Estimates of Size of Cycle in a PredatorPrey System
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Abstract
We consider a Rosenzweig–MacArthur predatorprey system which incorporates logistic growth of the prey in the absence of predators and a Holling type II functional response for interaction between predators and preys. We assume that parameters take values in a range which guarantees that all solutions tend to a unique limit cycle and prove estimates for the maximal and minimal predator and prey population densities of this cycle. Our estimates are simple functions of the model parameters and hold for cases when the cycle exhibits small predator and prey abundances and large amplitudes. The proof consists of constructions of several Lyapunovtype functions and derivation of a large number of nontrivial estimates which are of independent interest.
Keywords
Locating limit cycle Locating attractor Size of limit cycle Lyapunov function Lyapunov stabilityMathematics Subject Classification
Primary 34D23 34C05Introduction and Main Results
Rosenzweig–MacArthur systems incorporate logistic growth of the prey in the absence of predators and a Holling type II functional response (MichaelisMenten kinetics) for interaction between predators and preys. A literature survey shows that the model has been widely used in real life ecological applications, see e.g. [5, 17, 18, 19, 20], including the spatiotemporal dynamics of an aquatic community of phytoplankton and zooplankton [22] as well as dynamics of microbial competition [1, 8, 24].
From a mathematical point of view, the dynamics of systems of type (1.1) and (1.2) has been frequently studied, see e.g. [2, 3, 4, 6, 9, 11, 12, 13, 14, 15] and the references therein. In particular, system (1.2) always has a unique positive equilibrium at \(\left( x,s\right) \,=\, \left( \left( 1\lambda \right) \left( \lambda + a\right) , \lambda \right) \) which attracts the whole positive space when \(2\lambda + a \,>\, 1\). At \(2\lambda + a \,=\, 1\) there is a Hopf bifurcation in which the equilibrium loses stability and a stable limit cycle, surrounding the equilibrium, is created. In particular, for \(2\lambda + a \,<\, 1\) the equilibrium is a source and the cycle attracts the whole positive space (except the source) [2].
The above observations underscore the importance of understanding the dynamics of systems of type (1.2) under assumption (1.3). To further motivate our analytical estimates, we mention that it is nontrivial to obtain accurate numerical results by integrating the Eqs. (1.2) using standard numerical methods when a and \(\lambda \) are small, see section on numerical results in the end of the paper.
A nice and interesting study close to ours is [10] in which Hsu and Shi give estimates of the period of the cycle, and estimates of parts of this period when population is very small, large, increasing and decreasing. When death rate of predators tends to zero (which implies \(\lambda \rightarrow 0\)), they show that the limit cycle behaves similar to a nonlinear relaxation oscillator. When both \(\lambda \) and a tend to zero, they show that the prey (s) exhibits slow time scales when \(s \approx 0\) as well as when \(s \approx 1\). Hsu and Shi also show, without imposing restrictions on parameter values, that the prey is of order \(O(\lambda /b)\) (in our notations) and the predator is of order O(a) for a time scale of \(O(b/\lambda )\).
Before stating our main results, let us note that the above Rosenzweig–MacArthur systems are very simplified models of reality and therefore usually not directly applicable in biology without modifications. For example, it is clear from our main results that in model (1.2) predator and prey populations can decrease to unacceptable low abundances and still survive. However, even though our estimates are proved for such simple models, we believe that they are “good” in the sense of being useful when investigating dynamics also in more complex and realistic systems, such as, e.g., systems modeling the interactions of several predators and one prey, or seasonally dependent systems, see e.g. [1, 4, 16].
Our main results are summarized in the following theorem.
Theorem 1
Remark 1
In the last section of the paper we give some numerical results illustrating the precision of the estimates stated in Remark 1, see Figs. 9 and 10.
Corollary 1
Moreover, if \(\frac{H}{K}\) and \(\frac{d}{r}\) are small, then the estimate \(X_{min} \approx \frac{r K }{q}\exp {\left( \frac{q X_{max}}{r H}\right) }\) is good for the minimal predator density, and if \(\frac{H}{K}\) and \(\frac{dH}{rK}\) are small, then the estimate \(S_{min} \approx K \exp {\left( \frac{ q X_{max}}{d H }\right) }\) is good for the minimal prey density.
