Download Population Growth Models-Modeling and Simulation-Lecture Handouts and more Lecture notes Mathematical Modeling and Simulation in PDF only on Docsity! 1 CIS308 ‐ Modeling and Simulation Handout#6 Population Growth Models Thomas Malthus, an 18th century English scholar, observed in an essay written in 1798 that the growth of the human population is fundamentally different from the growth of the food supply to feed that population. He wrote that the human population was growing geometrically [i.e. exponentially] while the food supply was growing arithmetically [i.e. linearly]. He concluded that left unchecked, it would only be a matter of time before the world's population would be too large to feed itself. Mathematical Model 1. Malthusian Law of Population Growth (Exponential or Unlimited Growth) Malthus' model is commonly called the natural growth model or exponential growth model. For this model we assume that the population grows at a rate that is proportional to itself. If P represents such population then the assumption of natural growth can be written symbolically as dP/dt = k P Model Assumptions Limitation of resources and space have no effect Small population living in large environment Modeling Parameters t= time (independent variable) P = Population (dependent variable) k= growth‐rate co‐efficient o Proportionality constant between rate of grow of population and size of the population. Suppose we grow a population of some organism (say flies) in the laboratory. It seems reasonable that, on any given day, the population will change due to new births, so that it increases by the addition of a certain multiple f of the population. At the same time, a fraction d of the population will die. To track the population P of our laboratory organism, we focus on P, the change in population over a single day. P = f P − dP = (f − d)P What this means is simply that given a current population P (say P = 500) and the fecundity and death rates f and d, (say f = .1 and d = .03) we can predict the change in the population P = (.1 − .03)500 = 35 over a day. The population at the beginning of the next day is P + P = 500 + 35 = 535. A more general notation will make this simpler. Let Pt= P(t) = the size of the population measured on day t ΔPt = Pt+1− Pt is the change in population between two consecutive days. Pt+1 = Pt + ΔPt = Pt (f‐d) + Pt Pt+1 = λPt where (1+f‐d) is a constant – Population ecologists often refer to the constant λ as the finite growth rate of the population. For the values f = .1, d = .03, and P0 = 500 used previously, our entire model is now Pt+1 = 1.07Pt , P0 = 500 docsity.com 2 CIS308 ‐ Modeling and Simulation The first equation, relating Pt+1 and Pt is referred to as a difference equation and the second, giving P0 is its initial condition. With the two, it is easy to make a table of values of the population over time. In general, if the initial population is given as P0 at t = 0, then we have an initial value problem of the form dP/dt = kP ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐(I) P(0) = Po The solution to the above equation is P(t) = Poekt ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐(II) Exercises in MATLAB 1. Simulate the growth of a population according to equation (II). p0=100; % size of population at time 0 k=2; %population growth rate t=0:0.2:10; % timeperiod (in years) pt=p0*exp(k.*t); % the equation for pop. growth in continuous time figure % opens a new figure-window plot(t,pt) %plots the figure xlabel('Time(Years)') ylabel('Population Growth') grid on; 2. Consider a colony of micro‐organisms reproducing through simple cell division under ideal conditions of unlimited food supply and total absence of any predators. It is observed that the population increases q (10% every hour). Find a mathematical model (Using SIMULINK) that will also produce the same results. The unlimited growth model is given by P(t) = Poekt ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐(II) 0 1 2 3 4 5 6 7 8 9 10 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 x 10 10 Time(Years) P op ul at io n G ro w th docsity.com