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Comptes Rendus

Biological modelling / Biomodélisation
Permanence of a stage-structured predator-prey system with a class of functional responses
Comptes Rendus. Biologies, Volume 334 (2011) no. 12, pp. 851-854.

Résumé

Abstract

A stage-structured predator-prey system incorporating a class of functional responses is presented in this article. By analyzing the system and using the standard comparison theorem, the sufficient conditions are derived for permanence of the system and non-permanence of predators.

Métadonnées
Reçu le :
Accepté le :
Publié le :
DOI : 10.1016/j.crvi.2011.08.002
Mots clés : Predator-prey system, Stage structure, Generalized functional response, Permanence, Non-permanence
Zhihui Ma 1, 2 ; Shufan Wang 1 ; Wenting Wang 1 ; Zizhen Li 1, 2

1 School of Mathematics and Statistics, Lanzhou University, Lanzhou, 730000, China
2 Key Laboratory of Arid and Grassland Agroecology of Ministry of Education, Lanzhou, 730000, China
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     author = {Zhihui Ma and Shufan Wang and Wenting Wang and Zizhen Li},
     title = {Permanence of a stage-structured predator-prey system with a class of functional responses},
     journal = {Comptes Rendus. Biologies},
     pages = {851--854},
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Zhihui Ma; Shufan Wang; Wenting Wang; Zizhen Li. Permanence of a stage-structured predator-prey system with a class of functional responses. Comptes Rendus. Biologies, Volume 334 (2011) no. 12, pp. 851-854. doi : 10.1016/j.crvi.2011.08.002. https://comptes-rendus.academie-sciences.fr/biologies/articles/10.1016/j.crvi.2011.08.002/

Version originale du texte intégral

1 Introduction

The past decades have witnessed an enormous interest in predator-prey systems, with most of them focusing on homogeneous populations [1–5]. In fact, no natural population is truly homogeneous, many organisms undergo radical changes in many aspects such as the rates of survival, maturation, reproduction and predation while while they are processing their life history. Therefore, stage-structured predator-prey systems have received much attention recently [6–21].

However, these works [7–21] are largely using constant coefficient systems and have stage structures on only one of the interactive species. The research objectives mainly include the stability and permanence of the system, often with one particular type of functional response. In this article, we propose the following variable coefficient predator-prey system with time delays and stage structures for both interactive species, as well as different functional responses, Specifically, we will consider the condition for the permanence of the two species and the non-permanence condition for predators to become extinct:

x˙1(t)=r1(t)x2(t)d11x1(t)r1(t)ed11τ1x2(tτ1)x˙2(t)=r1(t)ed11τ1x2(tτ1)d12x2(t)b1(t)x22c1(t)ϕ(x2(t))x2(t)y2(t)y˙1(t)=r2(t)y2(t)d22y1(t)r2(t)ed22τ2y2(tτ2)y˙2(t)=r2(t)ed22τ2y2(tτ2)d21y2(t)b2(t)y22+c2(t)ϕ(x2(t))x2(t)y2(t)(1)
where x1(t) and x2(t) denote the immature and mature densities of the prey at time t, respectively. y1(t) and y2(t) denote the immature and mature densities of the predator at time t, respectively. ri(t), bi(t), ci(t) (i = 1, 2) are positive and continuous functions for all t ≥ 0.

The above system assumes that the mature predators only feed on the mature prey population, the immature are produced by the mature populations. The birth rate of prey and predators (ri(t) > 0; i = 1, 2) is proportional to the existing mature population sizes.

The other parameters have the following biological meanings: c1(t) denotes the capturing rate of mature predators at time t, and c2(t)/c1(t) is the rate of conversion of nutrients from the mature prey into the reproduction of the mature predator; bi(t) > 0 (i = 1, 2) represents the intraspecific competition rate of mature prey and predators at time t, respectively; d11 and d12 are the death rate of immature and mature population of prey respectively, d22 and d21 are the death rate of immature and mature population of predator respectively; τi > 0(i = 1, 2) is the length of time from the birth to maturity of the species i. The term r1(t)ed11τ1x2(t − τ1) represents the survived prey population born at time t − τ1, that is, the transition from the immature to mature prey. The term r2(t)ed22τ2y2(t − τ2) denotes the survived predator population born at time t − τ2, that is, the transition from the immature to mature predator.

The term φ(x2)x2, the number of prey captured per predator per unit time, is called the predator functional response and satisfies the following assumptions

0<ϕ(x2)<L<+,ddx2(ϕ(x2)x2)0(x2>0).(2)

The initial conditions of system (1) are given by

xi(θ)=ϕi(θ)>0,yi(θ)=ψi(θ)>0,ϕi(0)>0,ψi(0)>0(i=1,2),θ[τ,0],τ=max{τ1,τ2}.(3)

For the continuity of initial conditions, we require further that

x1(0)=τ10r1(s)ϕ2(s)ds,y1(0)=τ20r2(s)ψ2(s)ds.(4)

Again, we define that

rks=supt0rk(t)>0,cks=supt0ck(t)>0,bks=supt0bk(t)>0,rki=inft0rk(t)>0,cki=inft0ck(t)>0,bki=inft0bk(t)>0(k=1,2).(5)

2 Positivity and boundedness

Theorem 2.1

Solutions of system(1)with initial conditions(3) and (4)are positive and bounded for all t ≥ 0.