The proof of Theorem 1 consists of constructions of several Lyapunovtype functions and derivation of a large number of nontrivial estimates. We believe that these methods and constructions have values also beyond this paper as they present methods and ideas that, potentially, can be useful for proving analogous results for dynamics in similar systems as well as in more complex systems.
The proof is constructed in a way such that Theorem 1 is a direct consequence of four statements, namely Statements 1–4, which we prove in the following section. In addition to the estimates in Theorem 1 it is also possible to find, from these statements and lemmas, a positively invariant region trapping the unique limit cycle inside. In fact, the limit cycle will be inside an outer boundary consisting of the part of a trajectory \({\hat{T}}\) with initial condition \(x\left( 0\right) \,=\,1.6, \, s\,=\,\lambda \) and the part of \(s\,=\,\lambda \) between \(\left( 1.6, \lambda \right) \) and the next intersection with \(s\,=\,\lambda \) when \(x\,>\,h\left( \lambda \right) \). It will also be outside an inner boundary consisting of the part of a trajectory \(\check{T}\) with initial condition \(x\left( 0\right) \,=\,1, \, s\,=\,\lambda \) and the part of \(s\,=\,\lambda \) between \(\left( 1, \lambda \right) \) and the next intersection with \(s\,=\,\lambda \) when \(x\,>\,h\left( \lambda \right) \). Estimates for these boundaries can be found from given statements and lemmas, even if we do not write them explicitly here. We also point out that better but more complicated estimates than those summarized in Theorem 1 follow from lemmas which are used for the proofs of Statements 1–4 and Theorem 1.
The following section is devoted to the proof of Theorem 1, while we end the paper by giving a section on numerical results.
Proof of Theorem 1

Region 1, where \(x\,>\,h\left( s\right) , \ s\,>\,\lambda \) and x is growing and s decreasing.

Region 2, where \(x\,>\,h\left( s\right) ,\ s\,<\,\lambda \) and both x and s decrease.

Region 3, where \(x\,<\,h\left( s\right) ,\ s\,<\,\lambda \) and x decreases and s grows.

Region 4, where \(x\,<\,h\left( s\right) ,\ s\,>\,\lambda \) and both x and s increase.
Any trajectory starting in Region 1 will enter Region 2 from where it will enter Region 3 and then Region 4 and finally Region 1 again, and the behaviour repeats infinitely. Figure 2 illustrates the four regions together with isoclines and points which will be used in the proof of Theorem 1. Behaviour and estimates for trajectories in different regions are examined in different subsections. The main results in Regions 1–4 will be concluded in Statements 1–4.
Estimates in Region 1
We begin this section by proving a lemma which gives a bounded region into which all trajectories will enter after sufficient time and which will be used in several places in the proof of Theorem 1.
Lemma 1
Proof of Lemma 1
The maximal xvalue for a trajectory is attended when it escapes from Region 1 to Region 2. In this section we will give estimates for maximal xvalue, when trajectory starts on boundary of Region 1.
Statement 1
Any trajectory starting on the isocline \(x \,=\, h\left( s\right) , s \,>\, \lambda \) has a maximum \(x_0\) before it enters Region 2 and \(x_0 \,<\, 1.6\). Moreover, if the trajectory starts from a point where \(s \,>\, 0.9\), then \(x_0 \,>\, 1\).
We formulate the last part of the statement as a lemma with an own proof.
Lemma 2
Any trajectory starting on the isocline \(x\,=\,h\left( s\right) , s \,>\, 0.9\) has a maximum \(x_0\) before it enters Region 2 and \(x_0\,>\,1\).
Proof of Lemma 2
In Region 1 the xvalue on the trajectory is growing while the svalue is decreasing, and \(x'\) is smallest for greatest \(\lambda \) and \(s'\) is smallest for smallest a. This implies that in Region 1, for any \(a, \lambda \in [0,0.1)\), the xvalue for a trajectory of system (1.2) is always growing stronger than the xvalue for a trajectory of the system obtained for \(a\,=\,0\) and \(\lambda \,=\, 0.1\), since \(\vert \frac{dx}{ds}\vert \) will then be smallest. By this fact we are able to construct a bound for the minimal value of \(x_0\) by using system (1.2) with \(a\,=\,0\) and \(\lambda \,=\, 0.1\) fixed.
