permanence of the predator in patch 1 are twofold: one isdisadvantageous—contributed by the presence of d
12 in the denominator of B
4, while the other isadvantageous—contributed by the presence of d
12 in S
1. Similar conclusion can be drawn in patch 2 by comparing the condition (13) with (18). The dispersion of the prey from patch 2 to patch 1, measured by the dispersal rate d
21, has multiple effects, both advantageous and disadvantageous, on the permanence of the predator in patch 2.Remark 7.
From the two sets of permanence conditions in Theorems4 and 5, no conclusion can be drawn on the effects of d
21 on the permanence of the prey and the predator in patch 1; also the effects of d
12 on the permanence of the prey and the predator in patch 2 are unknown.


## 4.1. Effects of Dispersal Corridors on the Permanence of the Prey

We begin with conditions (9) and (15), which are mainly concerned with the permanence of the prey x in patch 1 under the situation of without or with dispersal corridors. Rewrite (9) as
(20)
r
11
>
a
12
(
1
)
M
1
=
a
12
(
1
)
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
≜
B
1
.
Also, rewrite (15) as
(21)
r
11
>
a
12
(
1
)
S
1
=
a
12
(
1
)
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
+
d
12
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
≜
B
2
.On the left sides of (20) and (21), r
11 is the intrinsic growth rate of the prey x in patch 1, which makespositive contribution to the population growth of the prey since it is greater than 0. The right sides of (20) and (21) can be understood as thenegative contribution, caused by the intracompetition of species x in patch 1, the predation by the predator y in patch 1, and also the dispersion to patch 2 if dispersal corridors exist, to the population growth of the prey. Thus, (
r
11
-
B
1
) and (
r
11
-
B
2
) can be interpreted as thenet reproduction rates of the prey x in patch 1 without or with dispersal corridors, respectively. Consequently, the biological interpretation of (20) or (21) is that the prey x in patch 1 is permanent if its net reproduction rate is positive.The only difference between (20) and (21) is whether the dispersal rate d
12 is involved or not. Recall that d
12
(
>
0
) is the dispersion rate of the prey x from patch 1 to patch 2. It is obvious that
(22)
B
1
>
B
2
,
or equivalently
(23)
r
11
-
B
1
<
r
11
-
B
2
.
This inequality shows that the net reproduction rate of the prey in patch 1 is larger when dispersal corridors are introduced. Hence, the existence of dispersal corridors for the prey x to migrate from patch 1 to patch 2 ishelpful to the permanence of the prey in patch 1. Moreover, if r
11
-
B
1
>
0, then r
11
-
B
2
>
0 always holds; but if r
11
-
B
2
>
0, then we are not certain whether r
11
-
B
1
>
0. In other words, with the intrinsic growth rate r
11 of the prey x in patch 1 keeping the same value regardless of the existence or nonexistence of dispersal corridors, if the prey in patch 1 can be permanent without dispersal corridors, then it is also permanent in patch 1 with dispersal corridors; but if the prey in patch 1 is permanent with dispersal corridors, it may or may not be permanent without the dispersal corridors. In fact, the population growth of the prey in patch 1 is density dependent; hence, an appropriate dispersion from patch 1 to patch 2 may counterbalance the effect of density dependency. Certainly only the dispersion rate d
12 in anappropriate range is helpful to the permanence of the prey in patch 1. If the dispersion rate is too large, then the residence of the prey in patch 1 will decrease to a very small number, which will lead to a decrease in the intrinsic growth rate r
11 and will consequently increase the value of a
22
(
1
), the intraspecific competition strength of the predator in patch 1, due to limited food sources. Hence, in the case of large dispersion rate d
12, the value of B
2 will decrease. If the decrease in r
11 is larger than the decrease in B
2, then the net reproduction rate of the prey in patch 1 may decrease and (21) may not hold; that is, the permanence of the prey in patch 1 may not be guaranteed.In summary, an appropriate degree of dispersion of the prey from patch 1 to patch 2, measured by the dispersion rated
12, ishelpful to the permanence of the prey in patch 1. Similar conclusion can be drawn in patch 2 by comparing the condition (12) with (17). An appropriate dispersion rate d
21 of the prey from patch 2 to patch 1 ishelpful to the permanence of the prey in patch 2.


