Each time 't' changing while running ode45. Consider naming 't' differently.
clc; clear all ;
%determine the behavior of the host population in
%the absence of a parasitoid population for different
%values of r
N1 = 20;
K = 50;
r1 = 1;
r2 = 2;
r3 = 3;
r4 = 4;
tspan = [1,1000];
N0 = 20;
[t1,N] = ode45(@(t,N) N(1)*exp(r1*(1-(N(1)/K))), tspan, N0);
plot(t1,N,'-o')
tspan = [1,1000];
M0 = 20;
[t2,M] = ode45(@(t,M) M(1)*exp(r2*(1-(M(1)/K))), tspan, M0);
plot(t2,M,'-o')
tspan = [1,1000];
G0 = 20;
[t3,G] = ode45(@(t,G) G(1)*exp(r3*(1-(G(1)/K))), tspan, G0);
plot(t3,G,'-o')
tspan = [1,1000];
H0 = 20;
[t4,H] = ode45(@(t,H) H(1)*exp(r4*(1-(H(1)/K))), tspan, H0);
plot(t4,H,'-o')
figure
subplot(2,2,1)
plot(t1,N,'-o')
title('r=1)')
subplot(2,2,2)
plot(t2,M,'-o')
title('r=2')
subplot(2,2,3)
plot(t3,G,'-o')
title('r=3')
subplot(2,2,4)
plot(t4,H,'-o')
title('r=4')