Moreover, for \(s\,=\,0.9\) we get \(f\left( s\right) \,=\,0.09\), meaning that the point \(\left( f\left( s\right) ,s\right) \,=\, \left( 0.09, 0.9\right) \) is on the isocline \(x \,=\, h\left( s\right) \) because \(h\left( 0.9\right) \,=\, 0.09\), see Fig. 4. We conclude that trajectories intersect the pieces of \(x\,=\,f\left( s\right) \) transversally going from the region defined by \(x\,<\,f\left( s\right) \) to region where \(x\,>\,f\left( s\right) \). The isocline of any system (1.2) under condition (1.3) is above the isocline for the system we considered, meaning x is greater and also trajectories cannot intersect \(x\,=\,f\left( s\right) \) before \(s\,<\,0.1\). Moreover \(f\left( 0.1\right) \,=\,1\). Thus any trajectory for any \(a,\lambda \in [0,0.1)\) under our conditions that start on the isocline \(x\,=\,h\left( s\right) \), \(s \,>\, 0.9\), will at \(s\,=\,0.1\) have an xvalue greater than 1 and consequently this holds also at \(s\,=\,\lambda \). Therefore, \(x_0 \,>\, 1\) and the proof of Lemma 2 is complete.
Proof of Statement 1
Estimates in Region 2 and Region 3
We consider a trajectory T of system (1.2) under condition (1.3) with initial condition \(x\left( 0\right) \,=\,x_0, \, s\left( 0\right) \,=\,\lambda \), where \(1\,<\,x_0\,<\,1.6\). We suppose \(c_1\) and \(c_2\) are such that \(0\,<\,c_2\,<\,c_1\,<\,\lambda \). If T intersects \(s\,=\,c_2\lambda \) before escaping Region 2 we denote the point of first intersection with \(s\,=\,c_1\lambda \) by \(P_1\,=\,\left( x_1, c_1\lambda \right) \) and the point of first intersection with \(s\,=\,c_2\lambda \) by \(P_2\,=\,\left( x_2, c_2\lambda \right) \). We denote the next intersection with the isocline \(x\,=\,h\left( s\right) \) by \(P_3\,=\,\left( x_3, s_3\right) \), where \(x_3\,=\,h\left( s_3\right) \). The second intersection with \(s\,=\,c_2\lambda \) we denote by \(P_4\,=\,\left( x_4, c_2\lambda \right) \) and the second intersection with \(s\,=\,c_1\lambda \) by \(P_5\,=\,\left( x_5, c_1\lambda \right) \). The next intersection with \(s\,=\,\lambda \) we denote by \(P_6\,=\,\left( x_6, \lambda \right) \). The lowest svalue of the trajectory before it escapes to Region 4 will be at \(P_3\) and the lowest xvalue at \(P_6\). The notations are illustrated in Fig. 2, where there are added also points used in Region 4. We point out that trajectory T is normally not a cycle, even though such case is illustrated in Fig. 2.
The main results in this section are given in Statements 2 and 3 which give main estimates in Regions 2 and 3. Statement 2 gives a lower and upper bound for minimal xvalue and Statement 3 gives a lower and upper bound for minimal svalue of the part of the trajectory in Regions 2 and 3. Lemma 3 gives a better upper estimate for lowest xvalue which is needed also in for the estimates in Region 4. These estimates will also serve as upper and lower estimates for the unique cycle of system (1.2) under condition (1.3). In the proofs of Statement 2 and Lemma 3 we assume \(c_1\,=\,e^ {2},\, c_2\,=\,e^{4}\).
We here give these three main results of this section.
Lemma 3
More general estimates than in Lemma 3 and Statement 2 are given in Lemmas 4 and 5. These are formulated for general choices of parameters \(c_1\) and \(c_2\), which are fixed in proofs of Lemma 3 and Statement 2.
Statement 2
From Statement 2 it follows that for small \(\frac{\lambda }{a}\) and a the estimate \(e^{\frac{x0}{a}}\) is good for the minimal xvalue on trajectory T.