## 4.2. Effects of Dispersal Corridors on the Permanence of the Predator

We begin with conditions (10) and (16), which are mainly concerned with the permanence of the predator y in patch 1 under the situation of without or with dispersal corridors. Rewrite (10) as
(24)
r
21
<
a
21
(
1
)
m
1
=
a
21
(
1
)
r
11
-
a
12
(
1
)
M
1
a
11
(
1
)
×
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
M
1
-
exp
⁡
⁡
(
r
11
-
1
)
]
≜
B
3
,
and rewrite (16) as
(25)
r
21
<
a
21
(
1
)
s
1
=
a
21
(
1
)
r
11
-
a
12
(
1
)
S
1
a
11
(
1
)
+
d
12
×
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
S
1
-
exp
⁡
⁡
(
r
11
-
1
)
]
≜
B
4
.
For convenience, we also list the expressions of M
1 and S
1 (refer to Theorems 2 and 5) as follows:
(26)
M
1
=
1
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
,
S
1
=
1
a
22
(
1
)
exp
⁡
⁡
[
-
r
21
+
a
21
(
1
)
a
11
(
1
)
+
d
12
exp
⁡
⁡
(
r
11
-
1
)
-
1
]
.On the left sides of (24) and (25), r
21 is the death rate of the predator y in patch 1. The right sides of (24) and (25) can be understood as thepositive contribution to the population growth of the predator in patch 1 due to the predation. Hence, (
B
3
-
r
21
) and (
B
4
-
r
21
) can be interpreted as thenet reproduction rates of the predator y in patch 1 without or with dispersal corridors, respectively. Consequently, the biological interpretation of (24) or (25) is that the predator y in patch 1 is permanent if its net reproduction rate is positive.There are two “places” in (25) where the dispersal rate d
12 appears; one is in the denominator of B
4, while another is in the term S
1. We will discuss the biological implications of them. First, it is clear that the d
12 that is explicit in the denominator of (25) has a diminishing effect on B
4. Therefore, this dispersal rate in the denominator isdisadvantageous to the net reproduction rate of the population growth, further the permanence, of the predator in patch 1. In fact, the migration of the prey from patch 1 to patch 2, while the predator remains fixed to patch 1, causes the decrease in the food sources of the predator, which in turn is disadvantageous to the permanence of the predator in patch 1.Next, we consider the effect of the dispersal rated
12 in the term S
1. It is obvious from (26) that S
1
<
M
1; thus
(27)
r
11
-
a
12
(
1
)
S
1
>
r
11
-
a
12
(
1
)
M
1
,
or equivalently (see (20) and (21))
(28)
r
11
-
B
2
>
r
11
-
B
1
,
which is exactly (23). We have observed earlier that the net reproduction rate of the prey in patch 1 with dispersal corridors is larger than that without dispersal corridors. In the following, we continue to analyze how this larger net reproduction rate of the prey in patch 1 impacts the net reproduction rate of the predator in patch 1.Consider the functiong
(
x
)
=
x
exp
⁡
(
x
-
a
) where a
>
0 in the interval (
0
,
+
∞
). The derivative of g
(
x
) is g
′
(
x
)
=
(
1
+
x
)
exp
⁡
(
x
-
a
)
>
0, so g
(
x
) is monotonically increasing in (
0
,
+
∞
). Thus, noting (27) we have
(29)
(
r
11
-
a
12
(
1
)
S
1
)
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
S
1
-
exp
⁡
(
r
11
-
1
)
]
>
(
r
11
-
a
12
(
1
)
M
1
)
exp
⁡
⁡
[
r
11
-
a
12
(
1
)
M
1
-
exp
⁡
(
r
11
-
1
)
]
.