Proof

First we show x2(t) > 0 for all t ≥ 0. Otherwise, noticing that x2(t) = φ2(t) > 0 for all −τ1 < t < 0.

Then there exists a t > 0, such that x2(t) = 0.

Now denoting t0 = inf  {t > 0 | x2(t) = 0}.

Then t0 > 0 and from system (1), we have

0t0τ1x˙2(t0)=r1(t0)ed11τ1ϕ2(t0τ1)>0.t0>τ1x˙2(t0)=r1(t0)ed11τ1x2(t0τ1)>0.
Hence x˙2(t0)>0. By the definition of t0, we have x˙2(t0)0. A contradiction.

Thus x2(t) > 0 for all t > 0.

Again, considering the following equation

u˙(t)=d11u(t)r1(t)ed11τ1u(tτ1)u(0)=x2(0)(6)

We can easily obtain that

u(t)=ed11tϕ2(0)0tr1(s)ϕ2(s)ds.
and
x1(t)>u(t)(0t<τ1).
From the initial conditions (3), we have u(τ1) = 0, then x1(t) > u(t) > 0 for 0 ≤ t < τ1.

By induction, we can show x1(t) > 0 for all t ≥ 0.

Similarly, we can prove that y1(t) > 0 and y2(t) > 0 for all t ≥ 0.

Next, we will prove the boundedness of the solutions of system (1).

Defining the function

V(t)=c2sx1(t)+c2sx2(t)+c1iy1(t)+c1iy2(t).
The derivative of V(t) along the positive solutions of system (1) is
V˙(t)=c2sx˙1(t)+c2sx˙2(t)+c1iy˙1(t)+c1iy˙2(t)(c2sr1sc2sd12)x2(t)c2sb1ix22(t)c2sd11x1(t)+(c1ir2sc1id21)y2(t)c1ib2iy22(t)c1id22y1(t).
For a positive constant d ≤ min  {d11, d22}, then
V˙(t)+dV(t)c2s(r1sd12+d)x2(t)c2sb1ix22(t)+c1i(r2sc1id21+d)y2(t)c1ib2iy22(t).
Hence there exists a positive number M, such that V˙(t)+dV(t)M. Then we get
V(t)Md1+(V(0)Md1)edt.
Therefore, the positive solutions of system (1) are bounded. This completes the Proof.  □

3 Permanence and non-permanence

Definition 3.1

If there exist positive constants mx, Mx, my and My, such that each solution (x1(t), x2(t), y1(t), y2(t)) of system (1) satisfies

0<mxliminft+xi(t)limsupt+xi(t)Mx(i=1,2),0<myliminft+yi(t)limsupt+yi(t)My(i=1,2).
Then system (1) is permanent. Otherwise, it is non-permanent.

Lemma 3.2 [22]

Consider the following equation:

u˙(t)=au(tτ)bu(t)cu2(t)
where a, b, c > 0;u(t) > 0 forτ ≤ t ≤ 0, we have
  • (1). If a > b, then limt+u(t)=abc,
  • (2). If a < b, then lim  t→+∞u(t) = 0.

Theorem 3.3

If the following assumptions hold

(H1).r1ix2i>r1sx2sed11τ1>0(H2).r2iy2i>r2sy2sed22τ2>0(7)
where
x2i=r1ied11τ1d12c1sLy2sb1s,x2s=r1sed11τ1d12b1i,y2i=r2ied22+c2iϕ(x2i)x2id21b2s,y2s=r2sed22τ2+c2sϕ(x2s)x2sd21b2i.
Then system(1)is permanent.

Proof

Let (x1(t), x2(t), y1(t), y2(t)) be any positive solution of system (1) for t ≥ 0. Firstly, from the second equation of system (1), we have

x˙2(t)r1(t)ed11τ1x2(tτ1)d12x2(t)b1(t)x22r1sed11τ1x2(tτ1)d12x2(t)b1ix22(t).
By Lemma 3.2 and standard comparison theorem, we get
limsupt+x2(t)r1sed11τ1d12b1iėx2s>0.
That is, for any ɛ > 0, there exists a T1 > 0, for any t > T1 > 0, such that x2(t)<x2s+ɛ.