Statement 3
From Statement 3 we see that for small \(\lambda ,\, a\) the estimate \(e^{\frac{x0}{\lambda }}\) is good for the minimal svalue on T.
The proof of Statement 2 is following from Lemma 3 and Statement 1 and a short Lemma 14. The proofs of Lemmas 3–5 are built on Lemmas 6–9. Lemmas 6–7 give estimates for trajectory from start to \(P_2\) (\(c_2\lambda \,<\,s\,<\, \lambda \) in Region 2). Lemma 8 gives estimate of the behaviour between \(P_2\) and \(P_4\) (\(s\,<\,c_2\lambda \)) and Lemma 9 for the behaviour between \(P_4\) and \(P_5\) (\(c_2\lambda \,<\,s\,<\, c_1\lambda \) in Region 3). Lemmas 6–9 use more new lemmas about which we inform later. The section ends with the proof of Statement 3.
Before we start with the proofs of the Statements and Lemma 3 we introduce Lemmas 4 and 5. Lemma 5 can be seen as corollary from Lemma 4. The proof of Lemma 3 is very similar to proof of Lemma 4. We wish to formulate the most general upper estimate for \(x_6\) in Lemma 4. We find such an estimate in the case T intersects \(s\,=\,c_2\lambda \) before escaping Region 2 using auxiliary estimates for \(x_1, \, x_2, \, x_4\) and \(x_5\). For the estimate we need some notations and assumptions.
Lemma 4
Lemma 5
Because \(C_i, \, i\,=\,1,2\) depend only on \(\lambda \), \(D^*_i\) only on \(\lambda \) and \(c_1\), \(\tilde{x}_ 2\) does not depend on a, only on \(\lambda ,\, c_1\) and \(x_0\). If we choose \(c_1\,=\,e^{2},\,c_2\,=\,e^{4}\) we are able to prove that assumptions (2.7), (2.9) and (2.12) are satisfied and get Lemma 3.
Lemma 4 is based on Lemmas 6, 8 and 9. We now give these lemmas and also Lemma 7 needed for Lemma 3. Lemma 7 can be seen as a corollary of Lemma 6.
Lemma 6
Using Lemma 6 for special values of \(c_i\) after calculating some quantities we get a corollary.
Lemma 7
Lemma 8
Lemma 9
Lemma 10
If the equation \(R\left( x\right) \,=\,C\) has a solution \(x\,=\,{\bar{x}}\), \(H \,<\, {\bar{x}} \,<\, u\), then the trajectory \(T_2\) intersects \(s\,=\,c_2^*\lambda \) before escaping Region 2 at a point \(\tilde{P} \,=\,\left( \tilde{x}, c_2^*\lambda \right) \), where \({\tilde{x}} \,>\, \bar{x}\).
Lemma 11
Proof of Lemma 10
Derivation with respect to time gives \(U'\left( x,s\right) \,=\, \left( h\left( s\right) H\right) \left( s\lambda \right) \,>\,0\) in Region 2. Thus, because \(U\left( x,s\right) \) increases in x, the trajectory \(T_2\) will remain in the region defined by \(x\,>\,x\left( s\right) \) until it intersects \(s\,=\,c_2^*\lambda \) at \({\tilde{P}}\) with \({\tilde{x}}\,>\, {\bar{x}}\). On trajectory part \(U\left( x,s\right) \,>\,U\left( x\left( s\right) ,s\right) \,=\,U\left( u,c_1^*\lambda \right) \). \(\square \)
Proof of Lemma 11
To prove the second inequality we note that the function Q is increasing in H for \(x\,<\,u\) and decreasing in u for \(x\,>\,H\), from which we conclude that \(x_+\) is decreasing in H and increasing in u and, therefore, \(\frac{H_m}{x_m}\,>\,\frac{H}{x_+}\) which implies (2.17). Thus, both inequalities of the lemma are proved and the proof is complete. \(\square \)
We can now prove Lemma 6.
