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Population model

  • #1

Homework Statement



The simplest useful model for fisher comes from the logistic model for population growth, together with a harvest h which is proportional to the current population P, that is,

h=EP,

where the constant E is called the effort. E measures the fraction of the population harvested, so that 0 <= E <= 1. This gives the model

dP/dt = kP(1-(P/a) - EP,

where P(t) is the number of these fish at time t year and k (the natural growth rate) and a( the carrying capacity) are constants for a particular fish polulation. In what follows take k= 1 and a= 4, for simplicity.

a) determine the equilibrium solutions for a given effort E.

my attempt.

dP/dt = P - P^2/4 - EP

dP/dt = ( P + E) (P -1/4)

equilibrium solutions are when dP/dt=0

so P(0)= -E, 1/4.
 
Last edited:

Answers and Replies

  • #2
ehild
Homework Helper
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1,854

Homework Statement



The simplest useful model for fisher comes from the logistic model for population growth, together with a harvest h which is proportional to the current population P, that is,

h=EP,

where the constant E is called the effort. E measures the fraction of the population harvested, so that 0 <= E <= 1. This gives the model

dP/dt = kP(1-(P/a) - EP,
You miss a parentheses: dP/dt = kP(1-(P/a)) - EP

where P(t) is the number of these fish at time t year and k (the natural growth rate) and a( the carrying capacity) are constants for a particular fish polulation. In what follows take k= 1 and a= 4, for simplicity.

a) determine the equilibrium solutions for a given effort E.

my attempt.

dP/dt = P - P^2/4 - EP

dP/dt = ( P + E) (P -1/4)
The last equation is wrong.

equilibrium solutions are when dP/dt=0

so P(0)= -E, 1/4.
Because of the faulty factorisation, the result is not correct (and physically impossible).

ehild
 

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