Location: Bowdoin / Mary Lou Zeeman / 3-D Lotka-Volterra Systems

# Mathematics

## Geometric analysis of Lotka-Volterra models of species competition in n-dimensions

The theme of this work is to translate the quadratic and monotone structures of the model into global geometric tools for predicting species coexistence, oscillations or extinction. Much of the work was inspired by visualization of the carrying simplex and dynamics in three dimensions. See the visualizations below which are joint with Michael Drew LaMar.

### Carrying Simplices for 3-D Lotka-Volterra Systems

Zeeman's 1993 paper classifies the dynamics of 3-D Lotka Volterra systems, modulo periodic orbits, using a geometric analysis of the nullclines of the system. Zeeman and LaMar have written software to visualise Carrying Simplices and their dynamics, using the Geometry Center's visualisation package Geomview.

The following tour through parameter space shows 21 example systems, one in each row. First the nullcline configuration of the system; then a face view of the carrying simplex with representative orbits; and finally a side view of the carrying simplex showing it's curvature. The tour includes birurcations at the boundary of the carrying simplex, and at the interior fixed point.

The coloring system for the axes is: red, green and blue (RGB) for x, y and z respectively. the coloring system for the fixed points (represented by colored dots) is red, green, blue and white for repelling, saddle, attracting and neutral fixed points respectively. Note that the fixed points are colored to represent their dynamics restricted to the carrying simplex.

Click on the images for an enlarged view and a caption. In the viewer mode, click on the image or use the N and P keys to view the Next and Previous photos.

#### Parameter values for the systems in the tour.

```Page 1.

1.   b1=12  a11=12  a12=12  a13=12

b2=12  a21= 6  a22= 6  a23= 6

b3=12  a31= 3  a32= 3  a33= 3

2. Perturb a13 to 6

3. Perturb a13 to 4

Page 2.

1. Perturb a13 to 3

2. Perturb a13 to 2

3. Perturb a13 to 1

So we're at: 12 12 12 1

12  6  6 6

12  3  3 3

Page 3.

1. Perturb a31 to 9

2. Perturb a31 to 10

3. Perturb a31 to 11

Page 4.

1. Perturb a31 to 12

2. Perturb a31 to 13

3. Perturb a31 to 14

Page 5.

1. Perturb a31 to 15

2. Perturb a31 to 18

3. Perturb a31 to 24

Page6.

OK, now we'll make a Hopf Bifurcation happen, giving rise to an

attracting periodic orbit, with a repelling fixed point inside.

Start from Page 4, number2:  12 12 12 1

12  6  6 6

12 13  3 3

1. Multiply the 1st row by .5

2. Multiply the 1st row by 1/3 (ie: 4 4 4 1/3)

3. Multiply the 1st row by 1/10 (ie: 1.2 1.2 1.2 .1)

Page 7.

Another Hopf Bifurcation.  This time giving rise to a repelling

periodic orbit, with an attracting fixed point inside.

1. Start from:  12 12 12 1

12  6  6 6

12 16  3 3

2. Now Multiply the 2nd row by 1/2 (1e: 6 3 3 3)

3. Multiply 2nd row by 1/3 (ie: 4 2 2 2)

```

Next we exhibit an example of E.C. Zeeman, which has two isolated periodic orbits. The inside one attracting, and the outside one repelling.

#### Parameter values for this system.

```These are the params for E.C. Zeeman's 2 periodic orbits example.

There is a neutral (weakly attracting) internal fixed point for the system

a11=5.3   a12=10.3   a13=1.73

a21=4.5   a22=7.5    a23=11.07

a31=10    a32=2      a33=7

With b's being the sums of the a's.

i.e.  b1=17.33,  b2=23.07  and  b3=19

The boundary of this system also attracts, and hence there is a repelling

periodic orbit in between.

Now we perturb along the diagonal, subtracting .001 from each diagonal

entry, and from each b, to keep the b's as the sum of the a's.  The

internal fixed point becomes repelling, giving rise to a small attracting

orbit through a Hopf Bifurcation.  The boundary continues to attract,

throughout the Hopf bifurcation, so we now have two periodic orbits.

One repelling and one attracting.

The attracting fixed point passes through (approx) 0.117, 0.602, 0.289

The repelling fixed point passes through (approx) 0.694 0.054 0.237

```