Proof of Lemma 6
From Lemmas 10 and 11 with \(c_1^*\,=\,1, \, c_2^*\,=\,c_1, C\,=\,C_1,\, H\,=\,H_0, \, u\,=\,x_0\) it follows that the trajectory T intersects \(s\,=\,c_1\lambda \) before escaping Region 2, and that for this intersection the first inequality in Lemma 6 holds. Using Lemmas 10 and 11 once again, this time with \(c_1^*\,=\,c_1, \, c_2^*\,=\,c_2,\, C\,=\,C_2,\, H\,=\,H_1, \, u\,=\,x_1\), we conclude that T also intersects \(s\,=\,c_2\lambda \) before escaping Region 2, and that for this intersection the second inequality in Lemma 6 holds. Indeed, to see that we can apply Lemma 11 here we observe that \( u \,=\, x_1 \,>\, x_1^+ \,>\, \sqrt{H_0 x_0} \,>\, a + \lambda \,>\, H_1. \) Finally, we notice that \(H_0\) and \(H_1\) take their maximal values for \(a\,=\,\lambda \,=\,0.1\). Thus, the possibility to replace \(x_i^+\) by \(x_i^*\) follows from inequality (2.17). \(\square \)
Proof of Lemma 7
We intend to use Lemma 6. Let \(c_1 \,=\, e^{2}\) and \(c_2 \,=\, e^{4}\). Using (2.5) and (2.6) we find \(C_1 \,>\, 1.135 \lambda ,C_2 \,>\, 1.883 \lambda ,H_0 \,<\, 0.2\) and \(H_1 \,<\, 0.1136\). Equation (2.8) with \(x_0 \,=\, 1\) yields \(x_1^* \,>\, 0.851\) and (2.10) with \(x_1^+ \,=\, 0.851\) yields \(x_2^* \,>\, 0.620\). Using these estimates we obtain \(D_1 \,=\, 1  \frac{H_0}{x_1^*} \,>\, 0.764, \, D_2\,=\, 1  \frac{H_1}{x_2^*} \,>\, 0.816\) and \(\frac{C_1}{D_1} + \frac{C_2}{D_2} \,>\, 3.8 \lambda \). Now, we note that the above estimates imply assumptions (2.7) and (2.9), and Lemma 7 now follows by an application of Lemma 6. \(\square \)
Lemma 12
Let \({\hat{x}}\) be the solution to \(\theta \left( x\right) \,=\,\theta \left( u\right) , \, x\,<\,H\,=\,a+\lambda ^*\) and \(\check{x}\) the solution to \(\theta \left( x\right) \,=\,\theta \left( u\right) , \, x\,<\,H\,=\,a\). Then for the next intersection of trajectory \(T^*\) with \(s \,=\, \lambda ^*\) at \(P\,=\,\left( v,\lambda ^*\right) \), it holds that \(\check{x}\,<\,v\,<\,{\hat{x}}\).
Next lemma gives estimate for the equation in previous lemma.
Lemma 13
Proof of Lemma 12
As in the proof of Lemma 10 we will make use of the function U defined in (2.18) to construct barriers for the trajectory \(T^*\). We note that U is decreasing in s, increasing in x for \(x \,>\, H\) and decreasing in x for \(x \,<\, H\). Moreover, \(\theta \left( x\right) \,=\, U\left( x,\lambda ^*\right) \).
We first prove the upper bound \(v \,<\, \hat{x}\). Let \(H\,=\,a+\lambda ^*\) and let \({\bar{s}}\left( x\right) \) be the level curve to U such that \(U\left( x, {\bar{s}}\left( x\right) \right) \,=\, \theta \left( u\right) \). The curve \({\bar{s}}\left( x\right) \) will have a minimum at \(x \,=\, H \,=\, a+\lambda ^*\) and intersect \(\lambda ^*\) at \(x \,=\, {\hat{x}}\) and also at \(x \,=\, u\). Observe that, since \(h\left( s\right) \,<\, h\left( \lambda ^*\right) \,<\, \lambda ^* + a \,=\, H\), the derivative of U with respect to time is positive: \(U'\,=\,\left( h\left( s\right) H\right) \left( s\lambda \right) \,>\, 0\). Therefore, the trajectory \(T^*\) must stay below the curve \({\bar{s}}\left( x\right) \). On trajectory \(T^*\) we have \(U\left( x,s\right) \,>\, U\left( x, {\bar{s}}\left( x\right) \right) \,=\, \theta \left( u\right) \,=\, \theta \left( {\hat{x}}\right) \). Hence, recalling that U is decreasing in x for \(x\,<\,H\), we have \(v \,<\, {\hat{x}}\) and the upper bound follows.