Noting the definitions of B
3 and B
4, inequality (29) shows that the dispersal rate d
12 which appeared in S
1 isadvantageous to the increase of the net reproduction rate of the predator in patch 1, which in turn ishelpful to the permanence of the predator in patch 1. In fact, from (27) (or (23)) this dispersion rate in S
1 increases the net reproduction rate of the prey in patch 1, which consequently increases the food supply for the predator in patch 1. As a result, this contributes positively to the net reproduction rate of the predator in patch 1, which further helps to sustain the permanence of the predator in patch 1.In summary, the effects of the dispersion of the prey from patch 1 to patch 2, measured by the dispersal rated
12, on the permanence of the predator in patch 1 are twofold: one isdisadvantageous—contributed by the presence of d
12 in the denominator of B
4, while the other isadvantageous—contributed by the presence of d
12 in S
1. Similar conclusion can be drawn in patch 2 by comparing the condition (13) with (18). The dispersion of the prey from patch 2 to patch 1, measured by the dispersal rate d
21, has multiple effects, both advantageous and disadvantageous, on the permanence of the predator in patch 2.Remark 7.
From the two sets of permanence conditions in Theorems4 and 5, no conclusion can be drawn on the effects of d
21 on the permanence of the prey and the predator in patch 1; also the effects of d
12 on the permanence of the prey and the predator in patch 2 are unknown.


## 5. Numerical Simulation

In this section, we present five numerical examples to confirm the theoretical results obtained earlier as well as to illustrate the effects of the dispersal corridors. All the examples are processed by MATLAB. The first example below confirms the correctness of Theorem4.Example 8.
Consider the following predator-prey model (3) in patchy environmentwithout dispersal corridors:
(30)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
⁡
[
0.3
-
0.2
x
1
(
n
)
-
0.1
y
1
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
⁡
[
-
0.06
+
0.15
x
1
(
n
)
-
0.3
y
1
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
⁡
[
0.32
-
0.21
x
2
(
n
)
-
0.1
y
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
⁡
[
-
0.07
+
0.14
x
2
(
n
)
-
0.29
y
2
(
n
)
]
.
Direct computation gives
(31)
r
11
-
a
12
(
1
)
M
1
=
0.1324
>
0
,
-
r
21
+
a
21
(
1
)
m
1
=
0.0090
>
0
,
r
12
-
a
12
(
2
)
M
2
=
0.1524
>
0
,
-
r
22
+
a
21
(
2
)
m
2
=
0.0023
>
0
.
Hence, the conditions of Theorem 4 are all satisfied. By Theorem 4, model (30) is permanent and this is illustrated in Figure 1.Figure 1
Permanence of model (30). The black solid (dotted) line is the population variation of the prey in patch 1 (patch 2). The green solid (dotted) line is the population variation of the predator in patch 1 (patch 2).The next example illustrates Theorem5.Example 9.
Consider the following predator-prey model (4) in patchy environmentwith dispersal corridors:
(32)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
[
0.3
-
0.05
x
1
(
n
)
-
0.1
y
1
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
-
0.1
x
1
(
n
)
+
0.15
x
2
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
[
-
0.07
+
0.15
x
1
(
n
)
-
0.3
y
1
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
+
0.15
*
0.15
x
2
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
[
0.52
-
0.35
x
2
(
n
)
-
0.15
y
2
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
+
0.1
x
1
(
n
)
-
0.15
x
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
[
-
0.08
+
0.3
x
2
(
n
)
-
0.25
y
2
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
+
0.3
*
0.1
x
1
(
n
)
]
.
In model (32), the dispersal rates are d
12
=
0.1 and d
21
=
0.15. Also, we set α
=
a
21
(
1
)
=
0.15 and β
=
a
21
(
2
)
=
0.3; that is, we assume that the predation rate of the predator on the prey which migrated from another patch is the same as the predation rate on the prey originally in the patch. By direct computation, we have all the conditions of Theorem 5 satisfied as follows:
(33)
r
11
-
a
12
(
1
)
S
1
=
0.1121
>
0
,
-
r
21
+
a
21
(
1
)
s
1
=
0.0063
>
0
,
r
12
-
a
12
(
2
)
S
2
=
0.2246
>
0
,
-
r
22
+
a
21
(
2
)
s
2
=
0.0109
>
0
.
