Predator-Prey Dynamics
Josh Baxter Alicia Sit
Final Project MAT3395
Department of Mathematics University of Ottawa April 28, 2017 Contents
1 Introduction 2
2 The model: Lotka-Volterra 2 2.1 Equilibria ...... 3 2.2 Stability ...... 3 2.3 Time evolution ...... 4
3 Modifying the model 5 3.1 Logistic growth ...... 5 3.2 Fuctional responses ...... 8 3.3 Competition ...... 9
4 Ecosystem 10 4.1 Food Web ...... 10 4.2 Creating the Model ...... 10 4.3 The Model ...... 12 4.4 Numerical Values ...... 14 4.5 Results ...... 15
5 Spatial Dependence 18 5.1 Solving this Model ...... 19 5.2 Initial Conditions ...... 21 5.3 Starting at the Same Position ...... 22 5.4 Starting at Di↵erent Positions ...... 25 5.5 Ideas For Improvement ...... 28
6 Conclusion 28
1 1 Introduction
The endeavor of this project is to investigate the evolution and dynamics of interacting animal popula- tions. Starting with a basic model with one predator and one prey population (Lotka-Volterra model), we will analyze in the usual way its equilibria points, stability and time evolution of the populations. In an e↵ort to bring this model closer to a realistic system, we will introduce various modifications that can be applied to the basic model. For example, limiting the growth rates of the prey, including the e↵ects of interspecies competition, and adding a functional response for the predation mechanism. Of course, in a realistic ecosystem, there are multiple predator and prey populations that interact. Combining the above mentionned modifications to the basic Lotka-Volterra equations, we numerically model the dynamics of a simple food web which include beetle, rabbit, fox and owl populations. As a bonus, we also present preliminary results if the animals are allowed to move around in a 2- dimensional space.
2 The model: Lotka-Volterra
Given two populations in which share a predator-prey relationship, we can describe their population dynamics using the basic Lotka-Volterra equations. This model was simultaneously and independently developed by Alfred Lotka and Vito Volterra in the 1920’s [1, 2].
dP = ↵P PV (1) dt dV = c PV V (2) dt where P and V are the prey and predator populations, respectively; ↵ is the prey birth rate; is the predation rate, i.e. the rate at which predators kill prey; c is the conversion rate from eaten prey to new predators; and is the predator death rate. All parameters are assumed to be positive. There are several assumptions built into this model. The first is that there is an unlimited food sup- ply for the prey (e.g. plants and vegetation for herbivores); in such a way, the prey maintain a constant birth rate, independent of population density. On the converse, the growth of the predator population relies solely on the presence of prey, and predators have limitless appetite. Another assumption is that prey do not live out their full life, but are inevitably eaten at some point for the duration of this
2 model. Predators, on the otherhand, can die of natural causes at a constant rate. Additionally, these assumptions mean that there are no seasonal variations that might negatively or postively a↵ect either population.
2.1 Equilibria
Let us investigate the potential equilibria of the Lotka-Volterrra equations. We begin setting Eq. 1 and Eq. 2 to zero:
dP ↵ 0 = = P(↵ V) = P¯ = 0 or V¯ = , (3) dt ) dV 0 = = V(c P ) = P¯ = or V¯ = 0. (4) dt ) c
We thus have two potential equilibria that give us both di↵erential equations zero simultaneously: ¯ ¯ ¯ ¯ ↵ (P = 0, V = 0) and (P = c , V = ). The first equilibrium is the morbid point where both populations are extinct; the second is a positive, real point.
2.2 Stability
To determine the stability of our two equilibria, we need to first calculate the Jacobian of the system. Recall that the Jacobian of our system is defined as,
@PP˙ @V P˙ J(P, V) = , (5) 2@PV˙ @V V˙ 3 6 7 6 7 46 57 where @ is shorthand for the partial derivative with respect to the subscripted symbol, and the dotted variables are shorthand for the time derivatives. The Jacobian is thus calculated to be,
↵ V V J(P, V) = . (6) 2 c Vc P 3 6 7 6 7 46 57 Evaluating this at our extinction equilirium point,
↵ 0 J(0, 0) = . (7) 20 3 6 7 6 7 46 57 3 Since this is a diagonal matrix, the eigenvalues are simply = ↵ and = . Both ↵ and are 1 2 positive by assumption, so we have a positive and negative equilibrium. Knowledge from our ordinary di↵erential equations (ODEs) classes tells us that (0, 0) is a saddle point, which is stable along one arm and unstable along the other in a P-V phase plane diagram. Thankfully (0, 0) is not completely stable! Otherwise no matter what our predator and prey would be doomed to extinction. We see that solutions with P = 0 will move towards (0, 0) along the stable arm; this can be interpreted as the predator population starving to death. On the other hand, solutions with V = 0 will move away from (0, 0) along the unstable arm, which occurs since the nothing is eating the prey and thus the population explodes. For non-zero populations, we need to analyze the second equilibrium point; the Jacobian evaluated at it is: ↵ 0 J , = c . (8) c 2 3 ! c↵ 0 6 7 6 7 Since this matrix is neither diagonal, nor upper triangle,46 we57 will calculate the characteristic equation to find the eigenvalues. To do so, we take the determinant,