We are concerned with positive solutions decaying to zero at infinity for the
logistic equation $-\Delta u=\lambda (V(x)u-f(u))$ in $\RR^N$, where $V(x)$ is
a variable potential that may change sign, $\lambda$ is a real parameter, and
$f$ is an absorbtion term such that the mapping $f(t)/t$ is increasing in
$(0,\infty)$. We prove that there exists a bifurcation non-negative number
$\Lambda$ such that the above problem has exactly one solution if $\lambda
>\Lambda$, but no such a solution exists provided $\lambda\leq\Lambda$.