The proof of the lower bound \(\check{x} \,<\, v\) is similar. Let \(H \,=\, a\) and let \(\underline{s}\left( x\right) \) be the level curve to U such that \(U\left( x, \underline{s}\left( x\right) \right) \,=\, \theta \left( u\right) \). In this case, the derivative of U with respect to time is negative, and thus the trajectory \(T^*\) must stay above the curve \(\underline{s}\left( x\right) \). On trajectory \(T^*\) we have \(U\left( x,s\right) \,<\, U\left( x, \underline{s}\left( x\right) \right) \,=\, \theta \left( u\right) \,=\, \theta \left( \check{x}\right) \), and it follows also that \(\check{x} \,<\, v\).
Proof of Lemma 13
It is clear that Eq. (2.19) must have a solution because \(\theta \left( H\right) \,<\,C\) and \(\theta \left( x\right) \rightarrow \infty \) for \(x\rightarrow 0_+\). The solution is unique because \(\theta \) is decreasing for \(x\,<\,H\). Moreover, since \(e^{C/H} \,<\, H\) and \(C \,<\, \theta \left( e^{C/H}\right) \), a solution \({\bar{x}}\) of \(\theta \left( x\right) \,=\, C\) must satisfy \({\bar{x}} \,>\, e^{C/H}\), which proves the first inequality in Lemma 13.
We are now ready with the proofs of Lemma 12 and 13 and can use them for proving Lemma 8. \(\square \)
Proof of Lemma 8
The result follows from Lemmas 12 and 13 by taking \(H \,=\, H^*\) and \(C \,=\, \tilde{\theta }\left( u\right) \). We observe that we will have \({\tilde{k}} \,>\,4\) because \(u\,>\,2.5H^*\).
Now only Lemma 9 is left to be proved in order to give the proofs of Lemmas 3–5.
Proof of Lemma 9
We have now finished the proofs of all auxiliary results needed for Lemmas 35 and we will now continue by proving these lemmas.
Proof of Lemma 4
Proof of Lemma 5
The proof is analogous to the proof of Lemma 4. We only use Lemma 6 so that we replace \(x_2^+\) by \(x_2^*\) and modify it by taking as \(H_0\) and \(H_1\) the values they get for \(a\,=\,\lambda \,=\,0.1\).
Proof of Lemma 3
In order to prove Statement 2 we need one more lemma. The proof of it follows by using Lemma 13 with \(C \,=\, \theta \left( u\right) \) and \(H \,=\, a\), but it can also be proved shortly directly.
Lemma 14
Equation \(\theta \left( x\right) \,=\,\theta \left( u\right) , \, u\,>\,1\), where \(\theta \left( x\right) \,=\,x a\ln \left( x\right) , \, a\,<\,0.1\), has a unique solution \(\bar{x}\) in \(\left( 0,a\right) \) and \({\bar{x}} \,>\, e^{\frac{u}{a}} \,=\, \check{x}\).
Proof
We first note that \(\theta \) is decreasing in \(\left( 0,a\right) \) and that \(\theta \) has its global minimum at a. Moreover, \(\theta \left( u\right) \,=\, u  a \ln \left( u\right) \,<\, u/a + e^{u/a} \,=\, \theta \left( \check{x}\right) \). Therefore, \( \theta \left( a\right) \,<\, \theta \left( u\right) \,<\, \theta \left( \check{x}\right) \) and thus there is a unique solution to \(\theta \left( x\right) \,=\, \theta \left( u\right) \) between \(\check{x}\) and a. \(\square \)
We have now finished the proofs of all auxiliary lemmas and will proceed to the proofs of our main results for this section; Statements 2 and 3.
Proof of Statement 2
Lemmas 12 and 14 together give the lower estimate in Statement 2 if we use \(\lambda ^*\,=\,\lambda \) and \(u\,=\,x_0\).
