## Volterra predator

You can report issue about the content on this page here Want to share your content on R-bloggers? So many know about the Lotka-Volterra model i. This model portrays two species, the predator y and the prey xinteracting each other in limited space. The prey grows at a linear rate and gets eaten by the predator at the rate of. The predator gains a certain amount vitality by eating the prey at a ratewhile dying off at another rate. Given this base, we can ask questions like, what parameterizations can we expect to find a coexistence between the fox and the hare for example?

These values assume a weaker growth of the rabbits relative to the strength of the death of foxes. Below, I simulated these values in R. And we get coexistence, they live happily forever after. With this simple model, we can play around by generalizing logistic growth of prey, etc.

I will put up some posts doing so. The way to do this in R is as follows just use the deSolve packagewhich will supersede the odesolve package :.

Want to share your content on R-bloggers? Never miss an update! Subscribe to R-bloggers to receive e-mails with the latest R posts. You will not see this message again.This example describes how to integrate ODEs with the scipy.

We will have a look at the Lotka-Volterra model, also known as the predator-prey equations, which is a pair of first order, non-linear, differential equations frequently used to describe the dynamics of biological systems in which two species interact, one a predator and the other its prey. The model was proposed independently by Alfred J. Lotka in and Vito Volterra inand can be described by. Before using! SciPy to integrate this system, we will have a closer look at position equilibrium.

Equilibrium occurs when the growth rate is equal to 0. This gives two fixed points:. We have to define the Jacobian matrix:. The origin is therefore a saddle point. Now we will use the scipy. This module offers a method named odeint, which is very easy to use to integrate ODEs:. Type "info odeint " if you want more information about odeint inputs and outputs. We will use Matplotlib's colormap to define colors for the trajectories.

These colormaps are very useful to make nice plots. Have a look at ShowColormaps if you want more information. This graph shows us that changing either the fox or the rabbit population can have an unintuitive effect.

If, in order to decrease the number of rabbits, we introduce foxes, this can lead to an increase of rabbits in the long run, depending on the time of intervention. Plotting iso-contours of IF can be a good representation of trajectories, without having to integrate the ODE. SciPy Cookbook latest. Definition of the equations:. The fox and rabbit populations are periodic as follows from further analysis. Integrating the ODE using scipy. We can now use Matplotlib to plot the evolution of both populations:.Put one predator in cages with different densities of prey and estimate prey mortality rate and corresponding k-value in each cage.

As we know, k-value equals to the instantaneous mortality rate multiplied by time. Thus, the predation rate a equals to the k-value divided by the duration of experiment. Example: lady-beetle killed 60 aphids out of in 2 days. Note: if a -values estimated at different prey densities are not close enough to each other, then the Lotka-Volterra model will not work!

However, the model can be modified to incorporate the relation of a to prey density. Estimation of parameters b and m: Keep constant density of prey e. Plot the intrinsic rate of predator population increase versus prey density: The linear regression of this line is: Note: If points do not fit to a straight line e.

Nowparameters b and m can be taken from this regression equation. How to solve differential equations There are two major approaches: analytical and numerical.

Analytical methods are complicated and require good mathematical skills.

## Competitive Lotka–Volterra equations

Also, many differential equations have no analytical solution at all. Numerical methods are easy and more universal however, there are problems with convergence.

The simplest and least accurate is the Euler's method. Consider a stationary differential equation: First we need initial conditions. We will assume that at time t o the function value is x t o. Now we can estimate x-values at later or earlier time using equation:. This is called Runge-Kutta method of second order. The most popular is the Runge-Kutta method of the fourth order.

However, for our purposes it is enough to use the second order method.Sure, I can give lots of them. For each of C, R, and F I will give several equations, increading in realisticness. This assumes the number of carrots is fixed they do not get killed, or propagatea is the total growth due to sunlight, and b is how much is eaten by a rabbit.

This implies unlimited exponential growth in the abscence of rabbits. This is called a 'logistic' equation, it assumes a 'maximum mass of carrots' called MaxC. This is appropriate in the likely event that you are dealing with a finite area for your ecology experiment.

This ODE gives an 'S shaped curve', rapidly growing from 0, and slowly approaching 1. This is the best equation to use, unless you are looking for an easy equation. This ignores the effect that if carrots get low, the rabbits will have a hard time finding them.

## R-bloggers

In many situations, the foxes keep the rabbits from eating too many carrots. Rabbit equations. The only good thing about that is it leads to the simplest nonlinear term.

This is the standard lame equation ignoring for the moment rabbit food. This is a pretty good equation for the rabbit food aspect, ignoring the foxes for the moment.