Hence, by Theorem 5 model (32) is permanent. Figure 2 illustrates the permanence of model (32).Figure 2
Permanence of model (32). The black solid (dotted) line is the population variation of the prey in patch 1 (patch 2). The green solid (dotted) line is the population variation of the predator in patch 1 (patch 2).Next, we present an example to show that dispersal corridors may bedisadvantageous to the permanence of the ecosystem.Example 10.
Consider the following two models:(34)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
[
0.29
-
0.2
x
1
(
n
)
-
0.1
y
1
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
[
-
0.06
+
0.15
x
1
(
n
)
-
0.3
y
1
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
[
0.32
-
0.21
x
2
(
n
)
-
0.1
y
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
[
-
0.07
+
0.14
x
2
(
n
)
-
0.29
y
2
(
n
)
]
,
(35)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
[
0.29
-
0.2
x
1
(
n
)
-
0.1
y
1
(
n
)
hhhhhhhhhhhhhhhhhh
-
0.61
x
1
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
[
-
0.06
+
0.15
x
1
(
n
)
-
0.3
y
1
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
[
0.32
-
0.21
x
2
(
n
)
-
0.1
y
2
(
n
)
hhhhhhhhhhhhhhhhhh
+
0.61
x
1
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
[
-
0.07
+
0.14
x
2
(
n
)
-
0.29
y
2
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
+
0.14
*
0.61
x
1
(
n
)
]
.
The difference between (34) and (35) is only whether dispersal corridors exist or not; all other coefficients are the same. Model (34) is without dispersal corridors, and a direct computation shows that all the conditions of Theorem 4 are satisfied as follows:
(36)
r
11
-
a
12
(
1
)
M
1
=
0.1230
>
0
,
-
r
21
+
a
21
(
1
)
m
1
=
0.0038
>
0
,
r
12
-
a
12
(
2
)
M
2
=
0.1542
>
0
,
-
r
22
+
a
21
(
2
)
m
2
=
0.0023
>
0
.
Hence, by Theorem 4 model (34) ispermanent. Figure 3(a) illustrates the permanence of model (34).The dynamics of models (34) and (35). (a) is the dynamics of (34). It is observed that model (34) is permanent. (b) is the dynamics of (35). It is observed that the predator in patch 1 is driven to extinction when dispersal corridors are introduced. The black solid (dotted) line is the population variation of the prey in patch 1 (patch 2). The green solid (dotted) line is the population variation of the predator in patch 1 (patch 2).
(a)
(b)Model (35) has dispersal corridors with d
12
=
0.61, d
21
=
0, and β
=
a
21
(
2
)
=
0.14. Checking the conditions of Theorem 5, we find
(37)
r
11
-
a
12
(
1
)
S
1
=
0.1635
>
0
,
-
r
21
+
a
21
(
1
)
s
1
=
-
0.0382
<
0
,
r
12
-
a
12
(
2
)
S
2
=
0.1542
>
0
,
-
r
22
+
a
21
(
2
)
s
2
=
0.0023
>
0
.
Hence, not all the conditions of Theorem 5 are satisfied and we cannot ensure the permanence of model (35). In fact, by numerical simulation we find that the predator in patch 1 is driven toextinction and so model (35) isnonpermanent. This is depicted in Figure 3(b).This example shows that the dispersion of the prey from patch 1 to patch 2 by the dispersal corridors may bedisadvantageous to the predator in patch 1, which has been discussed in Section 4.2. Further, it is observed from the above calculations that the introduction of d
12 increases the net reproduction rate (r
11
-
a
12
(
1
)
S
1) of the prey in patch 1 from 0.1230 to 0.1635, which has been noted in Section 4.1.Next, we give an example to show that dispersal corridors may behelpful to the permanence of the ecosystem.Example 11.
Consider the following two models:(38)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
[
0.05
-
0.5
x
1
(
n
)
-
0.16
y
1
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
[
-
0.02
+
0.2
x
1
(
n
)
-
0.5
y
1
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
[
0.52
-
0.35
x
2
(
n
)
-
0.15
y
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
[
-
0.08
+
0.3
x
2
(
n
)
-
0.25
y
2
(
n
)
]
,
(39)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
[
0.05
-
0.5
x
1
(
n
)
-
0.16
y
1
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
-
d
12
x
1
(
n
)
+
d
21
x
2
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
[
-
0.02
+
0.2
x
1
(
n
)
-
0.5
y
1
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
+
0.2
*
d
21
x
2
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
[
0.52
-
0.35
x
2
(
n
)
-
0.15
y
2
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
+
d
12
x
1
(
n
)
-
d
21
x
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
[
-
0.08
+
0.3
x
2
(
n
)
-
0.25
y
2
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
+
0.3
*
d
12
x
1
(
n
)
]
.
