buffer = 10;
N1 = linspace(-buffer,k+buffer);

% Nullcline for N1
plot(zeros(1,length(N1)),N1,'r');
hold on;
N2 = k - N1;
plot(N1,N2,'r');

% Nullcline for N2
plot(N1, zeros(1,length(N1)));
N2 = k - a*N1;
plot(N1,N2);

xlabel('N_1');
ylabel('N_2');
axis([-buffer k+buffer -buffer k+buffer]);

title('Nullclines in the absence of predators');