Proof of Statement 3
Estimates in Region 4
We again consider a trajectory T of system (1.2) under conditions (1.3) with initial condition \(x\left( 0\right) \,=\,x_0\,>\,1, \, s\left( 0\right) \,=\,\lambda \). We are interested in the behaviour of the trajectory in Region 4. The trajectory enters Region 4 at point \(P_6\,=\,\left( x_6,\lambda \right) \). We are interested in the next intersection of the trajectory with \(s\,=\,s_7\,>\,0.5\) at point \(P_7\,=\,\left( x_7,s_7\right) \) (if it occurs before escaping Region 4) and of the next intersection with the isocline \(x\,=\,h\left( s\right) \) at \(P_8\,=\,\left( x_8,s_8\right) \), where \(x_8\,=\,h\left( s_8\right) \). Lemma 3 from previous section gives an estimate for \(x_6\) and we are able to show that for such \(x_6\) the trajectory will intersect \(s\,=\,0.8\) before escaping Region 4 and the escaping occurs at \(P_8\), where \(s_8\,>\,0.9\).
The main result is Statement 4 which is based on the following two lemmas.
Lemma 15
The trajectory T after intersecting \(s\,=\,\lambda \) next time always intersects \(s\,=\,0.8\) at a point \(P_7\,=\,\left( x_7,0.8\right) \), where \(x_7 \,<\, 0.012\), before escaping Region 4.
Lemma 16
If trajectory T after intersecting \(s\,=\,\lambda \) next time intersects \(s\,=\,0.8\) at a point \(P_7\,=\,\left( x_7,0.8\right) \) where \(x_7 \,<\, 0.012\) then it intersects the isocline \(s'\,=\, 0\) next time for an svalue greater than 0.9.
From these lemmas follows
Statement 4
Trajectory T after intersecting \(s\,=\,\lambda \) at \(P_4\) escapes from Region 4 at an svalue greater than 0.9.
The trajectory in Region 4 is well estimated by \(x\,=\,x_6 B\) for \(s\,<\,0.8\), where B is defined in (2.26). For \(s\,>\,0.8\) expression (2.35) gives a onesided estimate for the trajectory while remaining in Region 4 and (2.36) gives an estimate for \(s_8\) substituting \(m\,=\,0.8\).
The proof of Lemma 16 is at the end of the section. The proof of Lemma 15 is based on some lemmas we provide here. Lemma 15 uses Lemma 18 and Lemma 19. Lemma 18 gives us necessary conditions in form of inequalities for the trajectory to intersect \(s\,=\,s_7\) before escaping Region 4 and an estimate for xvalue at intersection point \(P_7\). Lemma 19 tells us that we have to check the inequalities only for \(a,\lambda \,=\,0.1\) to be sure they hold for all other parameters. Lemma 18 is based on Lemmas 17 and 3, where Lemma 17 gives estimates in Region 4 and Lemma 3 takes care of estimates for trajectory in Regions 2 and 3. Lemma 17 is based on Lemmas 20 and 21. Lemma 20 gives us estimates for trajectory in a part of Region 4 and Lemma 21 tells us that we need to check these estimates only for \(s\,=\,\lambda \) and \(s\,=\,s_7\) in order to be sure the trajectory will stay in the region.
We now give Lemmas 17–19. Lemma 19 can be proved directly, but the proof of Lemma 17, which is needed for proving Lemma 18, needs more lemmas and is given later.
Lemma 17
Lemma 18
The following Lemma tells us that to prove that (2.25) holds for all \(a, \lambda \in [0,0.1)\), it is enough to prove the inequality for \(a \,=\, \lambda \,=\, 0.1\) fixed in the left hand side, i.e., \(\eta \left( 0.1, 0.1\right) \,<\, \left( 1k\right) \, h\left( s_7\right) \).
Lemma 19
The derivatives of \(\eta \left( a,\lambda \right) \) with respect to a and \(\lambda \) are positive if \(k\,\ge \, 0.9\).