The difference between (38) and (39) is only whether dispersal corridors exist or not; all other coefficients are the same. Model (38) is without dispersal corridors, and a direct computation yields
(40)
r
11
-
a
12
(
1
)
M
1
=
-
0.0847
<
0
,
-
r
21
+
a
21
(
1
)
m
1
=
-
0.0411
<
0
,
r
12
-
a
12
(
2
)
M
2
=
0.1737
>
0
,
-
r
22
+
a
21
(
2
)
m
2
=
0.0154
>
0
.
Hence, not all the conditions of Theorem 4 are satisfied and we cannot guarantee the permanence of (38). Indeed, by numerical simulation it is found that the predator in patch 1 will be driven toextinction. So model (38) isnonpermanent and this is illustrated in Figure 4(a).The dynamics of models (38) and (39) subject to (41). (a) is the dynamics of (38). It is observed that the predator in patch 1 will go to extinction. (b) is the dynamics of (39). It is observed that the predator in patch 1 is permanent now due to the dispersal corridors, and (39), as a whole, is permanent. The black solid (dotted) line is the population variation of the prey in patch 1 (patch 2). The green solid (dotted) line is the population variation of the predator in patch 1 (patch 2).
(a)
(b)Model (39) has dispersal corridors with d
12
>
0, d
21
>
0, α
=
a
21
(
1
)
=
0.2, and β
=
a
21
(
2
)
=
0.3. Suppose
(41)
d
12
=
0.55
,
d
21
=
0.21
.
By numerical simulation we find that the predator in patch 1 is now permanent, and model (39), as a whole, ispermanent. This is depicted in Figure 4(b). Note, however, that model (39) does not satisfy some of the conditions of Theorem 5, as
(42)
r
11
-
a
12
(
1
)
S
1
=
-
0.0742
<
0
,
-
r
21
+
a
21
(
1
)
s
1
=
-
0.0289
<
0
,
r
12
-
a
12
(
2
)
S
2
=
0.2362
>
0
,
-
r
22
+
a
21
(
2
)
s
2
=
0.0063
>
0
.
This simply reinforces the fact that the conditions in Theorem 5 aresufficient conditions.We observe that the introduction of dispersal corridors leads to (i) an increase in the net reproduction rates of both the prey and the predator in patch 1 as well as the prey in patch 2 and (ii) a decrease in the net reproduction rate of the predator in patch 2. The overall effect on the permanence of model (39) is positive. This example shows that the introduction of dispersal corridors to a patchy ecosystem may beadvantageous to the permanence of the ecosystem.We will further investigate the effects of dispersal corridors on model (39). For this, in Table 2, for different values of d
12 and d
21 we list the net reproduction rate R
i
x of the prey x in patch i, the net reproduction rate R
i
y of the predator y in patch i, i
∈
{
1,2
}, and the permanence status of model (39). Recall that, for a model with dispersal corridors,
(43)
R
1
x
=
r
11
-
a
12
(
1
)
S
1
,
R
1
y
=
-
r
21
+
a
21
(
1
)
s
1
,
R
2
x
=
r
12
-
a
12
(
2
)
S
2
,
R
2
y
=
-
r
22
+
a
21
(
2
)
s
2
.Table 2
Net reproduction rates and the permanence of model (39) under different dispersal rates.
