1. Lotka-Volterra Predator-Prey Problems
By: Alexandra Silva and Dani Hoover
Intro to Systems ESE 251
11/24/09

2. History Alfred Lotka Vito Volterra -American biophysicist
-Proposed the predator-prey model in 1925
-Italian mathematician
-Proposed the predator-prey model in 1926

3. 2-Species Models

4. Equations and Variables
X’ = ax – bxy
Y’ = -cy + dxy
X: the population of prey
Y: the population of predators
a: natural growth rate of prey in the absence of predation
b: death rate due to predation
c: natural death rate of predators in the absence of prey
d: growth rate due to predation

5. Assumptions
The prey always has an unlimited supply of food and reproduces exponentially
The food supply of the predators depend only on the prey population (predators eat the prey only)
The rate of change of the population is proportional to the size of the population
The environment does not change in favor of one species

6. Phase Plot of Predator vs. Prey
Set parameters a=b=c=d=1
Set initial conditions: x=2 (prey), y=2 (predators)
Equilibrium Point:
x=(c/d), y= (a/b)
Counter-clockwise motion
Equilibrium point (1,1)

7. Steady-State Orbit explanation
A = Too many predators. B = Too few prey. C = Few predator and prey; prey can grow. D= Few predators, ample prey.

8. Phase Plot: Case 2
When initial conditions equal the equilibrium point:
Parameters:
a=b=c=d=1
Initial conditions:
x=1 (prey), y=1 (predators)

9. Phase Plot: Case 3 When parameters are changed: Parameters:
a=c=d=1, b=2
*Increase the death rate due to predation
Initial Conditions:
x=2 (prey), y=2 (predator)

10. Phase Plot: Case 4 When a species dies out: Parameters:
a=1, b=c=d=1
Initial Conditions:
x=50 (prey), y=500 (predator)
Prey dies, therefore predator dies too.

11. 3-Species Model (Super-predator)

12. Equations and Variables (for 3-species model)
X’= ax-bxy (prey-- mouse)
Y’= -cy+dxy-eyz (predator-- snake)
Z’= -fz+gxz (super-predator-- owl)
a: natural growth rate of prey in the absence of predation
b: death rate due to predation
c: natural death rate of predator
d: growth rate due to predation
e: death rate due to predation (by super-predator)
f: natural death rate of super-predator
g: growth rate due to predation

13. Phase Plot of Prey vs. Predator vs. Super-predator
Parameters:
a=b=c=d=1, e=0.5, f=0.01, g=0.02
Initial Condition:
X=1, Y=1, Z=1

14. Problems with Lotka-Volterra Models
The Lotka-Volterra model has infinite cycles that do not settle down quickly. These cycles are not very common in nature.
Must have an ideal predator-prey system.
In reality, predators may eat more than one type of prey
Environmental factors

15. Thank you
Thank you to Anatoly for helping us with this presentation and helping us to make programs in MATLAB.

16. Questions?

17. Sources: