Leslie matrix: why is it enough to just evaluate det$(A-λI)$ to tell if the population grows or decays?

Question

There is a $3\times3$ Leslie matrix and we are told to evaluate $\det(A—λI)$ to tell if the population grows or decays when $λ=1$;

$λ=1,$ the population is stable when evaluating $\det(A—λI) = 1/48 \gt 0$ which implies that the real eigenvalues must be either less than 1 (population decays) or greater than $1$ (population grows)

How do I conclude if the population grows or decays by just looking at $p(λ) = \det(A—λI) = 1/48 > 0$? Thank you for your help!