Dispersal rates
R
1
x
R
1
y
R
2
x
R
2
y
Permanence of model (39)
d
12
=
0
,
d
21
=
0
−0.0847
−0.0411
0.1737
0.0154
No, predator in patch 1 is extinct
d
12
=
0.18
,
d
21
=
0
−0.0793
−0.0346
0.1737
0.0154
No, predator in patch 1 is extinct
d
12
=
0.36
,
d
21
=
0
−0.0762
−0.0312
0.1737
0.0154
No, predator in patch 1 is extinct
d
12
=
0.55
,
d
21
=
0
−0.0742
−0.0289
0.1737
0.0154
No, predator in patch 1 is extinct
d
12
=
1
,
d
21
=
0
−0.0715
−0.0260
0.1737
0.0154
No, predator in patch 1 is extinct
d
12
=
0.55
,
d
21
=
0.02
−0.0742
−0.0289
0.1835
0.0163
No, predator in patch 1 is extinct
d
12
=
0.55
,
d
21
=
0.07
−0.0742
−0.0289
0.2030
0.0157
Yes
d
12
=
0.55
,
d
21
=
0.14
−0.0742
−0.0289
0.2224
0.0116
Yes
d
12
=
0.55
,
d
21
=
0.21
−0.0742
−0.0289
0.2362
0.0063
Yes
d
12
=
0.55
,
d
21
=
0.28
−0.0742
−0.0289
0.2464
0.00086
YesFrom Table2, we observe the following.(a)
The effect of dispersal corridors on the net reproduction rate of prey is positive; specifically,R
1
x increases as d
12 increases and R
2
x increases as d
21 increases. This coincides with the discussion in Section 4.1.
(b)
The effects of dispersal corridors on the net reproduction rate of predator are twofold: advantageous and disadvantageous. For a fixedd
12
=
0.55, R
2
y first increases and then decreases as the value of d
21 increases from 0 to 0.28.
(c)
The entire ecosystem cannot be permanent by thesole effect of d
12 (i.e., d
21
=
0). If the value of d
12 increases to sufficiently large, even the prey in patch 1 will undergo extinction due to unpredicted environmental changes, as the population density of the prey in patch 1 will become very small. If the prey in patch 1 becomes extinct, so will the predator in patch 1, since there are no food sources.
(d)
Noting (c) above, we further observe that the ecosystem is also nonpermanent ifd
21 is too small; an appropriate value of d
21 is needed to induce a permanent (39).So far equilibrium dynamics have been observed in Examples8–11. Since it is well known that Ricker type model has complex dynamics, we give an example with more complex dynamics to illustrate our results.Example 12.
Consider the following two models:(44)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
[
2.4
-
0.2
x
1
(
n
)
-
0.1
y
1
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
[
-
0.1
+
0.15
x
1
(
n
)
-
0.3
y
1
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
[
0.4
-
0.21
x
2
(
n
)
-
0.1
y
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
[
-
0.07
+
0.14
x
2
(
n
)
-
0.69
y
2
(
n
)
]
,
(45)
x
1
(
n
+
1
)
=
x
1
(
n
)
exp
⁡
[
2.4
-
0.2
x
1
(
n
)
-
0.1
y
1
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
+
0.6
x
2
(
n
)
]
,
y
1
(
n
+
1
)
=
y
1
(
n
)
exp
⁡
⁡
[
-
0.1
+
0.15
x
1
(
n
)
-
0.3
y
1
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
+
0.15
*
0.6
x
2
(
n
)
]
,
x
2
(
n
+
1
)
=
x
2
(
n
)
exp
⁡
[
0.4
-
0.21
x
2
(
n
)
-
0.1
y
2
(
n
)
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
-
0.6
x
2
(
n
)
]
,
y
2
(
n
+
1
)
=
y
2
(
n
)
exp
⁡
[
-
0.07
+
0.14
x
2
(
n
)
-
0.69
y
2
(
n
)
]
.
The difference between (44) and (45) is only whether dispersal corridors exist or not; all other coefficients are the same. Model (44) is without dispersal corridors, and a direct computation yields
(46)
r
11
-
a
12
(
1
)
M
1
=
0.0772
>
0
,
-
r
21
+
a
21
(
1
)
m
1
=
-
0.0989
<
0
,
r
12
-
a
12
(
2
)
M
2
=
0.3283
>
0
,
-
r
22
+
a
21
(
2
)
m
2
=
0.1056
>
0
.