Again, from the fourth equation of system (1), there exists a T2 > T1 > 0, for any t > T2, we have

y˙2(t)r2(t)ed22τ2y2(tτ2)d21y2(t)b2(t)y22+c2(t)ϕ(x2s+ɛ)(x2s+ɛ)y2(t)r2sed22τ2y2(tτ2)d21y2(t)b2iy22+c2sϕ(x2s+ɛ)(x2s+ɛ)y2(t).
We obtain that using Lemma 3.2 and standard comparison theorem
limsupt+y2(t)r2sed22τ2+c2sϕ(x2s+ɛ)(x2s+ɛ)d21b2i.
Since ɛ is sufficiently small, then
limsupt+y2(t)r2sed22τ2d21+c2sϕ(x2s)x2sb2iėy2s>0.
Hence, for this ɛ > 0, there exists a T3 > T2 > 0, for any t > T3 > 0, such that y2(t)<y2s+ɛ.

Now, substituting it into the second equation of system (1), we have

x˙2(t)r1(t)ed11τ1x2(tτ1)d12x2(t)b1(t)x22(t)c1(t)(y2s+ɛ)ϕ(x2(t))x2(t)r1ied11τ1x2(tτ1)d12x2(t)b1sx22(t)c1sL(y2s+ɛ)x2(t).
By the Lemma 3.2 and the standard comparison theory, noticing that ɛ is sufficiently small, we obtain that
liminft+x2(t)r1ied11τ1d12c1sLy2sb1sėx2i>0.
That is, for this ɛ > 0, there exists a T4 > T3 > 0, for any t > T4 > 0, such that x2(t)x2iɛ.

From the fourth equation of system (1), there exists a T5 > T4 > 0, for any t > T5 > 0, we have

y˙2(t)r2(t)ed22τ2y2(tτ2)d21y2(t)b2(t)y22(t)+c2(t)ϕ(x2iɛ)(x2iɛ)y2(t)r2ied22τ2y2(tτ2)d21y2(t)b2sy22(t)+c2iϕ(x2iɛ)(x2iɛ)y2(t).
Similarly, we obtain that
liminft+y2(t)r2ied22d21+c2iϕ(x2i)x2ib2sėy2i>0.
That is, for this ɛ > 0, there exists a T6 > T5 > 0, for any t > T6 > 0, such that y2(t)y2iɛ.

Again, from the first equation of system (1), we have

x˙1(t)r1(t)(x2s+ɛ)d11x1(t)r1(t)ed11τ1(x2iɛ)r1s(x2s+ɛ)d11x1(t)r1ied11τ1(x2iɛ).
Noticing that ɛ is sufficiently small, we get
limsupt+x1(t)r1sx2sr1ied11τ1x2id11ėx1s>0.
According to the third equation of system (1) for any t > T6, and by Lemma 3.2, we have
limsupt+y1(t)r2sy2sr2ied22τ2y2id22ėy1s>0.
By the analogous way, we obtain that
liminft+x1(t)r1ix2ir1sed11τ1x2sd11ėx1i>0.liminft+y1(t)r2iy2ir2sed22τ2y2sd22ėy1i>0.
Therefore, by Definition 3.1, system (1) is permanent. This completes the proof.  □

Theorem 3.4

If (H1) and the following assumption hold

(H3).r2sed22τ2+c2sϕ(x2s)x2s<d21(8)
where
x2s=r1sed11τ1d12b1i.
Then the prey population is permanent while the predators are non-permanent.

Proof

Let (x1(t), x2(t), y1(t), y2(t)) be any positive solution of system (1) for t ≥ 0. In order to prove the non-permanence of predators and permanence of prey population, we only show that

limt+yi(t)=0(i=1,2).
If the assumption (H3) holds, then we obtain that y2s<0 and y2i<0.

From the Proof of Theorem 3.3, there exists a T7 > T6 > 0, for any t > T7, we have

limsupt+y2(t)0,liminft+y2(t)0.(9)
Hence, we get
limt+y2(t)=0.
Again, substituting it into third equation of the system (1), we have
d22y1(t)y1(t)d22y1(t).
Integrate it from 0 to t, we get
0<y1(o)ed22ty1(t)y1(o)ed22t.
Thus, we obtain that
limt+y1(t)=0.
Therefore, the prey population is permanent and predators are non-permanent. This completes the proof.  □

4 Discussion

In this article, we have considered a stage-structured (for both interactive populations) predator-prey system with a class of functional response incorporating discrete time delay, and obtain the sufficient conditions for the permanence of system (1) and the non-permanence of predators. Our work has provided some valuable suggestions for regulating populations and saving endangered species. We see that the predators only forage on mature prey as food resource. Consequently, when the prey population size is lower than a certain level predators will not sustain. This is especially true for mature predators which are responsible for recruitment. In other words, as long as the population sizes of prey and predators maintain at a certain positive level, the predator-prey interaction will continue.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No. 30970478,31100306).


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This work was supported by the Fundamental Research Funds for the Central Universities (No. lzujbky-2011-48).

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