Here we assume the foxes have no trouble finding rabbits, and each one eats h rabbits per unit time. Here -m is the death rate due to starvation when there are no rabbits around. Before working with this set of equations, simplify by 'scaling away parameters'. You can scale four paramiters C, R, F, and time which will allow you to eliminate 4 of the 11 parameters.

The simplist is to measure C in units of maxC, so that maxC becomes 1. There are many ways to simplify the remaining equations. Scaling time by a eliminates the a.

This leads to the parameters will now have different values :. The first step in the analysis is to find the nontrivial critical points, which are values of C,R,F where all right hand sides are zero.

The parameter n does not affect the critical point. Next you classify this critical point. Find the three eigenvalues of the frechet derivative this is a three by three matrix of partial derivatives evaluated at the CP.

If you are looking for a cyclical population situation, you probably need a complex conjugate pair with positive real part, or maybe a pure imaginary complex pair, and the third eigenvalue a positive real.

Finding the regions in b,e,f,h,m,p space where these two events happen would be nice, but likely involve a lot of UGLY work, maybe Monte Carlo simulation. Contact me for advice. It might be a good time to find some realistic value ranges for b, e, f, h, m, n, and p. Ralph Kelsey -- R Kelsey's contribution is much better than the article itself; both in terms of depth, as well as in clarifying the physical significance of each term.

What a shame R Kelsey is not accessible, as he deserves to be encouraged to adapt his contribution to the main article. I also regret that R Kelsey did not leave a reference. His last paragraph points out a way: I can surely anticipate the outcome, but would be glad to have a book as my companion enroute.

BTW my initial reaction is to be skeptical of the independence of the parameter n RK writes takes note that this is 'interesting'. I have not done the work, but I can't suppress suspicion that this may mask a computational fault. R Kelsey's contribution is a great example of a population model with three species. But it does not belong in this article.The Lotka—Volterra equationsalso known as the predator—prey equationsare a pair of first-order nonlinear differential equationsfrequently used to describe the dynamics of biological systems in which two species interact, one as a predator and the other as prey.

The populations change through time according to the pair of equations:. The Lotka—Volterra system of equations is an example of a Kolmogorov model[1] [2] [3] which is a more general framework that can model the dynamics of ecological systems with predator—prey interactions, competitiondisease, and mutualism.

The Lotka—Volterra predator—prey model was initially proposed by Alfred J. Lotka in the theory of autocatalytic chemical reactions in D'Ancona studied the fish catches in the Adriatic Sea and had noticed that the percentage of predatory fish caught had increased during the years of World War I — This puzzled him, as the fishing effort had been very much reduced during the war years.

Volterra developed his model independently from Lotka and used it to explain d'Ancona's observation. The model was later extended to include density-dependent prey growth and a functional response of the form developed by C.

Holling ; a model that has become known as the Rosenzweig—MacArthur model. In the late s, an alternative to the Lotka—Volterra predator—prey model and its common-prey-dependent generalizations emerged, the ratio dependent or Arditi—Ginzburg model. The Lotka—Volterra equations have a long history of use in economic theory ; their initial application is commonly credited to Richard Goodwin in [18] or The Lotka—Volterra model makes a number of assumptions, not necessarily realizable in nature, about the environment and evolution of the predator and prey populations: [21].

In this case the solution of the differential equations is deterministic and continuous. This, in turn, implies that the generations of both the predator and prey are continually overlapping.

If either x or y is zero, then there can be no predation. With these two terms the equation above can be interpreted as follows: the rate of change of the prey's population is given by its own growth rate minus the rate at which it is preyed upon. Note the similarity to the predation rate; however, a different constant is used, as the rate at which the predator population grows is not necessarily equal to the rate at which it consumes the prey.

Hence the equation expresses that the rate of change of the predator's population depends upon the rate at which it consumes prey, minus its intrinsic death rate. The equations have periodic solutions and do not have a simple expression in terms of the usual trigonometric functionsalthough they are quite tractable.

It is the only parameter affecting the nature of the solutions. Suppose there are two species of animals, a baboon prey and a cheetah predator. If the initial conditions are 10 baboons and 10 cheetahs, one can plot the progression of the two species over time; given the parameters that the growth and death rates of baboon are 1.

The choice of time interval is arbitrary. One may also plot solutions parametrically as orbits in phase spacewithout representing time, but with one axis representing the number of prey and the other axis representing the number of predators for all times. This corresponds to eliminating time from the two differential equations above to produce a single differential equation.An application has been performed on the Lotka--Volterra predator-prey system, turning to a strongly nonlinear differential equation in the phase variables.