Though not all the conditions of Theorem 4 are satisfied, model (44) ispermanent which is illustrated in Figure 5(a). It is observed from Figure 5(a) that the subsystem of patch 2 has astable positive equilibrium while the subsystem of patch 1 doesnot have such stable equilibrium though it is permanent. Further illustration of this subsystem of patch 1 is given in Figure 5(b). Figure 5(b) is the phase diagram of (
x
1
,
y
1
) and it is observed that the subsystem of patch 1 of model (44) has a periodic solution of period 3. In fact, the periodic solution is (
x
1
,
y
1
)
=
(
18.7232,1.1061
), (
4.3689,11.9115
), (
6.1079,0.5824
).
Model (45) has dispersal corridors with d
21
=
0.6, d
12
=
0, and α
=
a
21
(
1
)
=
0.15. The net reproduction rates are computed as follows:
(47)
r
11
-
a
12
(
1
)
S
1
=
0.0772
>
0
,
-
r
21
+
a
21
(
1
)
s
1
=
-
0.0989
<
0
,
r
12
-
a
12
(
2
)
S
2
=
0.3453
>
0
,
-
r
22
+
a
21
(
2
)
s
2
=
-
0.0213
<
0
.
The dynamics of (45) are presented in Figure 6(a). It is observed that the predator in patch 2 will becomeextinct and consequently model (45) isnot permanent. Figure 6(b) illustrates that the periodic solution observed in the subsystem of patch 1 of model (44) now vanishes due to the dispersion.
Comparing (46) and (47), we observe that when d
21
>
0, (i) the net reproduction rate of the prey in patch 2 increases, which has been noted in Section 4.1 and (ii) the net reproduction rate of the predator in patch 2 decreases its value from positive to negative; this shows that the dispersion of the prey from patch 2 to patch 1 has anadverse effect on the permanence of the predator in patch 2, which has been discussed in Section 4.2. This example shows that large dispersal rates of the prey may destroy the permanence of a predator-prey system.The dynamics of model (44). (a) is the dynamics of (44). It is observed that model (44) is permanent. The subsystem of patch 2 has a stable positive equilibrium, while oscillating population densities for both the prey and the predator are observed in the subsystem of patch 1. (b) is the dynamics of the subsystem of patch 1 illustrated in the phase diagram (
x
1
,
y
1
). A periodic solution of period 3 is observed.
(a)
(b)The dynamics of model (45). (a) is the dynamics of (45). It is observed that the predator in patch 2 will become extinct due to the dispersal corridors, and (45), as a whole, is not permanent. (b) is the dynamics of the subsystem of patch 1 illustrated in the phase diagram (
x
1
,
y
1
). The periodic solution in Figure 5(b) vanishes due to dispersion.
(a)
(b)


## 6. Conclusion

The paper is mainly involved with the effects of dispersal corridors on the permanence of discrete predator-prey models in patchy environment. We have introduced two models of Ricker type undergoing the effect of density dependency—model (3) is in patchy environment without dispersal corridors and model (4) is with dispersal corridors. Two sets of sufficient conditions to guarantee the permanence of the two models are then obtained through theoretical studies, respectively. In Theorem 4, conditions (9)–(13) guarantee the permanence of model (3); this result is illustrated by Example 8. In Theorem 5, conditions (15)–(18) ensure the permanence of model (4); this result is also numerically illustrated by Example 9.Through mathematical analysis of the two sets of sufficient conditions, combined with the actual biological background of models (3) and (4), we have discussed the effects of dispersal corridors on the permanence of discrete predator-prey model. It is found that the existence of dispersal corridors for the prey to migrate, such as from patch i to patch j, while the predator remains fixed to the original patch, ishelpful to the permanence of the prey in patch i under density dependent circumstances due to an increased net reproduction rate of the prey in patch i. Moreover, it is also found that the migration of prey, such as from patch i to patch j, is bothadvantageous anddisadvantageous to the predator in patch i because the dispersal rate d
i
j has both positive and negative effects on the net reproduction rate of the predator in patch i. Examples 10–12 are presented to illustrate the positive/negative effects of the dispersal corridors.



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*Source: 140902-2014-07-17.xml*