Proof
Lemma 20
Lemma 21
Suppose \(x_6\,<\,\left( 1k\right) \, h\left( \lambda \right) \) and \(x_6 \left( B_7 \right) ^{\frac{1}{k}}\,<\, \left( 1k\right) \, h\left( s_7\right) \), where \(B_7\) is the value B takes for \(s\,=\,s_7\,\ge \, 0.5\). Then the trajectory T intersects \(x\,=\,\left( 1k\right) \, h\left( s\right) \) next time after \(P_6\) for \(s\,>\,s_7\) and is inside the region determined by \(x\,<\,\left( 1k\right) \, h\left( s\right) \) before it intersects \(s\,=\,s_7\).
Proof of Lemma 20
Proof of Lemma 21
We are now ready with proofs of Lemmas 20 and 21 and can use them for getting proofs of Lemmas 17 and 18.
Proof of Lemma 17
The proof follows from Lemmas 20 and 21. Lemma 21 tells that the trajectory will be inside the region \(x\,<\,\left( 1k\right) \, h\left( s\right) \) and then Lemma 20 gives us the necessary estimates. \(\square \)
Finally, we are ready with all proofs of auxiliary results and can prove the main Lemmas 15 and 16 from which Statement 4 follows.
Proof of Lemma 15
We choose \(k\,=\,0.9\) and \(s_7\,=\,0.8\) and calculate \(\eta \left( 0.1,0.1\right) \,<\, 0.012 \,<\, \left( 1k\right) \, h\left( s_7\right) \) and then from Lemma 19 it follows that inequality (2.25) holds for all \(a,\lambda \in [0,0.1)\). Since \(\eta \left( a,\lambda \right) \,<\, 0.012\) it follows that \(x_6\, K_7 \left( \frac{1}{a+\lambda }\right) ^{\frac{1}{k}} \,<\, 0.012\) and because \(K_7 \,>\, 1\) we also get, using \(k \,=\, 0.9\), that \(x_6 \,<\, 0.012 \cdot \left( a+\lambda \right) \,<\, \left( 1k\right) \, h\left( \lambda \right) \). Lemma 15 now follows by an application of Lemma 18. \(\square \)
Proof of Lemma 16
Numerical Results
Before comparing our analytical estimates to numerical simulations, let us mention that to achieve accurate numerics of system (1.2) under assumption (1.3) we recommend transforming the equations (e.g. log transformations) to avoid variables taking on very small values. Imposing linear approximations near the unstable equilibria at \((x,s) = (0,0)\) and \((x,s) = (0,1)\) are also helpful. Indeed, using e.g. the MATLAB odesolver ODE45 directly on system (1.2) may result in trajectories not satisfying Theorem 1, when \(a \,\le \, 0.2\) and \(\lambda \,\le \, 0.2 a\), unless tolerance settings are forced to minimum values. The true trajectory approaches much smaller population densities and also spend more time at these very low population abundances. Therefore, one has to be careful, since such misleading numerical results would give, e.g., a far to good picture of the populations chances to survive from any perturbation.
In Fig. 8 the minimal predator and prey abundances, \(x_{min}\) and \(s_{min}\), are plotted for the unique limit cycle. We observe that the minimal predator and prey abundances decrease as a approaches zero, as well as when \(\lambda \) approaches zero. The analytical estimates for \(x_{min}\) and \(s_{min}\) are produced by using the corresponding estimate for the maximal xvalue, \(1< x_{max} < 1.6\), given in Theorem 1.
We end this section by plotting the functions \({\tau _{x}}\) and \({\tau _{s}}\), for small values of a, as functions of \(\lambda \) in Fig. 10 together with the analytical estimates for \({\tau _{x}}\) and \({\tau _{s}}\) given by \(\kappa _1\), \(\kappa _2\) and \(\kappa _3\) in Theorem 1. As \(\lambda \) and \(\frac{\lambda }{a}\) approaches zero, the lower estimate for \(\tau _s\) approaches 1 (\(\kappa _2 \rightarrow 1\)) while \(\kappa _1\) and \(\kappa _3\), giving upper estimates of \({\tau _{x}}\) and \({\tau _{s}}\), stays a bit away from 1 for all \(\lambda \).
Notes
Acknowledgements
We are very grateful to the referees for several helpful comments concerning the manuscript.
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