We might use a system of differential equations to model two interacting species, say where one species preys on the other. A good historical data are known for the populations of the lynx and snowshoe hare from the Hudson Bay Company. The Hudson Bay Company kept accurate records on the number of lynx pelts that were bought from trappers from to The company noticed that the number of pelts varied from year to year and that the number of lynx pelts reached a peak about every ten years.

The ten year cycle for lynx can be best understood using a system of differential equations.

**PD Tutorial 6-1: Predator and Prey model**

The primary prey for the Canadian lynx is the snowshoe hare. We will denote the population of hares by H t and the population of lynx by L twhere t is the time measured in years. We will make the following assumptions for our predator-prey model. The Lotka--Volterra system of equations is an example of a Kolmogorov model, which is a more general framework that can model the dynamics of ecological systems with predator-prey interactions, competition, disease, and mutualism.

The equations which model the struggle for existence of two species prey and predators bear the name of two scientists: Lotka and Volterra They lived in different countries, had distinct professional and life trajectories, but they are linked together by their interest and results in mathematical modeling.

The predator—prey model was initially proposed by Alfred J. Lotka in the theory of autocatalytic chemical reactions in Inhe utilized the equations to analyze predator-prey interactions. Lotka published almost a hundred articles on various themes in chemistry, physics, epidemiology or biology, about half of them being devoted to population issues.

He also wrote six books.

The same set of equations was published in by Vito Volterraa mathematician and physicist, who had become interested in mathematical biology because of the impact by the marine biologist Umberto D'Ancona He attended the University of Pisa, where he became professor of rational mechanics in His most famous work was done on integral equations. Inhe became professor of mechanics at the University of Turin and then, inprofessor of mathematical physics at the University of Rome La Sapienza.

The predator-prey system of equations was later extended by many researchers, including C. The critical points of the Lotka--Volterra system of equations are the solutions of the algebraic equations. We may try to find the general solution of the Lotka--Volterra system of equations. From both equations, we get. Notice that the predator population, Lbegins to grow and reaches a peak after the prey population, H reaches its peak.

As the prey population declines, the predator population also declines. Once the predator population is smaller, the prey population has a chance to recover that the cycle begins again. Dimitrov and H. Email: Prof. Vladimir Dobrushkin. Predator-Prey Equations Some situations require more than one differential equation to model a particular situation.The competitive Lotka—Volterra equations are a simple model of the population dynamics of species competing for some common resource.

They can be further generalised to include trophic interactions. The form is similar to the Lotka—Volterra equations for predation in that the equation for each species has one term for self-interaction and one term for the interaction with other species.

In the equations for predation, the base population model is exponential. For the competition equations, the logistic equation is the basis. The logistic population model, when used by ecologists often takes the following form:. Here x is the size of the population at a given time, r is inherent per-capita growth rate, and K is the carrying capacity. Given two populations, x 1 and x 2with logistic dynamics, the Lotka—Volterra formulation adds an additional term to account for the species' interactions.

Thus the competitive Lotka—Volterra equations are:. These values do not have to be equal. Also, note that each species can have its own growth rate and carrying capacity.

A complete classification of this dynamics, even for all sign patterns of above coefficients, is available, [1] [2] which is based upon equivalence to the 3-type replicator equation.

This model can be generalized to any number of species competing against each other. Then the equation for any species i becomes. A simple 4-Dimensional example of a competitive Lotka—Volterra system has been characterized by Vano et al.

This system is chaotic and has a largest Lyapunov exponent of 0. From the theorems by Hirsch, it is one of the lowest-dimensional chaotic competitive Lotka—Volterra systems.

The Kaplan—Yorke dimension, a measure of the dimensionality of the attractor, is 2. This value is not a whole number, indicative of the fractal structure inherent in a strange attractor. The coexisting equilibrium pointthe point at which all derivatives are equal to zero but that is not the origincan be found by inverting the interaction matrix and multiplying by the unit column vectorand is equal to. Note that there are always 2 N equilibrium points, but all others have at least one species' population equal to zero.

The eigenvalues of the system at this point are 0.

### Talk:Lotka–Volterra equations

This point is unstable due to the positive value of the real part of the complex eigenvalue pair. If the real part were negative, this point would be stable and the orbit would attract asymptotically. The transition between these two states, where the real part of the complex eigenvalue pair is equal to zero, is called a Hopf bifurcation. A detailed study of the parameter dependence of the dynamics was performed by Roques and Chekroun in.

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